ZXLD1350

350mA LED driver with internal switch

Description
The ZXLD1350 is a continuous mode The ADJ pin will accept either a DC voltage or a
inductive step-down converter, designed for PWM waveform. Depending upon the control
driving single or multiple series connected frequency, this will provide either a continuous
LEDs efficiently from a voltage source higher or a gated output current. The PWM filter
than the LED voltage. The device operates components are contained within the chip.
from an input supply between 7V and 30V and The PWM filter provides a soft-start feature by
provides an externally adjustable output controlling the rise of input/output current. The
current of up to 350mA. Depending upon soft-start time can be increased using an
supply voltage and external components, this external capacitor from the ADJ pin to ground.
can provide up to 8 watts of output power. Applying a voltage of 0.2V or lower to the ADJ
The ZXLD1350 includes the output switch and pin turns the output off and switches the device
a high-side output current sensing circuit, into a low current standby state.
which uses an external resistor to set the The device is assembled in a TSOT23-5 pin
nominal average output current. package.
Output current can be adjusted above, or
below the set value, by applying an external
control signal to the 'ADJ' pin.
Features Applications
• Simple low parts count • Low voltage halogen replacement LEDs
• Internal 30V NDMOS switch • Automotive lighting
www.DataSheet4U.com
• 350mA output current • Low voltage industrial lighting
• Single pin on/off and brightness control • LED back-up lighting
using DC voltage or PWM • Illuminated signs
• Internal PWM filter
• Soft-start
• High efficiency (up to 95%(*))
• Wide input voltage range: 7V to 30V
• 40V transient capability
• Output shutdown
• Up to 1MHz switching frequency
• Inherent open-circuit LED protection
• Typical 4% output current accuracy
(*) Using standard external components as specified under electrical characteristics. Efficiency is dependent upon the
number of LEDs driven and on external component types and values.
Pin connections Typical application circuit
VIN (12V - 30V) Rs
LX 1 5 VIN 0.33

GND 2
L1 47␮H
ADJ 3 4 ISENSE D1
ZLLS1000
C1 1␮F

TSOT23-5
Top view VIN ISENSE LX

N/C ADJ ZXLD1350

GND

GND

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Absolute maximum ratings (voltages to GND unless otherwise stated)

Input voltage (VIN) -0.3V to +30V (40V for 0.5 sec)
ISENSE voltage (VSENSE) +0.3V to -5V (measured with respect to VIN)
LX output voltage (VLX) -0.3V to +30V (40V for 0.5 sec)
Adjust pin input voltage (VADJ) -0.3V to +6V
Switch output current (ILX) 500mA
Power dissipation (Ptot) 450mW
(Refer to package thermal de-rating curve on page 18)
Operating temperature (TOP) -40 to 105°C
Storage temperature (TST) -55 to 150°C
Junction temperature (Tj MAX) 150°C
These are stress ratings only. Operation above the absolute maximum rating may cause device failure. Operation at
the absolute maximum ratings, for extended periods, may reduce device reliability.

Thermal resistance
Junction to ambient (R⍜JA) 200°C/W

Electrical characteristics (test conditions: VIN=12V, Tamb=25°C unless otherwise stated) (*)
Symbol Parameter Conditions Min. Typ. Max. Unit
VIN Input voltage 7 30 V
VSU Internal regulator start-up threshold VIN rising 4.8 V
IINQoff Quiescent supply current ADJ pin grounded
with output off 15 20 µA
IINQon Quiescent supply current ADJ pin floating
with output switching f=250kHz 250 500 µA
VSENSE Mean current sense threshold Measured on ISENSE pin with 95 100 105 mV
voltage respect to VIN
(defines LED current setting VADJ =1.25V
accuracy)
VSENSEHYS Sense threshold hysteresis ±15 %
ISENSE ISENSE pin input current VSENSE =VIN -0.1 1.25 10 µA
VREF Internal reference voltage Measured on ADJ pin with 1.21 1.25 1.29 V
pin floating
⌬VREF /⌬T Temperature coefficient of VREF 50 ppm/°C

VADJ External control voltage range on 0.3 2.5 V

ADJ pin for dc brightness control (†)
VADJoff DC voltage on ADJ pin to switch VADJ falling 0.15 0.2 0.25 V
device from active (on) state to
quiescent (off) state
VADJon DC voltage on ADJ pin to switch VADJ rising 0.2 0.25 0.3 V
device from quiescent (off) state to
active (on) state
RADJ Resistance between ADJ pin and 135 250 k⍀
VREF
ILXmean Continuous LX switch current 0.37 A

RLX LX Switch ‘On’ resistance 1.5 2 ⍀

ILX(leak) LX switch leakage current 1 µA

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Electrical characteristics (test conditions: VIN=12V, Tamb=25°C unless otherwise stated) (*) (continued)
Symbol Parameter Conditions Min. Typ. Max. Unit
DPWM(LF) Duty cycle range of PWM signal PWM frequency <500Hz 0.01 1
applied to ADJ pin during low PWM amplitude= VREF
frequency PWM dimming mode Measured on ADJ pin
Brightness control range 100:1

DPWM(HF) Duty cycle range of PWM signal PWM frequency >10kHz 0.16 1
applied to ADJ pin during high PWM amplitude= VREF
frequency PWM dimming mode Measured on ADJ pin
Brightness control range 5:1

TSS Soft start time Time taken for output current 500 µs
to reach 90% of final value
after voltage on ADJ pin has
risen above 0.3V
fLX Operating frequency ADJ pin floating
(See graphs for more detail) L=100µH (0.82⍀)
IOUT=350mA @ VLED=3.4V
Driving 1 LED 250 KHz
TONmin Minimum switch ‘ON’ time LX switch ‘ON’ 200 ns

TOFFmin Minimum switch ‘OFF’ time LX switch ‘OFF’ 200 ns

fLXmax Recommended maximum operating 1 MHz

frequency
DLX Recommended duty cycle range of 0.3 0.7
output switch at fLXmax

TPD Internal comparator propagation 50 ns

delay
NOTES:
(*) Production testing of the device is performed at 25°C. Functional operation of the device and parameters specified over
a -40°C to +105°C temperature range, are guaranteed by design, characterization and process control.
(†) 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE
threshold and output current proportionally.

Pin description
Name Pin No. Description
LX 1 Drain of NDMOS switch
GND 2 Ground (0V)
ADJ 3 Multi-function On/Off and brightness control pin:
• Leave floating for normal operation.(VADJ= VREF =1.25V giving nominal average output current
IOUTnom=0.1/RS)
• Drive to voltage below 0.2V to turn off output current
• Drive with DC voltage (0.3V<VADJ<2.5V) to adjust output current from 25% to 200%(†) of IOUTnom
• Drive with PWM signal from open-collector or open-drain transistor, to adjust output current.
Adjustment range 25% to 100% of IOUTnom for f>10kHz and 1% to 100% of IOUTnom for f<500Hz
• Connect a capacitor from this pin to ground to increase soft-start time. (Default soft-start
time=0.5ms. Additional soft-start time is approx.0.5ms/nF)
ISENSE 4 Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom=0.1/RS
(Note: RSMIN=0.27⍀ with ADJ pin open-circuit)
VIN 5 Input voltage (7V to 30V). Decouple to ground with 1µF or higher X7R ceramic capacitor close to
device

Ordering information
Device Reel size Reel width Quantity per reel Device mark
(mm) (mm)
ZXLD1350ET5TA 180 8 3,000 1350

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Block diagram

RS
VIN

L1
D1

VIN ISENSE LX

VIN
R1
Current sense circuit

Voltage 5V
regulator
-

+
Shutdown
circuit
C1 Comparator

ADJ + MN

R2
200k 4KHz

Vref 1.25V R3

GND

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Device description

The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a self-oscillating
continuous-mode buck converter.

Device operation (Refer to block diagram and Figure 1 - Operating waveforms)

Operation can be best understood by assuming that the ADJ pin of the device is unconnected and
the voltage on this pin (VADJ) appears directly at the (+) input of the comparator.
When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN
and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is
forced across internal resistor R1 by the current sense circuit and produces a proportional current
in internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of
the comparator. When this reaches the threshold voltage (VADJ), the comparator output switches
low and MN turns off. The comparator output also drives another NMOS switch, which bypasses
internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be
nominally 15% of VADJ.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The current
decays at a rate determined by the LED and diode forward voltages to produce a falling voltage
at the input of the comparator. When this voltage returns to VADJ, the comparator output switches
high again. This cycle of events repeats, with the comparator input ramping between limits of
VADJ ± 15%.

Switching thresholds
With VADJ =VREF, the ratios of R1, R2 and R3, define an average VSENSE switching threshold of
100mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is
then defined by this voltage and Rs according to:
IOUTnom=100mV/RS
Nominal ripple current is ±15mV/RS

Adjusting output current

The device contains a low pass filter between the ADJ pin and the threshold comparator and an
internal current limiting resistor (200k nom) between ADJ and the internal reference voltage. This
allows the ADJ pin to be overdriven with either DC or pulse signals to change the VSENSE
switching threshold and adjust the output current. The filter is third order, comprising three
sections, each with a cut-off frequency of nominally 4kHz.
Details of the different modes of adjusting output current are given in the applications section.

Output shutdown
The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuit
falls below the threshold (0.2V nom), the internal regulator and the output switch are turned off.
The voltage reference remains powered during shutdown to provide the bias current for the
shutdown circuit. Quiescent supply current during shutdown is nominally 15␮A and switch
leakage is below 1␮A.

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VIN

LX voltage

0V
Toff Ton

VIN
115mV 85mV 100mV
SENSE voltage VSENSE-
VSENSE+

IOUTnom +15%

Coil current IOUTnom

IOUTnom -15%
0V

0.15VADJ
Comparator
input voltage VADJ
0.15VADJ

5V
Comparator
output

0V

Figure 1 Operating waveforms

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Typical operating waveforms [VIN=12V, RS=0.3⍀, L=100µH]

Normal operation. Output current (Ch3) and LX voltage (Ch1)

Start-up waveforms. Output current (Ch3), LX voltage (Ch1) and VADJ (Ch2)

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Typical operating conditions

For typical application circuit driving 1W Luxeon® white LED(s) at VIN =12V and Tamb=25°C unless
otherwise stated.

Efficiency vs No. of LEDs Duty Cycle vs Input Voltage

L=100uH, Rs=0.33 Ohms L=100uH, Rs=0.33 Ohms

100 1.2

95 1

1 LED 1 LED
90 0.8
2 LED 2 LED
Efficiency (%)

Duty Cycle
3 LED 3 LED
4 LED 4 LED
85 0.6
5 LED 5 LED
6 LED 6 LED
7 LED 7 LED
80 0.4
8 LED 8 LED

75 0.2

70 0
5 10 15 20 25 30 5 10 15 20 25 30
VIN (V) VIN (V)

Operating Frequency vs Input Voltage Output current variation with Supply Voltage
L=100uH, Rs=0.33 Ohms L=100uH, Rs=0.33 Ohms

600 8

6
500
Deviation from nominal set current (%)

4
1 LED
400
2 LED
Frequency (kHz)

1 LED
3 LED 2
2 LED
4 LED 3 LED
300
5 LED 0 4 LED
6 LED 5 10 15 20 25 30 5 LED
7 LED 6 LED
200 -2
8 LED 7 LED

-4
100

-6
0
5 10 15 20 25 30 -8
VIN (V) VIN (V)

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Typical operating conditions (continued)

Efficiency vs No. of LEDs Duty Cycle vs Input Voltage

L=47uH, Rs=0.33 Ohms L=47uH, Rs=0.33 Ohms

100 1
95 1 LED 1 LED
0.8
Efficiency (%)

2 LED 2 LED

Duty Cycle
90 3 LED 0.6 3 LED
85 4 LED 4 LED
5 LED 0.4 5 LED
80
6 LED 6 LED
75 0.2
7 LED 7 LED
70 0
5 10 15 20 25 30 5 10 15 20 25 30
VIN (V) VIN (V)

Operating Frequency vs Input Voltage Output Current Variation vs Supply Voltage

L=47uH, Rs=0.33 Ohms L=47uH, Rs=0.33 Ohms
20
800
Deviation from nominal

700 15 1 LED
1 LED
Frequency (kHz)

set current (%)

600 2 LED 10 2 LED

500 3 LED 3 LED
5
400 4 LED 4 LED
300 5 LED 0 5 LED
200 6 LED 6 LED
-5 5 10 15 20 25 30
100 7 LED 7 LED
0 -10
5 10 15 20 25 30 -15
VIN (V) VIN (V)

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Typical operating conditions (continued)

Efficiency vs No. of LEDs Duty Cycle vs Input Voltage
L=220uH, Rs=0.33 Ohms L=220uH, Rs=0.33 Ohms

1
100

0.8
95
1 LED
1 LED
2 LED
2 LED 0.6
Efficiency (%)

Duty Cycle
90 3 LED
3 LED
4 LED
4 LED
5 LED
5 LED
0.4 6 LED
85 6 LED
7 LED
7 LED
8 LED
8 LED

80 0.2

75 0
5 10 15 20 25 30 5 10 15 20 25 30
VIN (V) VIN (V)

Operating Frequency vs Input Voltage Output Current Variation vs Input Voltage

L=220uH, Rs=0.33 Ohms L=220uH, Rs=0.33 Ohms

350 2

300 1
Deviation from nominal set current (%)

0
250
1 LED 5 10 15 20 25 30 1 LED
Frequency (kHz)

2 LED
-1 2 LED
200 3 LED
3 LED
4 LED
4 LED
5 LED -2
150 5 LED
6 LED
6 LED
7 LED -3 7 LED
100 8 LED
8 LED
-4
50
-5
0
5 10 15 20 25 30 -6
VIN (V) VIN (V)

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Typical operating conditions (continued)

Vref vs Vin over nominal supply voltage range Vref vs Vin at low supply voltage

1.2425 1.4
1.2
1
Vref (V)

Vref (V)
0.8
1.242
0.6
0.4
0.2
0
1.2415
0 1 2 3 4 5 6 7 8 9 10
5 10 15 20 25 30
Vin (V)
Vin (V)

Supply Current vs Vin (Operating) Supply Current vs Vin (Quiescent)

500 20

4 00
15

300
Iin (u A )

I in (uA)

10
200

5
1 00

0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Vin (V) Vin (V)

Output Current vs VADJ

35 0

300

250
Iout mean (mA)

20 0 Rs=0.3 Ohm
Rs=0.56 Ohm
Rs=1 Ohm
150

100

50

0
0 0.5 1 1.5 2 2.5 3
VADJ (V)

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Typical operating conditions (continued)

VADJ vs Temperature Output Current Change vs Temperature

L=100uH, Rs=0.33 Ohms VIN=7V, L=100uH, Rs=0.33 Ohms

1.255 2

Deviation from nominal set value

1.25 1

Vin = 7V
Vadj (V)

Vin = 9V
1.245

(%)
Vin = 12V 0
Vin = 30V -60 -40 -20 0 20 40 60 80 100 120 140

1.24
-1

1.235
-50 0 50 100 150 -2
Temperature (Deg C) Temperature (Deg C)

LX Switch 'On' Resistance vs Temperature Output Current Change vs Temperature

VIN=12V, L=100uH, Rs=0.33 Ohms
2.6

2.4
Deviation from nominal set value 0.5

2.2

0.25
2
Ohms

1.8
(%)

0
1.6 -60 -40 -20 0 20 40 60 80 100 120 140

1.4
-0.25
1.2

1
-60 -40 -20 0 20 40 60 80 100 120 140 160
-0.5
Temperature (Deg C) Temperature (Deg C)

Output Current Change vs Temperature

VIN=30V, L=100uH, Rs=0.33 Ohms

4
Deviation from nominal set value

3
(%)

0
-60 -40 -20 0 20 40 60 80 100 120 140
Temperature (Deg C)

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Application notes

Setting nominal average output current with external resistor RS

The nominal average output current in the LED(s) is determined by the value of the external
current sense resistor (RS) connected between VIN and ISENSE and is given by:
IOUTnom = 0.1/RS [for RS>0.27⍀]
The table below gives values of nominal average output current for several preferred values of
current setting resistor (RS) in the typical application circuit shown on page 1:

RS (⍀) Nominal average

output current (mA)
0.27 370
0.3 333
0.33 300
0.39 256
The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V).
Note that RS=0.27⍀ is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value.
It is possible to use different values of RS if the ADJ pin is driven from an external voltage. (See
next section).
Output current adjustment by external DC control voltage
The ADJ pin can be driven by an external dc voltage (VADJ), as shown, to adjust the output current
to a value above or below the nominal average value defined by RS.

ADJ ZXLD1350
+
GND
DC

GND

The nominal average output current in this case is given by:

IOUTdc = 0.08*VADJ/RS [for 0.3< VADJ <2.5V]
Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above
1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 370mA maximum.
The input impedance of the ADJ pin is 200k⍀ ±25%.

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Output current adjustment by PWM control

Directly driving ADJ input
A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as
shown below, to adjust the output current to a value above or below the nominal average value
set by resistor RS:

PWM
VADJ
ADJ ZXLD1350

0V GND

GND

Driving the ADJ input via open collector transistor

The recommended method of driving the ADJ pin and controlling the amplitude of the PWM
waveform is to use a small NPN switching transistor as shown below:

ADJ ZXLD1350
PWM
GND

GND

This scheme uses the 200k resistor between the ADJ pin and the internal voltage reference as a
pull-up resistor for the external transistor.
Driving the ADJ input from a microcontroller
Another possibility is to drive the device from the open drain output of a microcontroller. The
diagram below shows one method of doing this:

MCU
10k
ADJ ZXLD1350

GND

The diode and resistor suppress possible high amplitude negative spikes on the ADJ input
resulting from the drain-source capacitance of the FET. Negative spikes at the input to the device
should be avoided as they may cause errors in output current, or erratic device operation.
See the section on PWM dimming for more details of the various modes of control using high
frequency and low frequency PWM signals.

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Shutdown mode
Taking the ADJ pin to a voltage below 0.2V for more than approximately 100µs, will turn off the
output and supply current will fall to a low standby level of 15µA nominal.
Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above VREF will increase
output current above the 100% nominal average value. (See graphs for details).
Soft-start
The device has inbuilt soft-start action due to the delay through the PWM filter. An external
capacitor from the ADJ pin to ground will provide additional soft-start delay, by increasing the
time taken for the voltage on this pin to rise to the turn-on threshold and by slowing down the
rate of rise of the control voltage at the input of the comparator. With no external capacitor, the
time taken for the output to reach 90% of its final value is approximately 500µs. Adding
capacitance increases this delay by approximately 0.5ms/nF. The graph below shows the
variation of soft-start time for different values of capacitor.

Soft Start Time vs Capacitance from ADJ pin to Ground

10

8
Soft Start time (ms)

0
0 5 10 15 20 25
Capacitance (nF)

Inherent open-circuit LED protection

If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so
the device will not be damaged, unlike in many boost converters, where the back EMF may
damage the internal switch by forcing the drain above its breakdown voltage.
Capacitor selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in
series with the supply source impedance and lowers overall efficiency. This capacitor has to
supply the relatively high peak current to the coil and smooth the current ripple on the input
supply. A minimum value of 1␮F is acceptable if the input source is close to the device, but higher
values will improve performance at lower input voltages, especially when the source impedance
is high. The input capacitor should be placed as close as possible to the IC.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better
dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this
application and should NOT be used.
A table of recommended manufacturers is provided below:

Manufacturer Website
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Kemet www.kemet.com
AVX www.avxcorp.com

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Inductor selection

Recommended inductor values for the ZXLD1350 are in the range 47␮H to 220␮H.
Higher values of inductance are recommended at higher supply voltages in order to minimize
errors due to switching delays, which result in increased ripple and lower efficiency. Higher
values of inductance also result in a smaller change in output current over the supply voltage
range. (See graphs). The inductor should be mounted as close to the device as possible with low
resistance connections to the LX and VIN pins.
The chosen coil should have a saturation current higher than the peak output current and a
continuous current rating above the required mean output current.
Suitable coils for use with the ZXLD1350 are listed in the table below:

Part No. L DCR ISAT Manufacturer

(␮H) (⍀) (A)
DO1608C 47 0.64 0.5
47 0.38 0.56
CoilCraft
MSS6132ML 68 0.58 0.47
100 0.82 0.39
CD104-MC 220 0.55 0.53 Sumida
NP04SB470M 47 0.27 0.38 Taiyo Yuden
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times
within the specified limits over the supply voltage and load current range.
The following equations can be used as a guide, with reference to Figure 1 - Operating
waveforms.
LX Switch 'On' time
LΔI
T ON = ---------------------------------------------------------------------------------------
-
V IN – V LED – I avg ( R S + rL + R LX )
Note: TONmin>200ns
LX Switch 'Off' time
LΔI
T OFF = ----------------------------------------------------------------------
-
V LED + VD + I avg ( R S + rL )
Note: TOFFmin>200ns

Where:
L is the coil inductance (H)
rL is the coil resistance (⍀)
Iavg is the required LED current (A)
⌬I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RLX is the switch resistance (⍀)
VD is the diode forward voltage at the required load current (V)

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Example:

For VIN =12V, L=47␮H, rL=0.64⍀, VLED=3.4V, Iavg =350mA and VD =0.36V
TON = (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622␮s
TOFF = (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21␮s
This gives an operating frequency of 546kHz and a duty cycle of 0.34.
These and other equations are available as a spreadsheet calculator from the Zetex website.
Go to www.zetex.com/zxld1350
Note that in practice, the duty cycle and operating frequency will deviate from the calculated
values due to dynamic switching delays, switch rise/fall times and losses in the external
components.
Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal
supply voltage. This helps to equalize the undershoot and overshoot and improves temperature
stability of the output current.
Diode selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance
Schottky diode with low reverse leakage at the maximum operating voltage and temperature. The
recommended diode for use with this part is the ZLLS1000. This has approximately ten times
lower leakage than standard Schottky diodes, which are unsuitable for use above 85°C. It also
provides better efficiency than silicon diodes, due to a combination of lower forward voltage and
reduced recovery time.
The table below gives the typical characteristics for the ZLLS1000:

Diode Forward voltage Continuous Reverse Leakage Package Manufacturer

at 100mA current At 30V 85°C
(mV) (mA) (␮A)
ZLLS1000 310 1000 300 TSOT23 Zetex
If alternative diodes are used, it is important to select parts with a peak current rating above the
peak coil current and a continuous current rating higher than the maximum output load current.
It is very important to consider the reverse leakage of the diode when operating above 85°C.
Excess leakage will increase the power dissipation in the device.
The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will
increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to
ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed the
specified maximum value.

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Reducing output ripple

Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor Cled
across the LED(s) as shown below:

VIN Rs

LED Cled

L1
D1

VIN ISENSE LX

ZXLD1350

A value of 1␮F will reduce nominal ripple current by a factor three (approx.). Proportionally lower
ripple can be achieved with higher capacitor values. Note that the capacitor will not affect
operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of
LED voltage.
Operation at low supply voltage
The internal regulator disables the drive to the switch until the supply has risen above the start-
up threshold (VSU). Above this threshold, the device will start to operate. However, with the
supply voltage below the specified minimum value, the switch duty cycle will be high and the
device power dissipation will be at a maximum. Care should be taken to avoid operating the
device under such conditions in the application, in order to minimize the risk of exceeding the
maximum allowed die temperature. (See next section on thermal considerations).
Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient
to prevent the device from switching below approximately 6V. This will minimize the risk of
damage to the device.
Thermal considerations
When operating the device at high ambient temperatures, or when driving maximum load
current, care must be taken to avoid exceeding the package power dissipation limits. The graph
below gives details for power derating. This assumes the device to be mounted on a 25mm2 PCB
with 1oz copper standing in still air.

Maximum Power Dissipation

500

400

300
Power (mW)

200

100

0
-50 -30 -10 10 30 50 70 90 110 130
Ambient Temperature (Deg C)

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 ZXLD1350

Note that the device power dissipation will most often be a maximum at minimum supply
voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of
unsuitable coils, or excessive parasitic output capacitance on the switch output.
Thermal compensation of output current
High luminance LEDs often need to be supplied with a temperature compensated current in order
to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted
remotely from the device, so for this reason, the temperature coefficients of the internal circuits
for the ZXLD1350 have been optimized to minimize the change in output current when no
compensation is employed. If output current compensation is required, it is possible to use an
external temperature sensing network - normally using Negative Temperature Coefficient (NTC)
thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network
can be used to drive the ADJ pin in order to reduce output current with increasing temperature.

Layout considerations
LX pin
The LX pin of the device is a fast switching node, so PCB tracks should be kept as short as
possible. To minimize ground 'bounce', the ground pin of the device should be soldered directly
to the ground plane.
Coil and decoupling capacitors
It is particularly important to mount the coil and the input decoupling capacitor close to the device
to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important
to take account of any track resistance in series with current sense resistor RS.
ADJ pin
The ADJ pin is a high impedance input, so when left floating, PCB tracks to this pin should be as
short as possible to reduce noise pickup. A 100nF capacitor from the ADJ pin to ground will
reduce frequency modulation of the output under these conditions. An additional series 10k⍀
resistor can also be used when driving the ADJ pin from an external circuit (see below). This
resistor will provide filtering for low frequency noise and provide protection against high voltage
transients.

10k
ADJ ZXLD1350

GND
100nF

GND

High voltage tracks

Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage due to
board contamination. Any such leakage may raise the ADJ pin voltage and cause excessive
output current. A ground ring placed around the ADJ pin will minimize changes in output current
under these conditions.

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 ZXLD1350

Evaluation PCB
The picture below shows the top copper layout of the 3 LED ZXLD1350EV2 evaluation board. This
board and other evaluation boards for the ZXLD1350 are available upon request.

k
ZXLD1350EV2
EVALUATION BOARD
D1
LED k
a
JP1 JP2

+VIN C1 SD1 k
C3 RS

U1 C2 D2
L1
GND R1
a

JP3
ADJ
k
LED a
D3

a
Bare board: ZDB308R2
Copyright Zetex Plc 2006

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 ZXLD1350

Dimming output current using PWM

Low frequency PWM mode

When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltage
VADJ and a low level of zero, the output of the internal low pass filter will swing between 0V and
VADJ, causing the input to the shutdown circuit to fall below its turn-off threshold (200mV nom)
when the ADJ pin is low. This will cause the output current to be switched on and off at the PWM
frequency, resulting in an average output current IOUTavg proportional to the PWM duty cycle.
(See Figure 2 - Low frequency PWM operating waveforms).

VADJ

Ton Toff
PWM Voltage

0V

VADJ

Filter Output
300mV
200mV
0V

IOUTnom 0.1/Rs

Output Current IOUTavg

Figure 2 Low frequency PWM operating waveforms

The average value of output current in this mode is given by:
IOUTavg 0.1DPWM/RS [for DPWM >0 01]
This mode is preferable if optimum LED 'whiteness' is required. It will also provide the widest
possible dimming range (approx. 100:1) and higher efficiency at the expense of greater output
ripple.
Note that the low pass filter introduces a small error in the output duty cycle due to the difference
between the start-up and shut-down times. This time difference is a result of the 200mV shutdown
threshold and the rise and fall times at the output of the filter. To minimize this error, the PWM
duty cycle should be as low as possible consistent with avoiding flicker in the LED.

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 ZXLD1350

High frequency PWM mode

At PWM frequencies above 10kHz and for duty cycles above 0.16, the output of the internal low
pass filter will contain a DC component that is always above the shutdown threshold. This will
maintain continuous device operation and the nominal average output current will be
proportional to the average voltage at the output of the filter, which is directly proportional to the
duty cycle. (See Figure 3 - High frequency PWM operating waveforms). For best results, the PWM
frequency should be maintained above the minimum specified value of 10kHz, in order to
minimize ripple at the output of the filter. The shutdown comparator has approximately 50mV of
hysteresis, to minimize erratic switching due to this ripple. An upper PWM frequency limit of
approximately one tenth of the operating frequency is recommended, to avoid excessive output
modulation and to avoid injecting excessive noise into the internal reference.

VADJ

Ton Toff
PWM voltage

0V

VADJ

Filter output

200mV
0V

0.1/RS
Output current IOUTnom

Figure 3 High frequency PWM operating waveforms

The nominal average value of output current in this mode is given by:
IOUTnom ≈0.1DPWM/RS [for DPWM >0.16]
This mode will give minimum output ripple and reduced radiated emission, but with a reduced
dimming range (approx.5:1). The restricted dimming range is a result of the device being turned
off when the dc component on the filter output falls below 200mV.

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 ZXLD1350

Package outline - TSOT23-5

DIM Millimeters Inches

Min. Max. Min. Max.
A - 1.00 - 0.0393
A1 0.01 0.10 0.0003 0.0039
A2 0.84 0.90 0.0330 0.0354
b 0.30 0.45 0.0118 0.0177
c 0.12 0.20 0.0047 0.0078
D 2.90 BSC 0.114 BSC
E 2.80 BSC 0.110 BSC
E1 1.60 BSC 0.062 BSC
e 0.95 BSC 0.0374 BSC
e1 1.90 BSC 0.0748 BSC
L 0.30 0.50 0.0118 0.0196
L2 0.25 BSC 0.010 BSC
a° 4° 12° 4° 12°
Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches

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 ZXLD1350

Definitions
Product change
Zetex Semiconductors reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or
service. Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights
arising from such use or otherwise. Zetex does not assume any legal responsibility or will not be held legally liable (whether in contract,
tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business, contract,
opportunity or consequential loss in the use of these circuit applications, under any circumstances.
Life support
Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express written
approval of the Chief Executive Officer of Zetex Semiconductors plc. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
Reproduction
The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the
company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a
representation relating to the products or services concerned.
Terms and Conditions
All products are sold subjects to Zetex’ terms and conditions of sale, and this disclaimer (save in the event of a conflict between the two
when the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.
For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Zetex sales office.
Quality of product
Zetex is an ISO 9001 and TS16949 certified semiconductor manufacturer.
To ensure quality of service and products we strongly advise the purchase of parts directly from Zetex Semiconductors or one of our
regionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com/salesnetwork
Zetex Semiconductors does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels.
ESD (Electrostatic discharge)
Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.
The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
Green compliance
Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or exceeding
regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to
reduce the use of hazardous substances and/or emissions.
All Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance with
WEEE and ELV directives.
Product status key:
“Preview” Future device intended for production at some point. Samples may be available
“Active” Product status recommended for new designs
“Last time buy (LTB)” Device will be discontinued and last time buy period and delivery is in effect
“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete” Production has been discontinued
Datasheet status key:
“Draft version” This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version” This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
“Issue” This term denotes an issued datasheet containing finalized specifications. However, changes to
specifications may occur, at any time and without notice.

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© 2006 Published by Zetex Semiconductors plc

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© Zetex Semiconductors plc 2007