HV9912 Supertex, Inc., HV9912 Datasheet - Page 10

• Manufacturer Part Number: HV9912
• Description: Switch-mode Led Driver Ic With High Current Accuracy And Hiccup Mode Protection
• Manufacturer: Supertex, Inc.

Linear Dimming
Linear dimming can be accomplished by varying the voltage
at the IREF pin, as the output current is proportional to the
voltage at the IREF pin. This can be done either by using a
potentiometer from the REF pin or by applying an external
voltage source at the IREF pin.
In the HV9911, due to the offset voltage of the short circuit
comparator as well as the non-linearity of the X2 gain stage,
pulling the IREF pin very close to GND will cause the internal
short circuit comparator to trigger and shut down the IC.
LED string
disconnects
Output Current
Output Cap
Voltage
COMP
FLT
100V
90V
80V
5V
1V
O VP ON
Fig. 4 Open Circuit Protection
t RC
O VP O FF
LED string
reconnects
t hiccup1
10
To overcome this in the HV9912, the minimum output of the
gain stage is limited to 125 ~ 250mV, allowing the IREF pin
to be pulled all the way to 0V without triggering the short cir-
cuit comparator. Note: Since this control IC is a peak current
mode controller, pulling the IREF pin to zero will not cause
the LED current to become zero. The converter will still be
operating at its minimum on-time causing a very small cur-
rent to fl ow through the LEDs. To get zero LED current, the
PWMD input has to be pulled to GND.
t hiccup2
LED string voltage
Preliminary
HV9912