Supertex inc.
Supertex inc.
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
AN-H50
Application Note
Constant frequency, peak current controlled buck converters
(Fig. 1) are an excellent choice for driving LEDs for a number
of reasons:
►
Reasonable regulation of LED current over wide
variations in input and output voltages.
►
Simple to design as no feedback compensation
is required.
►
PWM dimming response of the converters is
almost instantaneous.
However, peak current controlled buck converters go into
sub-harmonic oscillations at duty cycles over 50%. These
oscillations cause the average output current to drop, while
the output ripple current increases. The only way to avoid
these problems is by adding slope compensation circuitry
externally. The slope compensation adds an upward slope
on to the current sense signal and the converter can be
stabilized by varying the slope of the added ramp (Fig.
2). This added ramp causes an error between the sensed
current (as seen at the CS pin of the HV9910B) and the
actual LED current.
Although this error can be compensated for by changing the
sensed resistor appropriately, the converter’s rejection of
the input and output voltage variations will be significantly
degraded. Thus, changing the input or output voltage will
significantly change the LED current, without additional
feedback circuitry for regulating the LED current. This
makes the peak current controlled buck converter practically
useless for cases where the input voltage is less than twice
the output voltage.
This problem can be overcome by changing the control
method to a constant off-time operation. In this case, the off-
time is fixed by design, the on-time is based on the current
sense signal and the switching time-period adjusts to be
equal to the on-time plus the off-time. This change will allow
the converter to work with greater than 50% duty cycles and
still have the advantages of the peak current controlled buck
converter given above.
The information in this Application Note also applies to the
Supertex HV9910.
Constant, Off-time, Buck-based, LED Drivers
Using the HV9910B
C
1
C
2
R
1
R
2
Q1
L1
D1
LED(s)
HV9910B
VIN
RT
GATE
CS
VDD
LD
PWMD
GND
+
-
Fig. 1. Constant Frequency, Peak Current Controlled LED Driver