HV9982 datasheet(10/20 Pages) MICROCHIP

10 / 20 page



2014-2026 Microchip Technology Inc. and its subsidiaries

DS20005295C-page 10

HV9982

When S1 is high and the HV9982 is operating in the

analog control of PWM dimming mode, the PWM dim-

ming frequency is set by a capacitor connected at the

RAMP pin. The RAMP frequency range is 100Hz-

1.0kHz and the capacitor can be selected as:

When the PWM signal is high, the GATE and FLT pins

are enabled and the output of the transconductance

op-amp is connected to the external compensation net-

work. 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 FLT

pin goes low, turning off the disconnect switch.

The output capacitor of the converter determines the

PWM-dimming response of the converter, because it is

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 capaci-

tor 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 out-

put capacitor is prevented from being discharged and

thus the PWM-dimming response of the boost con-

verter Improves dramatically.

Disconnecting the LED load during PWM dimming

causes the energy stored in the inductor to be dumped

into the output capacitor. The filter capacitor should be

chosen large enough so that it can absorb the inductor

energy without significant change to the voltage across it.

3.9

Fault Conditions

The HV9982 is a robust controller which can protect the

LEDs and the LED driver in case of fault conditions.

The HV9982 includes both open LED protection and

output short circuit protection. In both cases, the

HV9982 shuts down and attempts a restart. The hiccup

time can be programmed by a single external capacitor

at the SKIP pin.

During start-up, or when a fault condition is detected,

both GATE and FLT outputs are disabled, the COMP

pins and SKIP pins are pulled to GND. Once the volt-

age at the SKIP pin falls below 0.1V and the fault con-

dition(s) have disappeared, the capacitor at the SKIP

pin is released and is charged slowly by a 10μA current

source. When the capacitor is charged to 5.0V, the

COMP pins are released and GATE and FLT pins are

allowed to turn on. If the hiccup time is long enough, it

will ensure that the compensation networks are all

completely discharged and that the converters start at

minimum duty cycle.

The hiccup timing capacitor can be programmed as:

3.10 Short Circuit Protection

When a short circuit condition is detected (output cur-

rent becomes higher than twice the steady state cur-

rent), the GATE and FLT outputs are pulled low. As

soon as the disconnect FET is turned off, the output

current goes to zero and the short circuit condition dis-

appears. At this time, the hiccup timer is started (Fig.

3). Once the timing is complete, the converter attempts

to restart. If the fault condition still persists, the con-

verter shuts down and goes through the cycle again. If

the fault condition is cleared, due to a momentary out-

put short, the converter will start regulating the output

current normally. This allows the LED driver to recover

from accidental shorts without having to reset the IC.

During short circuit conditions, there are two conditions

that determine the hiccup time.

The first condition is the time required to discharge the

compensation capacitors. Assuming a pole-zero R-C

network at the COMP pin (series combination of RZ

and CZ in parallel with CC),

where n refers to the channel number.

If the compensation networks are only type 1 (single

capacitor), then:

Thus, the maximum compensation time required can

be computed as:

The second condition is the time required for the induc-

tors to completely discharge following a short circuit.

This time can be computed as:

Note:

In the following description of the PWM-

dimming performance the PWMD signals

refer to the internal PWM dimming signal

and not to the signal applied at the PWMD

pins

f HZ





1.0s

CRAMP

-----------------

CRAMP

10A tHICCUP



4.9V

---------------------------------------

tCOMP n

3 RZn CZn



tCOMP n

3 650 CZn



tCOMP max



max tCOMP1 tCOMP2 tCOMP3



tIND N





--- LN CON



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