6
HV9966
Supertex inc.
www.supertex.com
Doc.# DSFP-HV9966
A051412
VCC, VDD and Gate Drivers
The external voltage applied at VCC is used to power the IC.
The voltage at this pin is typically 12V+/-15% with a 16V abs
max rating.
An internal linear regulator is used to generate 3.3V at VDD
pin which is used to power the low voltage analog circuit. The
voltage at VDD pin can also be used as a reference to set the
LED current using a resistor divider from VDD to the REF pin.
Both VCC and VDD have built-in UVLO to disable the IC in
case the voltages at the pins are lower than expected.
The gate drivers are powered off directly from the VCC pin.
The switching gate driver currents are supplied from the low
ESR capacitor connected externally at the VCC pin.
GM Amplier and PWM dimming
The GM amplier is used to control the LED current. The current
level is set by the voltage at the REF pin and the LED current is
sensed by a current sense resistor and the voltage across the
sense resistor is fed into the FB pin. The compensation capaci-
tor is connected between COMP and GND.
When PWMD is high, the OTA is allowed to control the voltage
at the COMP pin. When PWMD is low, the OTA is disconnected
from the COMP pin. The leakage current at the COMP pin due
to all circuitry connected to it (ESD protection, pull down tran-
sistor and disconnect switch) should be less than 10nA.
The pull down FET at the COMP pin is used to discharge the
COMP capacitor at startup and during fault conditions.
Boost FET current sense
The current sense pin has a built in 100 - 250ns blanking time.
It also has a pull down FET which is turned on whenever the GT
is off. This is to facilitate slope compensation using an external
resistor/capacitor network. Although most applications with this
IC are expected to be DCM boost circuits which do not need
slope compensation, the pull-down FET is included for to make
the part usable for CCM boost converters as well.
Hiccup timer
Hiccup timing is achieved by using an internal 8-bit counter
which counts 1024 clock cycles. This makes the hiccup time
dependent on the switching frequency.
Startup
When power is initially applied to the IC, POR1 goes high. At
this point, two latches – Fault latch and POR1 latch – are set
and FLT goes high. The POR1 latch output will be high untill the
rst PWM pulse is applied at PWMD. This keeps the counter
cleared and enabled till the PWMD pulse is applied. Once the
rst PWM dimming pulse is applied, the counter is allowed to
count to 256, at which point the gate drivers and COMP pin
are released, and the converter can start regulating the LED
current.
Over-Voltage and Over-Current Fault
Over-voltage is detected using the voltage at the OVP pin.
When the voltage at OVP exceeds 2.0V, over-voltage is trig-
gered and the over-voltage condition is said to exist untill the
voltage at OVP drops below 1.8V (10% lower).
Over-current condition is detected by the over-current compar-
ator, which compares the voltage at the FB pin with two-times
the voltage at the REF pin.
As long as these fault conditions exist, they set the Fault latch,
which turns off the gate drivers and clears the 8-bit counter and
keeps the counter cleared. Once the fault disappers, the coun-
ter is allowed to count and the operation of the IC is identical to
the startup case.
Short Circuit to Chassis Condition
A wiring fault condition occurs during the manufacturing pro-
cess involving large LED strings. Consider a case of three LED
light bars connected in series and driven from the same boost
converter. The input to the boost converter is typically about
120V. Assume each LED light bar has a forward drop of about
80V (25 LEDs with 3.2V/per LED). If one of the connections be-
tween the LED light bars is shorted to ground (see Fig.1), then
the boost converter looses the feedback signal since all the cur-
rent ows through the short bypassing the sense resistor.
Fig.1 : Short Circuit to Chassis
In this case, COMP will rail to VDD and the boost converter
will operate at its maximum power limit. This excessive current
through the rst two light bars could damage the LEDs.
A1
C1
C2
C3
Q2
R2