HV9925
Initial Release
Programmable-Current LED Lamp Driver IC
with PWM Dimming
Features
Programmable Output Current to 50mA
PWM Dimming / Enable
Universal 85-264VAC Operation
Fixed OFF-Time Buck Converter
Internal 500V Power MOSFET
Over Temperature Protection with Hysteresis
General Description
The HV9925 is a pulse width modulated (PWM) high-efficiency
LED driver control IC with PWM dimming capabilities. It
allows efficient operation of high brightness LED strings from
voltage sources ranging up to 400VDC. The HV9925 includes
an internal high-voltage switching MOSFET controlled with a
fixed off-time T
OFF
of approximately 10µs. The LED string is
driven at constant current, thus providing constant light output
and enhanced reliability. Selecting a value of a current sense
resistor can externally program the output LED current of the
HV9925. 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. The HV9925 is designed with a built in thermal
shutdown to prevent excessive power dissipation in the IC.
Applications
Decorative Lighting
Low Power Lighting Fixtures
Typical Application Circuit
~
AC
~
BR1
L1
C
IN
C
O
LED
N
D1
LED
1
6
Enable
3
4
C
DD
Drain
PWMD
7
Drain
8
Drain
HV9925
V
DD
R
SENSE
U1
GND
1
R
SENSE
2
NR021506
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HV9925
Ordering Information
DEVICE
HV9925
-G indicates package is RoHS compliant (‘Green’)
Package Options
8-Pin SOIC w/ Heat Slug
HV9925SG-G
Absolute Maximum Ratings*
Parameter
Supply Voltage, V
DD
Supply Current, I
DD
PWMD, R
SENSE
Voltage
Operating Ambient Temperature Range
Operating Junction Temperature Range
Storage Temperature Range
Power Dissipation @ 25°C
All voltages referenced to GND pin.
**The power dissipation is given for the standard minimum pad without a
heat slug, and based on R
θJA
=125°C/W. R
θJA
is the sum of the junction-to-
case and case-to-ambient thermal resistance, where the latter is deter-
mined by the user’s board design. The junction-to-ambient thermal resis-
tance is R
θJA
= 105°C/W when the part is mounted on a 0.04 in
2
pad of 1 oz
copper, and R
θJA
= 60°C/W when mounted on a 1 in
2
pad of 1 oz copper.
Pin Configuration
Value
-0.3 to +10V
-0.3 to +10V
+5mA
-40°C to +85°C
-40°C to +125°C
-65°C to +150°C
800mW**
R
SENSE
GND
PWMD
V
DD
1
2
3
4
HV9925SG
8
7
6
5
Drain
Drain
Drain
NC
top view
SO-8 + Heat Slug
(Heat Slug Potential is at ground)
DRAIN (6,7,8)
– This is a drain terminal of the output switching
MOSFET and a linear regulator input.
V
DD
(4)
– This is a power supply pin for internal control circuits.
Bypass this pin with a 0.1uF low impedance capacitor.
R
SENSE
(1)
– This is a source terminal of the output switching
MOSFET provided for current sense resistor connection.
GND (2)
– This is a common connection for all circuits.
PWMD (3)
– This is the PWM Dimming input to the IC.
Electrical Characteristics
Symbol
V
DD
V
UVLO
ΔV
UVLO
I
DD
V
BR
V
DRAIN
R
ON
C
DRAIN
I
SAT
V
TH
T
BLANK
T
ON(MIN)
Parameter
V
DD
regulator output
(The
*
denotes the specifications which apply over the full operating temperature range of -40°C < T
A
< +85°C, otherwise the specifications are at
T
A
= 25°C. V
DRAIN
= 100V, unless otherwise noted)
Min
-
5.0
-
-
500
20
-
-
100
0.44
200
-
Typ
7.5
-
200
300
-
-
100
1.0
150
0.47
300
-
Max
-
-
-
500
-
-
200
5.0
-
0.50
400
650
Units
V
V
mV
μA
V
V
Ω
pF
mA
V
ns
ns
Conditions
--
--
--
V
DD(EXT)
= 8.5V
* --
--
I
DRAIN
= 50mA
V
DRAIN
= 400V
--
--
* --
--
V
DD
undervoltage threshold
V
DD
undervoltage lockout hysteresis
Operating supply current
Breakdown voltage
V
DRAIN
supply voltage
ON resistance
Output capacitance
DRAIN saturation current
Threshold voltage
Leading edge blanking delay
Minimum ON time
Output (DRAIN)
Current Sense Comparator
NR021506
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HV9925
Functional Description
The HV9925 is a PWM peak current control IC for driving
a buck converter topology in continuous conduction mode
(CCM). The HV9925 controls the output current (rather than
output voltage) of the converter that can be programmed by
a single external resistor (R
SENSE
), for the purpose of driving a
string of light emitting diodes (LED). An external enable input
(PWMD) is provided that can be utilized for PWM dimming of
an LED string. The typical rising and falling edge transitions
of the LED current when using the PWM dimming feature of
the HV9925 are shown in Fig. 6 and Fig. 7.
When the input voltage of 20 to 400V appears at the DRAIN
pin, the internal linear regulator seeks to maintain a voltage
of 7.5VDC at the V
DD
pin. Until this voltage exceeds the
internally programmed under-voltage threshold, no output
switching occurs. When the threshold is exceeded, the
integrated high-voltage switch turns on, pulling the DRAIN
low. A 200mV hysteresis is incorporated with the under-
voltage comparator to prevent oscillation.
When the voltage at R
SENSE
exceeds 0.47V, the switch turns
off and the DRAIN output becomes high impedance. At the
same time, a one-shot circuit is activated that determines
the off-time of the switch (10µs typ.).
A “blanking” delay of 300ns is provided upon the turn-on of
the switch that prevents false triggering of the current sense
comparator due to the leading edge spike caused by circuit
parasitics.
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
HV9925 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
HV9925.
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
)
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 rating V
R
of the diode must be greater than the maximum input voltage
of the LED lamp.
The total parasitic capacitance present at the DRAIN output
of the HV9925 can be calculated as:
C
P
=
C
DRAIN
+
C
PCB
+
C
L
+
C
J
(3)
When the switch turns on, the capacitance C
P
is discharged
into the DRAIN output of the IC. The discharge current is
limited to about 150mA typically. However, it may become
lower at increased junction temperature. The duration of the
leading edge current spike can be estimated as:
T
SPIKE
=
V
IN
⋅
C
P
+
t
rr
I
SAT
(4)
Application Information
Selecting L1 and D1
The required value of L1 is inversely proportional to the ripple
current ∆I
O
in it. Setting the relative peak-to-peak ripple to
20~30% is a good practice to ensure noise immunity of the
current sense comparator.
V
⋅
T
L1
=
O OFF
(1)
∆
I
O
V
O
is the forward voltage of the LED string. T
OFF
is the off-
time of the HV9925. The output current in the LED string (I
O
)
is calculated then as:
I
O
=
V
TH
R
SENSE
−
1
⋅ ∆
I
O
2
(2)
where V
TH
is the current sense comparator threshold, and
R
SENSE
is the current sense resistor. 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 HV9925, the ripple
current is nearly independent of the input AC or DC voltage
NR021506
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