Datasheet

UNISONIC TECHNOLOGIES CO., LTD
USL3531
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LINEAR INTEGRATED CIRCUIT
NON-ISOLATED BUCK
OFFLINE LED DRIVER

DESCRIPTION
The UTC USL3531 is a high performance, high precision and
low cost constant current LED driver for non-isolated buck
application. The device operates in critical conduction mode and
is suitable for 85V~265V universal input offline LED lighting. It
operates in critical conduction mode, the output current does not
change with the inductance and LED output voltage.
The UTC USL3531 integrates a 500V power MOSFET. It can
achieve excellent constant current performance with very few
external components, so the system cost and size are minimized.
The UTC USL3531 offers rich protection functions to improve
the system reliability, including LED open circuit protection, LED
short circuit protection, VCC under voltage protection, CS resistor
short circuit protection and thermal regulation function.

SOP-8
FEATURES
* Critical conduction mode operation
* Internal 500V Power MOSFET
* Ultra low operating current
* No auxiliary winding
* ±5% LED output current accuracy
* LED short protection
* LED open protection
* VCC under voltage protection
* Current sensing resistor short protection
* Thermal regulation function
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ORDERING INFORMATION
Ordering Number
USL3531G-S08-R
USL3531G-S08-R
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Package
SOP-8
(1)Packing Type
(1) R: Tape Reel
(2)Package Type
(2) S08: SOP-8
(3)Green Package
(3) G: Halogen Free and Lead Free
Packing
Tape Reel
MARKING
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Copyright © 2015 Unisonic Technologies Co., Ltd
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PIN CONFIGURATION
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PIN DESCRIPTION
PIN NO.
1
2
3
4
5, 6
7, 8
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PIN NAME
GND
ROVP
NC
VCC
DRAIN
CS
LINEAR INTEGRATED CIRCUIT
DESCRIPTION
Ground
Over voltage protection setting pn. Connect a resistor to GND
No connection. Should be connected to GND(Pin1)
Power supply pin
Internal HV Power MOSFET Drain.
Current sense pin. Connect a sense resistor between this pin and GND pin.
BLOCK DIAGRAM
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LINEAR INTEGRATED CIRCUIT
ABSOLUTE MAXIMUM RATING
PARAMETER
SYMBOL
RATINGS
UNIT
VCC Pin Maximum Sink Current
ICC_MAX
5
mA
Internal HV MOSFET Drain Voltage
DRAIN
-0.3~500
V
Current Sense Pin Input Voltage
CS
-0.3~6
V
Over-Voltage Setting Pin Voltage
ROVP
-0.3~6
V
Power Dissipation
PD
0.45
W
Junction Temperature
TJ
-40~+150
°C
Storage Temperature
TSTG
-55~+150
°C
Note: Absolute maximum ratings are those values beyond which the device could be permanently damaged.
Absolute maximum ratings are stress ratings only and functional device operation is not implied.

RECOMMENDED OPERATION CONDITIONS
PARAMETER
Output LED Current @ VOUT=72V
(Input Voltage 176V~265V)
Output LED Current @ VOUT=36V
(Input Voltage 176V~265V)
Minimum LED Loading Voltage
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RATINGS
UNIT
ILED1
160
mA
ILED2
220
mA
VLED min
>15
V
RATING
150
UNIT
°C/W
THERMAL RESISTANCES CHARACTERISTICS
PARAMETER
Junction to Ambient

SYMBOL
SYMBOL
θJA
ELECTRICAL CHARACTERISTICS (Notes 1, 2) (Unless otherwise specified, VCC=14V and TA=25°C)
PARAMETER
SYMBOL
TEST CONDITIONS
MIN TYP MAX UNIT
Supply Voltage Section
VCC Clamp Voltage
VCC_CLAMP 1mA
16.8
V
VCC Turn On Threshold
VCC_ON VCC Rising
13.8
V
VCC Turn Off Threshold
VCC_UVLO VCC Falling
9
V
VCC Startup Current
IST
VCC= VCC-ON - 1V
180 280
µA
VCC Operating Current
IOP
FOP=70KHz
200 300
µA
Current Sense Section
Threshold Voltage for Peak Current Limit
VCS_peak
390 400 410
mV
Threshold Voltage for Peak Current Limit When
VCS_SHORT Output Short
200
mV
Output Short
Leading Edge Blanking Time for Current Sense
TLEB
300
ns
Switch Off Delay Time
TDELAY
200
ns
Internal Time Control Section
Minimum OFF Time
TOFF_MIN
4.5
µs
Maximum OFF Time
TOFF_MAX
220
µs
Maximum On Time
TON_MAX
35
µs
ROVP Pin Voltage
VROVP
0.5
V
MOSFET Section
Static Drain-source On-resistance
RDS_ON VGS=15V/IDS=0.4A
14
Ω
Drain-Source Breakdown Voltage
BVDSS VGS=0V/IDS=250µA
500
V
Power MOSFET Drain Leakage Current
IDSS
VGS=0V/VDS=500V
1
µA
Thermal Regulation Section
Thermal Regulation Temperature
TREG
150
°C
Notes: 1. Production testing of the chip is performed at 25°C.
2. The maximum and minimum parameters specified are guaranteed by test, the typical value are guaranteed
by design, characterization and statistical analysis.
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LINEAR INTEGRATED CIRCUIT
APPLICATION INFORMATION
The UTC USL3531 is a high performance non-isolated Buck converter specially designed for LED lighting. The
device integrates a 500V 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 USL3531 integrates a 17V zener
diode to clamp the VCC voltage.
Constant Current Control
Cycle by Cycle current sense is adjusted in UTC USL3531, 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:
IPK =
400
(mA)
RCS
Where, RCS is the current sense resistor value.
The current in LED can be calculated by the equation:
ILED =
IPK
200
=
(mA)
2
RCS
Where, IPK is the Peak current of the inductor.
Inductor Selection
The UTC USL3531 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:
ton 
L  I PK
V IN - V LED
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:
toff 
L  I PK
V LED
The inductance can be calculated by the equation:
L 
V LED  (V IN - V LED )
freq  I PK  V IN
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 USL3531 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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LINEAR INTEGRATED CIRCUIT
APPLICATION INFORMATION(Cont.)
Over Voltage Protection
The over voltage protection can be programmed by the ROVP pin resistor. The ROVP pin voltage is 0.5V.
When the LED is open circuit, the output voltage increases gradually, and the demagnetization time gets shorter.
The demagnetization time at OVP---- Tovp can be calculated by the open circuit protection voltage:
TOVP ≈
L  I PK
VOVP
Where,
Vovp is the open circuit protection voltage
And then the Rovp resistor value can be calculated by the equation:
ROVP ≈ 19  Top  10 6
(kΩ)
Protection Function
The UTC USL3531 offers rich protection functions to improve the system reliability, including LED open/short
protection, CS resistor short protection, VCC under voltage protection, thermal regulation. When the LED is open
circuit, the system will trigger the over voltage protection and stop switching.
When the LED short circuit is detected, the system works at low frequency (about 5kHz), and the CS pin turn off
threshold is reduced to 200mV. So the system power consumption is very low. At some catastrophic fault condition,
such as CS resistor shorted or inductor saturated, the internal fast fault detection circuit will be triggered, the system
stops switching immediately.
After the system enters into fault condition, the VCC voltage will decrease until it reaches the UVLO threshold,
then the system will re-start again. If the fault condition is removed, the system will recover to normal operation.
Thermal Regulation
The UTC USL3531 integrates thermal regulation function. When the system is over temperature, the output
current is gradually reduced; the output power and thermal dissipation are also reduced. The system temperature is
regulated and the system reliability is improved. The thermal regulation temperature is set to 150°C internally.
PCB Layout
The following rules should be followed in UTC USL3531 PCB layout:
Bypass Capacitor
The bypass capacitor on VCC pin should be as close as possible to the VCC Pin and GND pin.
ROVP Pin
The ROVP resistor should be as close as possible to the ROVP Pin.
Ground Path
The power ground path for current sense should be short, and the power ground path should be separated from
small signal ground path before connecting to the negative node of the bulk capacitor.
The Area of Power Loop
The area of main current loop should be as small as possible to reduce EMI radiation, such as the inductor, the
power MOSFET, the output diode and the bus capacitor loop.
NC pin
The NC pin should be connected to GND (pin1).
Drain Pin
To increase the copper area of DRAIN pin for better thermal dissipation. However too large copper area may
compromise EMI performance.
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LINEAR INTEGRATED CIRCUIT
TYPICAL APPLICATION CIRCUIT
AC
UTC
USL3531
VCC
DRAIN
ROVP
DRAIN
NC
CS
GND
CS
UTC assumes no responsibility for equipment failures that result from using products at values that
exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or
other parameters) listed in products specifications of any and all UTC products described or contained
herein. UTC products are not designed for use in life support appliances, devices or systems where
malfunction of these products can be reasonably expected to result in personal injury. Reproduction in
whole or in part is prohibited without the prior written consent of the copyright owner. The information
presented in this document does not form part of any quotation or contract, is believed to be accurate
and reliable and may be changed without notice.
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