HV9910 Ver la hoja de datos (PDF) - Supertex Inc

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Fabricante

HV9910

Universal High Brightness LED Driver

Supertex Inc 

HV9910

Knowing the nominal rectified input voltage VIN = 120V*1.41

= 169V, the switching duty ratio can be determined, as:

D = VLEDs /VIN = 30/169 = 0.177

Then, given the switching frequency, in this example f =

OSC

50KHz, the required on-time of the MOSFET transistor can

be calculated:

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.

Output Open Circuit Protection

TON = D/fOSC = 3.5 microsecond

The required value of the inductor is given by:

L = (VIN - VLEDs) * TON /(0.3 * ILED) = 4.6mH

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.

Input Bulk Capacitor

DC/DC Low Voltage Applications

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 = 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

connected capacitors at the place of calculated CMIN. Each of

these identical capacitors should be rated for ½ of the input

voltage and have twice as much capacitance.

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

limitation is related to the output current instability that may

develop when the HV9910 buck converter operates at a

duty cycle greater than 0.5. This instability reveals itself as

an oscillation of the output current at a sub-harmonic of the

switching frequency.

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