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HV9910BLG-G

Universal High Brightness LED Driver

Supertex Inc 

HV9910B

Current Sense

Linear Dimming

The current sense input of the HV9910B goes to the non-

inverting inputs of two comparators. The inverting terminal

of one comparator is tied to an internal 250mV reference

whereas the inverting terminal of the other comparator is

connected to the LD pin. The outputs of both these com-

parators are fed into an OR GATE and the output of the OR

GATE is fed into the reset pin of the flip-flop. Thus, the com-

parator which has the lowest voltage at the inverting terminal

determines when the GATE output is turned off.

The outputs of the comparators also include a 150-280ns

blanking time which prevents spurious turn-offs of the exter-

nal FET due to the turn-on spike normally present in peak

current mode control. In rare cases, this internal blanking

might not be enough to filter out the turn-on spike. In these

cases, an external RC filter needs to be added between the

external sense resistor (RCS) and the CS pin.

Please note that the comparators are fast (with a typical

80ns response time). Hence these comparators are more

susceptible to be triggered by noise than the comparators

of the HV9910. A proper layout minimizing external induc-

tances will prevent false triggering of these comparators.

Oscillator

The oscillator in the HV9910B is controlled by a single re-

sistor connected at the RT pin. The equation governing the

oscillator time period tOSC is given by:

The Linear Dimming pin is used to control the LED current.

There are two cases when it may be necessary to use the

Linear Dimming pin.

► In some cases, it may not be possible to find the exact

RCS value required to obtain the LED current when the

internal 250mV is used. In these cases, an external volt-

age divider from the VDD pin can be connected to the LD

pin to obtain a voltage (less than 250mV) corresponding to

the desired voltage across RCS.

► Linear dimming may be desired to adjust the current

level to reduce the intensity of the LEDs. In these cases,

an external 0-250mV voltage can be connected to the LD

pin to adjust the LED current during operation.

To use the internal 250mV, the LD pin can be connected to

VDD.

Note: Although the LD pin can be pulled to GND, the output

current will not go to zero. This is due to the presence of a

minimum on-time (which is equal to the sum of the blanking

time and the delay to output time) which is about 450ns.

This will cause the FET to be on for a minimum of 450ns and

thus the LED current when LD = GND will not be zero. This

current is also dependent on the input voltage, inductance

value, forward voltage of the LEDs and circuit parasitics. To

get zero LED current, the PWMD pin has to be used.

PWM Dimming

If the resistor is connected between RT and GND, HV9910B

operates in a constant frequency mode and the above equa-

tion determines the time-period. If the resistor is connected

between RT and GATE, the HV9910B operates in a constant

off-time mode and the above equation determines the off-

time.

GATE Output

The GATE output of the HV9910B is used to drive an ex-

ternal FET. It is recommended that the GATE charge of the

external FET be less than 25nC for switching frequencies

≤100kHz and less than 15nC for switching frequencies >

100kHz.

PWM Dimming can be achieved by driving the PWMD pin

with a low frequency square wave signal. When the PWM

signal is zero, the GATE driver is turned off and when the

PWMD signal if high, the GATE driver is enabled. Since the

PWMD signal does not turn off the other parts of the IC,

the response of the HV9910B to the PWMD signal is almost

instantaneous. The rate of rise and fall of the LED current is

thus determined solely by the rise and fall times of the induc-

tor current.

To disable PWM dimming and enable the HV9910B perma-

nently, connect the PWMD pin to VDD.

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