INTEGRATED CIRCUITS DIVISION Application Note AN-300

MXHV9910
Design Considerations

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INTEGRATED CIRCUITS DIVISION AN-300

1 Off-line LED Driver using MXHV9910

This application note provides general guidelines for externally set at the LD pin. The lower of these two
designing an off-line LED driver using the MXHV9910. thresholds determines the LED peak current in
conjunction with the current sense resistor (RSENSE) at
The MXHV9910 is a constant frequency buck converter
specifically designed to provide a low cost, minimal the CS pin. A linear dimming function can be
external component solution for off-line LED accomplished by adjusting the current sense threshold
applications. The converter operates in a continuous- voltage up to the internal current threshold range.
conduction, peak-current control mode with no slope When the linear dimming function is not used, it is
compensation. recommended that the LD pin be connected to VDD.

When designing an LED driver with the MXHV9910, Figure 1 shows the functional block diagram of the
the duty cycle must be restricted to less than 50% in MXHV9910 device. Figure 2 shows a schematic of a
order to prevent subharmonic oscillations. typical application circuit for the device, and is referred
to in all the discussions that follow.
The MXHV9910 has two current sense thresholds: one
is internally set at 240mV, and the other can be

Figure 1 MXHV9910 Block Diagram

VDD 6
VIN 1 Voltage
Regulator

Voltage
Reference
250mV

RT OSC
8
- PWM
+ Control
4 GATE
LD 7 -
+

PWMD 5
GND 3 CS
2

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Figure 2 Application Circuit Diagram

D1 LEDs

L1

BR VIN FET
VIN VDD GATE
FUSE
PWMD
CC
CS
LD
CBULK CVDD
ROSC PGND

MXHV9910
NTC1 RSENSE
RT

2 Typical Design Parameters

• DC Bulk Voltage at Low and High Line
Parameter Symbol Min Typ Max Units
AC Input Voltage V DC_bulk_min = 2  V AC-min
Minimum Voltage VAC-min 90 - - V DC_bulk_min = 127.3V
Vrms
Maximum Voltage VAC-max - - 130
AC Input Frequency fac 50 - 60 Hz V DC_bulk_max = 2  V AC-max

LED String Voltage VLEDstring - 60 - V V DC_bulk_max = 183.8V

LED String Current ILEDmax - - 350 mA
• Average Input Current
Estimated Efficiency  - 0.90 - -
Oscillator Frequency fS - 64 - kHz P in 23.33W
I in_avg = ------------------------------- = ------------------
Duty Cycle Dmax_spec - - 0.5 - V DC_bulk_min 127.3V

I in_avg = 0.183A
• Output Power Calculation
P OUT = V LEDstring  I LEDmax • Peak Input Current
P OUT = 60V  350mA I in_pk = 5  I in_avg
P OUT = 21W
I in_pk = 0.915A
• Input Power Calculation
P OUT Note: During a surge, the current could be as much as
P IN = -------------
 5 times higher, hence the multiplier.
21W
P IN = -----------
0.90
P IN = 23.33W

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3 Switching Frequency and Resistor RT Selection

It is recommended that the switching frequency range design, RT is selected to be 402k, which sets the
for off-line applications ranges from 30kHz to 120kHz. oscillator frequency to about 64kHz. Figure 3 below
The MXHV9910 requires an external resistor, RT , that shows the typical oscillator frequency for a given RT
sets the internal RC oscillator frequency. For this resistor value.

Figure 3 Oscillator Frequency vs. Resistor Value

Oscillator Frequency, fS, vs. RT

(TA=27ºC)
250

200
Frequency (kHz)

150

100

50

0
0 200 400 600 800 1000 1200
RT (kΩ)

4 Selecting Fuse and NTC1 Thermistor 5 Diode Bridge Rectifier

The fuse protects the circuit from input current surges The selection of the diode bridge rectifier is based on
during turn-on. Choose a fuse that is rated five times DC blocking voltage, forward current, and surge
the peak input current. current.
I fuse = 5  I in_pk V rb = V DC_bulk_max
I fuse = 4.575A V rb = 183.8V

The thermistor in series with the input bridge rectifier The diode forward current rating should be set to 1.5
limits the inrush charging current into the input bulk times the input average current.
capacitor during startup. The value is determined by:
I fb = 1.5  I in_avg
2  V AC_max I fb = 0.2745A
R th_cold = ----------------------------------
I in_pk
R th_cold = 200.87 The diode bridge can be subjected to currents as high
as 5 times the forward current, and the diode bridge
should be rated accordingly.
I fsb = 5  I fb
I fsb = 1.3725A

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6 Input Bulk Capacitor, CBULK, and CC 7 Bypass Capacitor, CVDD

The AC line voltage is filtered by the input bulk The VDD pin is the internal regulator output pin and
capacitor (CBULK), which is selected based on the must be bypassed by a low-ESR capacitor (typically
minimum peak rectifier input line voltage and peak-to- 0.1F or higher) to provide a low-impedance path for
peak ripple voltage. Assuming a 20% ripple: high-frequency switching noise.
r DC_bulk = 0.2
V in_min =  1 – r DC_bulk   V DC_bulk_min =  1 – 0.2    127.3 
8 Duty Cycle and ON-Time
V in_min = 101.8V From the design requirements, the duty cycle and
ON-time can be calculated as:
P in V LEDstring
C bulk = ------------------------------------------------------------------------------
- 60V
2 2 D max_buck = ------------------------------- = -----------------
f AC   V DC_bulk_min – V in_min  V DC_bulk_min 127.3V
D max_buck = 0.471
23.33W
C bulk = ---------------------------------------------------------------------
2 2
-
60Hz   127.3V – 101.8V  D max_buck 0.471
t on.max_buck = ------------------------ = ----------------
fS 64kHz
C bulk = 66.70F
t on.max_buck = 7.366s

For this example, the voltage rating of the capacitor

should be more than VDC_bulk_max with some safety Dmax_buck is less than 50% and meets the
margin factored in. An electrolytic capacitor with a subharmonic oscillation requirement.
250V, 68F rating would be adequate.
9 Inductor Design
Note that electrolytic bulk capacitors contain parasitic
elements that cause their performance to be less than The inductor (L1) value is determined based on desired
ideal. One important parasitic is the capacitor’s LED ripple current and the switching frequency. 64 kHz
Equivalent Series Resistance (ESR), which causes was chosen as the optimum switching frequency to
internal heating as the ripple current flows into and out minimize switching losses and to reduce circuit power
of the capacitor. In order to select a proper capacitor, dissipation at the expense of larger inductor size.
the designer should consider capacitors that are Assuming a 30% peak-to-peak ripple in LED current,
specifically designed to endure the ripple current at the one can calculate the inductor requirements:
maximum temperature, and that have an ESR that is
guaranteed within a specific frequency range (usually r iout = 0.3
provided by manufacturers in the 120Hz to 100kHz  V DC_bulk_min – V LEDstring   t on.max_buck
range). L min_buck = ---------------------------------------------------------------------------------------------------
r iout  I LEDmax
The Effective Series Inductance (ESL) is another  127.3V – 60V   7.366s
L min_buck = ----------------------------------------------------------------
parasitic that limits the effectiveness of the electrolytic 0.3  350mA
capacitor at high frequencies. L min_buck = 4.7mH
The combination of the variation of ESR over
temperature and a high ESL may require adding a Inductor peak current rating:
parallel film or tantalum capacitor (CC) to absorb the I Lmax = I LEDmax   1 +  0.5  r iout  
high-frequency ripple component. This keeps the
combined ESR within the required limit over the full I Lmax = 350mA   1 +  0.5  0.3  
design temperature range. I Lmax = 0.403A

In some cases, when the design requires a higher

current rating and there is no standard inductor
available, a custom-made inductor should be
considered.

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10 Power MOSFET and Diode Selection 11 Current Sense Resistor, RSENSE

Peak voltage seen by the discrete power MOSFET The current sense resistor (RSENSE) is selected based
(FET) and diode (D1) are equal to the maximum bulk on the desired LED current. In this case, the maximum
voltage. For safety reasons assume an additional 50% LED current is set at 350mA. Note that there is a
margin by design. difference between the peak current and the average
V FET_BVDSS_buck = 1.5  V DC_bulk_max current in the inductor. This ripple difference should be
included in resistor calculations. The current sense
V FET_BVDSS_buck = 1.5  183.8V
threshold is given in the MXHV9910 data sheet.
V FET_BVDSS_buck = 275.771V
Assuming 30% ripple:
V Diode_r_buck = 1.5  V DC_bulk_max V cs(high) = 250mV
V Diode_r_buck = 1.5  183.8V r iout = 0.3

V Diode_r_buck = 275.771V V cs(high) 250mV

R sense = ------------------------------------------------------------------ = ---------------------------------------------------------------
 1 +  0.5  r iout    I LEDmax  1 +  0.5  0.3    350mA
Maximum RMS current though the FET depends on R sense = 0.621
the maximum duty cycle seen by the FET. In this buck
converter, the maximum duty cycle is set slightly less
than 50%. Choose a MOSFET with a rating of 3 times Note that since the current sense threshold voltage of
this current. the MXHV9910 (Vcsth) is specified between 200mV
I FET_rms_buck = 0.5  I LEDmax and 280mV, 250mV, the nominal value, is used in the
formula above.
I FET_rating_buck = 3  I FET_rms_buck
I FET_rating_buck = 0.743A
Power dissipation across the sense resistor:
2
P = I LEDmax  R sense
Average current though the diode is one-half of the
LED current. Choose a diode with a rating 3 times this P = 0.076W
current.
I Diode_buck = 0.5  I LEDmax = 0.5  350mA = 0.175A In practice, select a resistor power rating that is at least
twice the calculated value.
I Diode_rating_buck = 3  I Diode_buck
I Diode_rating_buck = 0.525A 12 Layout Considerations
In all switching converters, proper grounding and trace
For this design, the IXTA8N50P external power FET, in
length are important considerations. The LED driver
the SMD D2-Pak package, was selected from IXYS’
operates at a high frequency, and the designer must
family of Polar N-channel devices. The Polar process
keep trace length from the MXHV9910 GATE pin to the
features 30% reduction of RDS(on) and substantial
external power MOSFET as short as possible. Doing
reduction of total gate charge, QG. This helps with
this helps to avoid such undesired performance
improved LED driver efficiency by minimizing characteristics as ringing and spiking.
conduction and switching losses. In addition, the Polar
power FET family has very low thermal resistance, In high-frequency switching, current tends to flow near
RJC, which improves the device’s power dissipation. the surface of a conductor, so ground traces on the PC
The IXA8N50P can be used with an external heat sink board must be wide in order to avoid any problems due
similar to Aavid Thermalloy’s part number 573100. to parasitic trace inductance. If possible, one side of the
PC board should be used as a ground plane.
The high frequency switching of the buck LED driver
requires the use of a fast recovery diode. The The current sense resistor, Rsense, should be kept
BYV26_B series diode, in the SOD 57 package, was close to the CS pin in order to prevent noise coupling to
chosen for this design. the internal high-speed voltage comparator, which
would affect IC performance. In addition, RT should be
placed away from the inductor and away from any PCB
trace that is close to switching noise.

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13 Design Idea
This design idea features an inexpensive, off-the-shelf 110VAC or 220VAC operation simply by changing the
Triac Dimmer Controller used with the MXHV9910 LED value of resistor, R3. For a 220VAC application,
driver. The simple circuit is a voltage divider that feeds decrease the value of R3 to 7.8k.
into the LD pin. The voltage divider can be adjusted for

VIN
TRIAC DIMMER
120VAC CONTROLLER
EXT. POWER

VIN FUSE F2
2A
R1
402k VDD MONITOR
AC - LD MONITOR
X1
AC + C1
0.01μF R3 R4
df04s
400V 17k 10M
D1 C2
NTC1 MXHV9910
BYV26B 2.2μF
8-PIN SOIC
1 8 16V
VIN RT
2 2 DUT1 7
R5 CS LD
L1
100k 3 6
4.7mH PGND VDD
Q1 1 4 5
IXTA8N50P GATE PWMD C3*
3 10μF
LED+
pwmd
LED CONNECTION
1 2 3
LED-
R2 X2 PWMD
0.56Ω

* C3 can be between 0.1μF and 10μF

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Specification: AN-300-R02
©Copyright 2012, IXYS Integrated Circuits Division
All rights reserved. Printed in USA.
12/13/2012

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