HV9910NG データシートの表示（PDF） - Supertex Inc

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HV9910NG

Universal High Brightness LED Driver

Supertex Inc 

HV9910

Output Open Circuit Protection

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.

However, in the case of the buck-boost or the

Flyback topology the HV9910 may cause excessive

voltage stress of the switching transistor and the

rectifier diode and potential failure. In this case, the

HV9910 can be disabled by pulling the PMW_D pin

to ground when the over voltage condition is

detected.

DC/DC Low Voltage Applications

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.

Flyback (Buck-Boost) Operation

This power conversion topology can be used when

the forward voltage drop of the LED string is higher,

equal or lower than the input supply voltage. For

example, the buck-boost topology can be

appropriate when input voltage is supplied by an

automotive battery (12V) and output string consists

of three to six HB LEDs, as the case may be for tail

and break signal lights.

In the buck-boost converter, the energy from the

input source is first stored in the inductor or a

Flyback transformer when the switching transistor

is ON. The energy is then delivered to the output

during the OFF time of the transistor. When the

energy stored in the Flyback inductor is not fully

depleted by the next switching cycle (continuous

conduction mode) the DC conversion between

input and output voltage is given by:

VOUT = - VIN*D/(1-D)

The output voltage can be either higher or lower

than the input voltage, depending on duty ratio.

Let us discuss the above example of an automotive

LED driver that needs to drive three HB LEDs at

350mA.

Knowing the nominal input voltage VIN=12V, the

nominal duty ratio can be determined, as

D=VLEDs/(VIN+VLEDs)=9/(12+9)=0.43

Then, given the switching frequency, in this

example fOSC=50KHz, the required on-time of the

MOSFET transistor can be calculated:

Ton=D/fOSC=8.6 microsecond

The required value of the inductor is given by:

L = VIN* Ton/(0.3 * Iled) = 0.98mH, use 1mH

Output Capacitor

Unlike the buck topology, the buck-boost converter

requires an output filter capacitor to deliver power

to the LED string during the ON time of switching

the transistor, when the Flyback inductor current is

diverted from the output of the converter.

In order to average the current in the LED, this

capacitor must present impedance to the switching

output AC ripple current that is much lower than the

dynamic impedance ROUT of the LED string. If we

assume ROUT=3 Ohm in our example, in order to

attenuate the switching ripple by a factor of 10, a

capacitor with equivalent series resistance (ESR) of

0.3 Ohm is needed. A chip SMT tantalum capacitor

can be selected for this purpose.

7

C110504

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