11

HV9911

PWM Dimming

PWM dimming can be achieved by driving the PWMD pin

with a TTL compatible source. The PWM signal is connected

internally to the three different nodes – the transconductance

amplifier, the FAULT output, and the GATE output.

When the PWMD signal is high, the GATE and FAULT pins

are enabled, and the output of the transconductance opamp

is connected to the external compensation network. Thus,

the internal amplifier controls the output current. When the

PWMD signal goes low, the output of the transconductance

amplifier is disconnected from the compensation network.

Thus, the integrating capacitor maintains the voltage across

it. The GATE is disabled, so the converter stops switching

and the FAULT pin goes low, turning off the disconnect

switch.

The output capacitor of the converter determines the

PWM dimming response of the converter, since it has to

get charged and discharged whenever the PWMD signal

goes high or low. In the case of a buck converter, since the

inductor current is continuous, a very small capacitor is used

across the LEDs. This minimizes the effect of the capacitor

on the PWM dimming response of the converter. However,

in the case of a boost converter, the output current is

discontinuous, and a very large output capacitor is required

to reduce the ripple in the LED current. Thus, this capacitor

will have a significant impact on the PWM dimming response.

By turning off the disconnect switch when PWMD goes low,

the output capacitor is prevented from being discharged,

and thus the PWM dimming response of the boost converter

improves dramatically.

Note:

Disconnecting the capacitor might cause a sudden spike

in the capacitor voltage as the energy in the inductor is

dumped into the capacitor. This might trigger the OVP

comparator if the OVP point is set too close to the maximum

operating voltage. Thus, either the capacitor has to sized

slightly larger or the OVP set point has to be increased.

Note:

The HV9911 IC might latch-up if the PWMD pin is pulled

0.3V below GND, causing failure of the part. This abnor-

mal condition can happen if there is a long cable between

the PWM signal and the PWMD pin of the IC. It is recom-

mended that a 1.0kΩ resistor be connected between the

PWMD pin and the PWM signal input to the HV9911. This

resistor, when placed close to the IC, will damp out any

ringing that might cause the voltage at the PWMD pin to

go below GND.

Avoiding False Shutdowns of the HV9911

The HV9911 has two fault modes which trigger a latched

protection mode, an over current (or short circuit) protection,

and an over voltage protection.

To prevent false triggering due to the tripping of the over

voltage comparator, (due to noise in the GND traces on

the PCB), it is recommended that a 1.0 - 10nF capacitor

be connected between the OVP pin and GND. Although

this capacitor will slow down the response of the over

voltage protection circuitry somewhat, it will not affect the

overall performance of the converter, as the large output

capacitance in the boost design will limit the rate of rise of

the output voltage.

In some cases, the over current protection may be triggered

during PWM dimming, when the FAULT goes high and the

disconnect switch is turned on. This triggering of the over

current protection is related to the parasitic capacitance of

the LED string (shown as a lumped capacitance C

4).

During normal PWM dimming operation, the HV9911

maintains the voltage across the output capacitor (C

turning off the disconnect switch and preserving the charge

in the output capacitance when the PWM dimming signal

is low. At the same time, the voltage at the drain of the

disconnect FET is some non-zero value V

dimming signal goes high, FET Q

the voltage at the drain of the FET (V

zero. Assuming a constant output voltage V

i

= -C

In this case, the rate of fall of the drain voltage of the

disconnect FET is a large value (since the FET turns on very

quickly) and this causes a spike of current through the sense

resistor, which could trigger the over current protection

(depending on the parasitic capacitance of the LED string).

To prevent this condition, a simple RC low pass filter network

can be added as shown in Fig. 5. Typical values are R

1.0kΩ and C

seeing the turn-on spike and normalize the PWM dimming

operation of the HV9911 boost converter. This will have

minimal effect on the stability of the loop but will increase

the response time to an output short. If the increase in the

response time is large, it might damage the output current

sense resistor due to exceeding its peak-current rating.