HV9922 Supertex, Inc., HV9922 Datasheet - Page 4

• Manufacturer Part Number: HV9922
• Description: 3-pin Switch-mode Led Lamp Driver Ic
• Manufacturer: Supertex, Inc.

Functional Description
The HV9922 is a PWM peak current controller for controlling
a buck converter topology in continuous conduction mode
(CCM). The output current is internally preset at 50mA.
When the input voltage of 20 to 400V appears at the
DRAIN pin, the internal high-voltage linear regulator seeks
to maintain a voltage of 7.5VDC at the VDD pin. Until this
voltage exceeds the internally programmed under-voltage
threshold, the output switching MOSFET is non-conductive.
When the threshold is exceeded, the MOSFET turns on. The
input current begins to fl ow into the DRAIN pin. Hysteresis
is provided in the under-voltage comparator to prevent
oscillation.
When the input current exceeds the internal preset level,
a current sense comparator resets an RS fl ip-fl op, and the
MOSFET turns off. At the same time, a one-shot circuit is
activated that determines the duration of the off-state (10.5µs
typ.). As soon as this time is over, the fl ip-fl op sets again.
The new switching cycle begins.
A “blanking” delay of 300ns is provided that prevents false
triggering of the current sense comparator due to the leading
edge spike caused by circuit parasitics.
Application Information
The HV9922 is a low-cost off-line buck converter IC
specifi cally designed for driving multi-LED strings. It can
be operated from either universal AC line range of 85 to
264VAC, or 20 to 400VDC, and drives up to tens of high
brightness LEDs. All LEDs can be run in series, and the
HV9922 regulates at constant current, yielding uniform
illumination. The HV9922 is compatible with triac dimmers.
The output current is internally fi xed at 50mA. This part is
available in space saving TO-92 and SOT-89 packages.
Selecting L1 and D1
There is a certain trade-off to be considered between
optimal sizing of the output inductor L1 and the tolerated
output current ripple. The required value of L1 is inversely
proportional to the ripple current ∆I
L1 = (V
V
time of the HV9922. The output current in the LED string (I
is calculated then as:
I
O
O
= I
is the forward voltage of the LED string. T
TH
- (ΔI
O
• T
O
OFF
/ 2)
) / ΔI
O
O
in it.
OFF
(1)
(2)
is the off-
O
)
4
where I
ripple current introduces a peak-to-average error in the
output current setting that needs to be accounted for. Due to
the constant off-time control technique used in the HV9922,
the ripple current is independent of the input AC or DC line
voltage variation. Therefore, the output current will remain
unaffected by the varying input voltage.
Adding a fi lter capacitor across the LED string can reduce
the output current ripple even further, thus permitting a
reduced value of L1. However, one must keep in mind that
the peak-to-average current error is affected by the variation
of T
be sacrifi ced at large ripple current in L1.
Another important aspect of designing an LED driver with
the HV9922 is related to certain parasitic elements of the
circuit, including distributed coil capacitance of L1, junction
capacitance and reverse recovery of the rectifi er diode D1,
capacitance of the printed circuit board traces C
capacitance C
elements affect the effi ciency of the switching converter and
could potentially cause false triggering of the current sense
comparator if not properly managed. Minimizing these
parasitics is essential for effi cient and reliable operation of
the HV9922.
Coil capacitance of inductors is typically provided in the
manufacturer’s data books either directly or in terms of the
self-resonant frequency (SRF).
SRF = 1 / (2π √(L • C
where L is the inductance value, and C
capacitance.) Charging and discharging this capacitance
every switching cycle causes high-current spikes in the LED
string. Therefore, connecting a small capacitor C
recommended to bypass these spikes.
Using an ultra-fast rectifi er diode for D1 is recommended to
achieve high effi ciency and reduce the risk of false triggering
of the current sense comparator. Using diodes with shorter
reverse recovery time t
achieves better performance. The reverse voltage rating V
of the diode must be greater than the maximum input voltage
of the LED lamp.
The total parasitic capacitance present at the DRAIN pin of
the HV9922 can be calculated as:
C
P
= C
OFF
. Therefore, the initial output current accuracy might
DRAIN
TH
is the current sense comparator threshold. The
+ C
DRAIN
PCB
+C
of the controller itself. These parasitic
L
L
))
+C
rr
and lower junction capacitance C
J
HV9922
L
PCB
O
is the coil
(3)
and output
(~10nF) is
R
J