2
AN-H50
Supertex inc.
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
The unique design of the oscillator in the HV9910B allows
the IC to be configured for either constant frequency or con
-
stant off-time based on how one resistor, connected to the
RT pin, is wired. For normal operation as constant frequency
converter, the resistor at the RT pin is connected to GND
(Fig. 3a). For operation as a constant off-time converter, the
resistor is connected between the RT and GATE pins (Fig.
3b). In both cases, the equation to determine the resistor is
given by:
T
OSC
(µs) =
R
T
(kΩ) + 22
25
For constant frequency operation T
OSC
is set to the switching
time period and for constant off-time operation, T
OSC
is set to
the required off-time.
Fig. 2. Slope Compensation to Eliminate Subharmonic Oscillation
I
R2
Current Through
Sense Resistor
t
V
CS
t
Error due to
Slope Compensation
Added
External Slope
Voltage Across
Sense Resistor
Fig. 3a. Constant Frequency Operation
Fig. 3b. Constant Off-Time Operation
HV9910B
VIN
RT
GATE
CS
VDD
LD
PWMD
GND
C
2
R
1
R
2
Q1
Connected
to GATE
Connected
to GND
HV9910B
VIN
RT
GATE
CS
VDD
LD
PWMD
GND
C
2
R
2
Q1
Fig. 4 Circuit Diagram
C
1
C
2
R
1
R
2
Q1
L1
D1
LED(s)
+
-
HV9910B
VIN
RT
GATE
CS
VDD
LD
PWMD
GND