Supertex HV9910B, Quick Start Manual, Page: 38 / 53

Supertex inc.
Supertex inc.
● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
AN-H48
Application Note
Fundamental Buck Converter topology is an excellent
choice for LED drivers in off-line (as well as low-voltage)
applications as it can produce a constant LED current at
very high efficiencies and low cost. A peak-current-controlled
buck converter can give reasonable LED current variation
over a wide range of input and LED voltages and needs
little effort in feedback control design. Coupled with the fact
that these converters can be easily designed to operate at
above 90% efficiency, the buck-based driver becomes an
unbeatable solution to drive High Brightness LEDs.
The Supertex HV9910B provides a low-cost, low component
count solution to implement the continuous mode buck
converter. HV9910B has two current sense threshold voltages
– an internally set 250mV and an external voltage at the
LD pin. The actual threshold voltage will be the lower of the
internal 250mV and the voltage at the LD pin. The low sense
voltage allows the use of low current sense resistor values.
HV9910B operates down to 8V input, which is required for
automobile applications, and can take a maximum of 450V
input, which makes it ideal for off-line applications. It also
has an internal regulator that supplies power to the IC from
the input voltage, eliminating the need for an external low
voltage power supply. It is capable of driving the external
FET directly, without the need for additional driver circuitry.
Linear or PWM dimming can also be easily implemented
using the HV9910B.
This Application Note discusses the design of a buck-based
LED driver using the HV9910B with the help of an off-line
application example. The same procedure can be used to
design LED drivers with any other lower voltage AC or DC
input; 12V for example.
The information in this Application Note also applies to the
Supertex HV9910.
Buck-based LED Drivers
Using the HV9910B
Circuit Diagram
Q1
R
2
R
1
L1
D2
LED(s)
C
3
C
2
C
1
D1
NTC1
HV9910B
VIN
RT
VDD
LD
GATE
CS
PWMD
GND
AC Input Voltage Range Expected LED string voltage Stabilized LED current Expected Efficiency
V
NOM,AC
= 120V rms V
O,MIN
= 20V
I
O,MAX
350mA η = 0.9
V
MIN,AC
= 90V rms
V
O,MAX
= 40V V
MAX,AC
= 135V rms
freq = 60Hz