HV9910
Knowing the nominal rectified input voltage VIN = 120V*1.41
= 169V, the switching duty ratio can be determined, as:
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.
D = VLEDs /VIN = 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:
=
Output Open Circuit Protection
TON = D/fOSC = 3.5 microsecond
When the buck topology is used, and the LED is connected
in series with the inductor, there is no need for any protection
against an open circuit condition in the LED string. Open LED
connection means no switching and can be continuous.
The required value of the inductor is given by:
L = (VIN - VLEDs) * TON /(0.3 * ILED) = 4.6mH
DC/DC Low Voltage Applications
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:
Buck Converter Operation
The buck power conversion topology can be used when
the LED string voltage is needed to be lower than the input
supply voltage. The design procedure for a buck LED driver
outlined in the previous chapters can be applied to the low
voltage LED drivers as well. However, the designer must
keep in mind that the input voltage must be maintained higher
than 2 times the forward voltage drop across the LEDs. This
CMIN = ILED*VLEDs*0.06/VIN^2
CMIN = 22 µF, a value 22µF/250V can be used.
A passive PFC circuit at the input requires using two series limitation is related to the output current instability that may
connected capacitors at the place of calculated CMIN. Each of develop when the HV9910 buck converter operates at a
these identical capacitors should be rated for ½ of the input duty cycle greater than 0.5. This instability reveals itself as
voltage and have twice as much capacitance.
an oscillation of the output current at a sub-harmonic of the
switching frequency.
6
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