LD5850

11/30/2012

Boost Controller for LED Backlight

REV: 00

General Description Features

The LD5850 is a wide-input asynchronous current mode  Wide Input Range: 9V to 28V
boost controller, capable to operate in the range between  Current Mode Control
9V and 28V and to generate 14V of voltage to the GATE  0.3V LED Feedback Current Sensing Reference
pin of MOSFET to reduce thermal loss. The current mode  Fixed Switching Frequency
control architecture enhances transient response and  PWM Dimming Input
simplifies the loop compensation. The DIM input allows  Internal Slope Compensation
the brightness control for LED Backlight or LED lighting.  Cycle-by-Cycle Current Limit
 Over Temperature Protection
The Device also features internal slope compensation,
input voltage under-voltage lockout, output voltage short Applications
circuit protection, cycle-by-cycle current limit and thermal  LED TV Backlight
shutdown protection.  LED Monitor Backlight
 LED lighting

Typical Application
VIN VOUT

RA

RB

RCS
VCC

1 2 4
VCC DRV CS
DIM
8 DIM LD5850 FB 5

OVP COMP GND RFB

7 6 3

COVP

Fig. 1 Application circuit

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Pin Configuration
SOP-8 (TOP VIEW)
VCC

COMP
OVP
DIM

FB
8 7 6 5

TOP MARK YY: Year code

YYWWPP WW: Week code
PP: Production code

1 YYWWPP
2 3 4
DRV
VCC

GND
CS

Ordering Information
Part number Package TOP MARK Shipping

LD5850 GS SOP-8 LD5850 GS 2500 /tape & reel

The LD5850 is ROHS compliant/ Green Packaged.

Pin Descriptions
PIN NAME FUNCTION
1 VCC Power source VCC pin.
2 DRV Gate drive output to drive the external MOSFET.
3 GND Ground.
Current Sense pin. Connect with an external current sensing resistor to GND. CS
4 CS pin voltage is used to provide current feedback in the control loop and detect an
overcurrent condition.
5 FB LED output current feedback through a current sense resistor.
6 COMP Compensation of the error amplifier for voltage loop compensation.
7 OVP Over-voltage protection.
PWM Dimming Input. If the DIM pin voltage is below logic Voltage threshold, the
8 DIM DRV control would be stopped immediately until DIM pin voltage rises to high
again.

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Block Diagram
VCC

DIM DIM

UVLO Internal Bias

Vref
VCC OK & Vref
8.7V/7.0V

Vref OK
OVP
SCP
0.2V

2.5V
OVP
SCP Driver
DIM Stage
DRV
LED SHORT

14V

Osillator S Q
Slope
Compensation
PWM R
Comparator

CS LEB
0.6V
0.5V OCP LED SHORT
Comparator

GND
Voltage Divider 0.3V
& GM
Buffer FB

COMP

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Absolute Maximum Ratings
VCC, DRV, CS, FB, OVP 30V
All Other Pins -0.3V ~ 5.5V
Power Dissipation, PD@85C, SOP-8 250mW
Package Thermal Resistance, SOP-8, JA 160°C/W
Maximum Junction Temperature 150C
Operating Junction Temperature -40C to 125C
Operating Ambient Temperature -40C to 85C
Storage Temperature Range -55C to 125C
Lead Temperature (Soldering, 10sec) 260C
ESD Level (Human Body Model) 2.0KV
ESD Level (Machine Model) 200V

Recommended Operating Conditions

Input Supply Voltage 9V to 28V
DIM Frequency 100Hz to 800Hz
Dimming duty cycle 1% ~ 100%
DIM PIN voltage 0V ~ 5V

Caution:
Stress exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stress above Recommended Operating Conditions may affect
device reliability.

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Electrical Characteristics
(VCC=12V, TA=25C, unless otherwise noted.)
PARAMETER CONDITIONS MIN. TYP. MAX. UNITS
Input Power (VCC)
Turn On Level UVLO(on) 8.7 V
Turn Off Level UVLO(off) 7 V
Shutdown Current DIM=Low, over 200ms 40 A
Standby Current DIM=Low 1 mA
DIM=High,
Operating Current 5 mA
Switching at no load
Boost Converter
Switching frequency 162 180 198 KHz
Boost Maximum duty cycle Switching frequency=180KHz 85 90 95 %
Source current, VIN=12V 1.2 A
DRV gate drive
Sink current, VIN=12V 1.3 A
Output High Clamp Level VCC=24V 14 V
DRV pin Rising Time DRV pin load=1nF 75 ns
DRV pin Falling Time DRV pin load=1nF 75 ns
COMP clamp voltage 3.6 3.8 4.0 V
Feedback (FB)
Reference Voltage 0.3 V
Tolerance of Reference Voltage -3 3 %
PWM Dimming (DIM)
Enable 2 V
DIM Voltage threshold
Disable 0.8 V
Resistance from DIM pin to GND 600 K
PWM dimming frequency 0.1 1 KHz
Dimming Duty-Cycle 0 100 %
Shutdown Recover Delay Time
(Note 1) 200 ms
(TDOWN)
Current Sensing (CS)
Current Sense Input Threshold
0.45 0.5 0.55 V
Voltage
LEB time 275 nS

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PARAMETER CONDITIONS MIN. TYP. MAX. UNITS
Over Voltage Protection
Over Voltage Threshold OVP 2.375 2.5 2.625 V
LED Over Voltage Hysteresis OVP Hysteresis 1 V
Output Short Circuit Protection
Output short Voltage Threshold OSP 0.17 V
Over Temperature Protection
OTP Trip Point 150 °C
De-bounce Point 30 °C

Note 1:

ton toff 200ms

Tdown

DIM

Shutdown

ILED_full

LED current
Per channel Shutdown-
0mA

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Typical Performance Characteristics
17 LEDs in series, 500mA/string, unless otherwise noted.

ILED
VIN

VCC

VOUT VOUT

VDRV

VDIM

ILED

Fig. 2 VIN=24V, “DIM Turn On, Duty=100%” Fig. 3 VIN=24V, “Steady State, Duty=100%”

VIN VIN
VOUT VOUT

ILED

ILED
VDIM

VDIM

Fig. 4 VIN=24V, “fDIM=240Hz ,Dim Duty=100%” Fig. 5 VIN=24V, “ fDIM=240Hz, Duty=10%

VIN
VOUT

VOVP

VOUT
ILED

VCOMP

VDIM
VDRV

Fig. 6 VIN=24V, “fDIM=240Hz, Dim Duty=100%” Fig. 7 VIN=24V, “LED Open Protection , Duty=100%

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Application Information
Operation Overview LED Open Protection and OVP Trip Point
The LD5850 is designed for current-mode control power If there’s open in LED string, VOUT will start to boost up
converters. It provides all the advantages of current-mode OVP voltage. Once it rises up to the threshold of around
control, including cycle-by cycle current limit and the 2.5V, the MOSFET drive output (DRV) will turn off and the
simplified loop compensation. IC restart it only after OVP voltage drops below 1.5V.
R  RA
VOVP  2.5  B
Output Drive Stage RB

An output stage of a CMOS buffer, with typical driving Place the bypass capacitor (COVP) between OVP and

capability of 1.2A/-1.3A, is incorporated to drive a power signal ground as close as possible. It’s superior to

MOSFET directly. The output voltage is clamped at 14V suppress the noise and protect OVP from abnormal

to protect the MOSFET gate even when the VCC voltage condition.

is higher than 14V.

Programming the LED Current
Under Voltage Lockout (UVLO) Select an external current sense resistor (RFB) to set the

An UVLO comparator is implemented in it to detect the LED current.

0.3V
voltage across the VCC pin. It would detect if the supply RFB 
ILED
voltage is enough to turn on the LD5850 and further to
drive the power MOSFET. As shown in Fig. 1, a
Dimming Control
hysteresis is built in to prevent the shutdown from the
The LED brightness control is accomplished by the PWM
voltage dip during start up. The turn-on and turn-off
signal from DIM pin in proportion to the various duty
threshold level are set at 10.0V and 8V, respectively.
cycles. LD5850 can apply the external PWM signal to DIM
Vcc
pin in the range from 100Hz to 1 KHz with a swing voltage

UVLO(on) of 0V to a level greater than 2.0V.

UVLO(off)
Current Sensing and Leading-edge
Blanking
t The LD5850 detects the primary MOSFET current across

I(Vcc) the CS pin for the protection of cycle-by-cycle current limit.

Standby Current
(~ mA) The voltage threshold of the current sensing pin is set at
0.5V maximum. The MOSFET peak current can be
obtained as below.
Shutdown Current
0.5V
(~uA) IPEAK (MAX) 
R CS
t
Fig. 1 A 275ns leading-edge blanking (LEB) time is programed
in the input of CS pin to prevent the false-triggering from
the current spike. The R-C filter is eliminable in those low
power applications, features pulse width of the turn-on

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spikes below 275ns and the negative spike of the CS pin PCB Layout Guideline
below -0.5V. It’s recommended to separate the high frequency

However, the pulse width of the turn-on spike is switching current from the low-level control signals in

determined according to the output power, circuit design layout. The high switching current (MOSFET, inductor,

and PCB layout. It is strongly recommended to add a gate driver and FB return ends) may disturb the other

smaller R-C filter for large power application to avoid CS low-level signals in the feedback loop and protection

pin from being damaged by the negative turn-on spike. circuitry. As a result, it may cause the control function to
275ns behave abnormally. To avoid these side effects, a few
blanking
time guidelines are recommended for the PCB layout as below.
DRV
1. Route the VIN bypass capacitor and the signal ground
close to the IC as possible. The traces between
OCP capacitor and VIN pin should be short as possible to
Signal CS
avoid noise interference.
R CS
2. Use broader traces for VIN, VOUT and power ground.
0.5V
Those components connected to VIN, VOUT and
Removable if the negative power ground carry high input/output current, such as
Spike is not over spec (-0.5V)
Fig. 2 power MOSFET and decoupling capacitors. To
The VRCS need be checked again by below equation to minimize power loss in these traces, the resistance of
avoid the output power be limited by VCOMP_CLAMP. traces should be kept as low as possible.
1.5  (1.3  Duty(%) )  (3  IL(Peak )  RCS )  3.5V
3. Use broader traces between power MOSFET drain,
inductor and diode since they often carry high current
in these traces. To minimize power loss in these traces,

CS
Sense the resistance of traces should be minimized as
Amplifier
x3
possible.
RCS

4. Keep the gate drive traces short and broad around the
VSLOPE S
COMP IC driver output, DRV pin, and the power MOSFET.
VCOMP_clamp=3.5V The driving traces have a high current spike during

FB EA
inverter operation. To minimize power MOSFET
switching loss or oscillation voltage in the gate driver
signal, the drive traces should be as broad and short
as possible to minimize resistance and parasitic
Thermal Protection inductance.
Thermal protection limits the whole power dissipation in
this device. When the junction temperature reaches 150°C
approximately, the thermal sensor will send the signal of
shutdown logic to turn off this device and resume the
operation after the IC’s junction temperature cools by 30°
C

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Package Information
SOP-8

Dimensions in Millimeters Dimensions in Inch

Symbols
MIN MAX MIN MAX

A 4.801 5.004 0.189 0.197

B 3.810 3.988 0.150 0.157

C 1.346 1.753 0.053 0.069

D 0.330 0.508 0.013 0.020

F 1.194 1.346 0.047 0.053

H 0.178 0.254 0.007 0.010
I 0.102 0.254 0.004 0.010
J 5.791 6.198 0.228 0.244
M 0.406 1.270 0.016 0.050
θ 0° 8° 0° 8°

Important Notice
Leadtrend Technology Corp. reserves the right to make changes or corrections to its products at any time without notice. Customers
should verify the datasheets are current and complete before placing order.

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Revision History

Rev. Date Change Notice

00 11/30/2012 Original Specification

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