HV9921/22
Initial Release
3-Pin Switch-Mode LED Lamp Driver IC
Features
Constant Output Current:
o
HV9921 – 20mA
o
HV9922 – 50mA
Universal 85-264VAC Operation
Fixed OFF-Time Buck Converter
Internal 500V Power MOSFET
General Description
The HV9921/22 are pulse width modulated (PWM)
high-efficiency LED driver control ICs. They allow
efficient operation of LED strings from voltage
sources ranging up to 400VDC. The HV9921/22
include an internal high-voltage switching MOSFET
controlled with fixed off-time T
OFF
of approximately
10µs. The LED string is driven at constant current,
thus providing constant light output and enhanced
reliability. The output current is internally fixed at
20mA for HV9921 and 50mA for HV9922. The peak
current control scheme provides good regulation of
the output current throughout the universal AC line
voltage range of 85 to 264VAC or DC input voltage
of 20 to 400V.
Applications
Decorative Lighting
Low Power Lighting Fixtures
Typical Application Circuit
~
AC
~
C
IN
LED
1
BR1
C
0
-
LED
N
D1
L1
3
V
DD
DRAIN
1
U1
C
DD
HV9921/22
GND
2
NR042205
NR030805
HV9921/22
Ordering Information
Package Options
DEVICE
TO-92
HV9921
HV9922
HV9921N3
HV9922N3
SOT-89
HV9921N8
HV9922N8
Absolute Maximum Ratings
Supply Voltage, V
DD
Supply Current, I
DD
Operating Ambient Temperature Range
Operating Junction Temperature Range
Storage Temperature Range
Power Dissipation @ 25°C, TO-92
Power Dissipation @ 25°C, SOT-89
-0.3 to +10V
+5mA
-40°C to +85°C
-40° to +125°C
-65° to +150°C
740mW
1600mW
†
† Mounted on FR4 board, 25mm x 25mm x 1.57mm.
Stresses beyond those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond
those indicated in the operational sections of the specifications is not
implied. Exposure to absolute maximum rating conditions for extended
periods may affect device reliability.
(The * denotes the specifications which apply over the full operating junction temperature
range of -40
°
C < T
A
< +85
°
C, otherwise the specifications are at T
A
=25
°
C, V
DRAIN
=50V, unless otherwise noted)
Electrical Characteristics
Regulator (VDD)
Symbol
V
DD
V
DRAIN
V
UVLO
∆V
UVLO
I
DD
Parameter
V
DD
Regulator Output
V
DRAIN
Supply Voltage
Min
7.1
20
5.0
Typ
7.5
Max
7.8
Units
V
V
V
Conditions
V
DD
Undervoltage Threshold
V
DD
Undervoltage Lockout
Hysteresis
Operating Supply Current
200
200
Min
500
Typ
110
100
1
100
150
350
Max
210
200
5
mV
µA
Units
V
Ω
Ω
pF
mA
V
DD(EXT)
= 8.5V, V
DRAIN
= 40V
Conditions
I
DRAIN
= 20mA
I
DRAIN
= 50mA
V
DRAIN
= 400V
Output (DRAIN)
Symbol
Parameter
V
BR
Breakdown Voltage
R
ON
R
ON
C
DRAIN
I
SAT
ON Resistance – HV9921
ON Resistance – HV9922
Output Capacitance
MOSFET Saturation Current
Current Sense Comparator
Symbol
Parameter
I
TH
Threshold Current – HV9921
I
TH
T
BLANK
T
ON(MIN)
Threshold Current – HV9922
Leading Edge Blanking Delay
Minimum ON Time
Min
20.5
52
200
300
Typ
Max
25.5
63
400
650
Units
mA *
mA
ns
ns
*
*
Conditions
OFF-Time Generator
Symbol
Parameter
T
OFF
OFF Time
Min
8
Typ
10.5
2
Max
13
Units
µs
Conditions
NR040805
HV9921/22
Functional Description
HV9921 and HV9922 are PWM peak current
controllers for controlling a buck converter topology
in continuous conduction mode (CCM). The output
current is internally preset at 20mA (HV9921) or
50mA (HV9922).
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 7VDC at the V
DD
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 flow 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 flip-
flop, 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 flip-flop 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.
V
O
is the forward voltage of the LED string. T
OFF
is
the off-time of the HV9921/22. The output current in
the LED string (I
O
) is calculated then as:
I
O
=
I
TH
−
1
⋅ ∆
I
O
,
2
(2)
where I
TH
is the current sense comparator threshold.
The 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 HV9921/22, 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 filter 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
OFF
. Therefore,
the initial output current accuracy might be sacrificed
at large ripple current in L1.
Another important aspect of designing an LED driver
with HV9921/22 is related to certain parasitic
elements of the circuit, including distributed coil
capacitance of L1, junction capacitance and reverse
recovery of the rectifier diode D1, capacitance of the
printed circuit board traces C
PCB
and output
capacitance C
DRAIN
of the controller itself. These
parasitic elements affect the efficiency 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 efficient and reliable operation of
HV9921/22.
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 /(2
π
L
⋅
C
L
)
,
Application Information
The HV9921/22 is a low-cost off-line buck converter
IC specifically 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 HV9921/22 regulates at
constant current, yielding uniform illumination. The
HV9921/22 is compatible with triac dimmers. The
output current is internally fixed at 20mA for HV9921
and 50mA for HV9922. Both parts are 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
O
in it.
L
1
=
V
O
⋅
T
OFF
∆
I
O
where L is the inductance value, and C
L
is the coil
capacitance.) Charging and discharging this
capacitance every switching cycle causes high-
current spikes in the LED string. Therefore,
connecting a small capacitor C
O
(~10nF) is
recommended to bypass these spikes.
Using an ultra-fast rectifier diode for D1 is
recommended to achieve high efficiency and reduce
the risk of false triggering of the current sense
comparator. Using diodes with shorter reverse
recovery time
t
rr
and lower junction capacitance C
J
achieves better performance. The reverse voltage
5
NR040805
(1)
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