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USL3631 Fiches technique(PDF) 4 Page - Unisonic Technologies

No de pièce USL3631
Description  NON-ISOLATED BUCK OFFLINE LED DRIVER
Download  6 Pages
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Fabricant  UTC [Unisonic Technologies]
Site Internet  http://www.utc-ic.com
Logo UTC - Unisonic Technologies

USL3631 Fiches technique(HTML) 4 Page - Unisonic Technologies

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USL3631
Preliminary
LINEAR INTEGRATED CIRCUIT
UNISONICTECHNOLOGIESCO.,LTD
4 of 6
www.unisonic.com.tw
QW-R103-076.d
APPLICATION INFORMATION
The UTC USL3631 is a high performance non-isolated Buck converter specially designed for LED lighting. The
device integrates a 600V power MOSFET. With very few external components, the converter achieves excellent
constant current control. And it does not need auxiliary winding for powering the IC or voltage sensing, thus the
system size and cost is greatly reduced.
Start Up
When system powered up, the VCC pin capacitor is charged up by the start up resistor. When the VCC pin voltage
reaches the start up threshold, the internal circuits start operating. The UTC USL3631 integrates a 17V zener
diode to clamp the VCC voltage.
Constant Current Control
Cycle by Cycle current sense is adjusted in UTC USL3631, the CS pin is connected to the current sense
comparator, and the voltage on CS pin is compared with the internal 400mV reference voltage. The MOSFET will be
switched off when the voltage on CS pin reaches the Peak Current threshold. The CS comparator includes a 350ns
leading edge blanking time. After the MOSFET switched off, the OFF time controller would open the MOSFET again
at time up. At that moment, CS pin voltage is 0mV usually.
The Delta inductor current is given by:
PK
CS
400
I=
(mA
R
Where, RCS is the current sense resistor value.
The current in LED can be calculated by the equation:
PK
LED
CS
I
200
I=
=
(mA
2R
Where, IPK is the Peak current of the inductor.
Inductor Selection
The UTC USL3631 works under inductor current critical conduction mode. When the power MOFET is switched
on, the current in the inductor rises up from zero, the on time of the MOSFET can be calculated by the equation:
LED
IN
PK
on
V
V
I
L
t
-
Where,
L is the inductance value
VIN is the DC bus voltage after the rectifier bridge
VLED is the voltage on the LED
After the power MOSFET is switched off, the current in the inductor decreases. When the inductor current
reaches zero, the power MOSFET is turned on again by IC internal logic. The off time of the MOSFET is given by:
LED
PK
off
V
I
L
t
The inductance can be calculated by the equation:
IN
PK
LED
IN
LED
V
I
freq
V
V
V
L
)
-
(
The freq is the system switching frequency, which is proportional to the input voltage. So the minimum switching
frequency is set at lowest input voltage, and the maximum switching frequency is set at highest input voltage.
The minimum and maximum off time of UTC USL3631 is set at 4.5µs and 240µs, respectively. Referring to the
equation of tOFF calculation, if the inductance is too small, the tOFF may be smaller than the minimum off time, system
will operate in discontinuous conduction mode and the output current will be smaller than the designed value. If the
inductance is too large, the tOFF may be larger than the maximum off time, the outputt current will not be constant
follow design value. So it is important to choose a proper inductance, we recommend user to regulate the inductance
value for 70KHz application.


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