HV9985 Supertex, HV9985 Datasheet - Page 9

• Manufacturer Part Number: HV9985
• Description: LED Driver ICs
• Manufacturer: Supertex

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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:
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
where n refers to the channel number.
C
t
COMP,n
RAMP
=
= 3 • R
5μA • t
Zn
2V
• C
HICCUP
Zn
Z
and C
●
Z
1235 Bordeaux Drive, Sunnyvale, CA 94089
in parallel with C
C
),
9
In case the compensation networks are only of Type 1 (sin-
gle capacitor), then:
Thus, the maximum COMP discharge time required can be
computed as:
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:
where L and C
of each power stage.
Thus, the maximum time required for the inductors to dis-
charge can be computed as:
The hiccup time is then chosen as:
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
t
t
t
t
t
COMP,n
COMP,MAX
IND,n
IND,MAX
HICCUP
DETECT1
=
= 3 • 300Ω • C
> max (t
●
= max (t
π
= t
4
= max (t
Tel: [phone]
BLANK
√ L
O
are the input inductor and output capacitor
n
• C
COMP,MAX
+ t
IND1
COMP1
DELAY
ON
, t
IND2
Zn
, t
, t
≈ 900ns(max)
, t
COMP2
IND,MAX
●
IND3
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, t
)
)
COMP3
)
HV9985