HV9910
Inductor Design
Referring to the Typical Application Circuit below
the value can be calculated from the desired peak-
to-peak LED ripple current in the inductor.
Typically, such ripple current is selected to be 30%
of the nominal LED current. In the example given
here, the nominal current I
LED
is 350mA.
The next step is determining the total voltage drop
across the LED string. For example, when the
string consists of 10 High-Brightness LEDs and
each diode has a forward voltage drop of 3.0V at its
nominal current; the total LED voltage V
LEDS
is 30V.
Knowing the nominal rectified input voltage
V
IN
=120V*1.41=169V, the switching duty ratio can
be determined, as:
D= V
LEDs
/V
IN
=30/169=0.177
Then, given the switching frequency, in this
example fosc=50KHz, the required on-time of the
MOSFET transistor can be calculated:
Ton=D/fosc=3.5 microsecond
The required value of the inductor is given by:
L = (Vin - V
LEDs
) * Ton/(0.3 * I
LED
) = 4.6mH
Input Bulk Capacitor
An input filter capacitor should be designed to hold
the rectified AC voltage above twice the LED string
voltage throughout the AC line cycle. Assuming
15% relative voltage ripple across the capacitor, a
simplified formula for the minimum value of the bulk
input capacitor is given by:
Cmin = I
LED
*V
LEDs
*0.06/V
IN
^2
Cmin = 22
µF,
a value 22uF/250V can be used.
A passive PFC circuit at the input requires using
two series connected capacitors at the place of
calculated C
MIN
. Each of these identical capacitors
should be rated for �½ of the input voltage and have
twice as much capacitance.
Enable
The HV9910 can be turned off by pulling the
PWM_D pin to ground. When disabled, the HV9910
draws quiescent current of less than 1mA.
Figure 1: Typical Application Circuit
2A,
250 V
AC Input
85-135V AC
coilcraft
BUSH-2820R5B
1N4004
1N4004
BYV26B
68µ
F,
160 V
1N4004
VIN
VDD
1N4004
1µF,
10V
HV9910
GATE
VN2224
220nF,
400 V
LEDs
RT
280 kΩ
750µΗ
,
0.1µ
F,
250 V
2R
NTC
1n F,
250 V
0.1µ
F,
250 V
1n F,
250 V
1N4004
1N4004
68µ
F,
160 V
LD
CS
0.2 Ω
optional for PFC
GND
PW M_D
LED(s) – a string of HB LEDs , 16 diodes
6
C110504
SUPERTEX [ Supertex, Inc ]
