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HV9985

Supertex inc.

www.supertex.com

Doc.# DSFP-HV9985

C051513

Note that 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 with-

out significant change of the voltage across it.

Fault Conditions

The HV9985 is a robust controller which can protect the

LEDs and the LED driver in case of fault conditions. The out-

puts of the HV9985 LED driver are protected from both an

open and a short LED condition. In both cases, the HV9985

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 and SKIP

pins are pulled to GND. Once the voltage at the SKIP pin falls

below 0.1V, and the fault condition(s) have disappeared, the

capacitor at the SKIP pin is released, and it begins charging

slowly from a 5.0μA current source. Once the capacitor is

charged to 2.0V, the COMP pins are released, and the gate

driver outputs (GATE and FLT) are allowed to turn on. If the

hiccup time is long enough, it will ensure that the compen-

sation networks are all completely discharged and that the

converters start at minimum duty cycle.

The hiccup timing capacitor can be programmed as:

C

5μA • t

HICCUP

2V

Output Short Circuit Protection

When a short circuit condition is detected (output current be-

comes higher than twice the steady state current), 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 disappears. At this time, the hiccup

timer is started. Once the timing is complete, the converter

attempts to restart. If the fault condition still persists, the

converter shuts down and goes through the cycle again. If

the fault condition is cleared (a momentary output short) the

converter will start regulating the output current normally.

This allows the LED driver to recover from accidental shorts

without the need to reset the IC.

During short circuit conditions, there are two factors that de-

termine the hiccup time.

The first is the time required to discharge the compensation

capacitors. Assuming a pole-zero R-C network at the COMP

pin (series combination of R

t

where n refers to the channel number.

In case the compensation networks are only of Type 1 (sin-

gle capacitor), then:

t

Thus, the maximum COMP discharge time required can be

computed as:

t

The second factor is the time required for the inductors to

discharge completely after the short circuit condition has

been cleared. This time can be computed as:

t

π

√ L

4

where L and C

of each power stage.

Thus, the maximum time required for the inductors to dis-

charge can be computed as:

t

The hiccup time is then chosen as:

t

False Triggering of the Short Circuit Comparator During

PWM Dimming

During PWM dimming, the parasitic capacitance of the LED

string causes a spike in the output current when the discon-

nect FET is turned on. If this spike is detected by the short

circuit comparator, it will cause the IC to falsely detect an

over current condition and shut down.

In the HV9985, to prevent these false triggerings, a built-in

500ns blanking network for the short circuit comparator is in-

cluded. This blanking network is activated when the PWMD

input goes high. Thus, the short circuit comparator will not

see the spike in the LED current during the turn-on transi-

tion of the PWM Dimming. Once the blanking time is over,

the short circuit comparator will start monitoring the output

current. Thus, the total delay time for detecting a short circuit

will depend on the condition of the PWMD input.

If the output short circuit exists before the PWM dimming

signal goes high, the total detection time will be:

t