UM1361S
Hysteretic Buck High Brightness LED Driver with Internal Switch
UM1361S SOT23-5
General Description
The UM1361S is a PWM step-down converter with internal power switch, designed for driving
single or multiple series connected LEDs efficiently from a voltage source higher than the LED
voltage. The device operates from an input supply between 6V and 40V and employs hysteretic
control with a high side current sense resistor to set the constant output current up to 1A. The
device is well suited for applications requiring a wide input range. The high side current sensing
and an integrated current sensing circuitry minimize the number of external components while
delivering an accurate average output current. Depending upon supply voltage and external
components, this device can provide up to 30 watts of output power.
Output current can be adjusted below the set value, by applying an external control signal to the
VSET pin. The VSET pin will accept either a DC voltage or a PWM waveform. Dedicated pulse
width modulation (PWM) input enables pulsed LED dimming over a wide range of brightness
levels. A hysteretic control method ensures excellent input supply rejection and fast response
during load transients and PWM dimming.
Applying a voltage of 0.2V or lower to the VSET pin turns the output off and switches the device
into a low current standby state.
The UM1361S comes in small SOT23-5 package. It is ideal for industrial and general lighting
applications.
Applications
Features
Low Voltage Halogen Replacement
LEDs
Low Voltage Industrial Lighting
LED Back-Side Lighting
Illuminated Signs
DC/DC or AC/DC LED Driver
Application
General Purpose, Constant Current
Source
Hysteretic Control with High Side Current
Sensing
Integrated 40V 0.5Ω NDMOS
> 90% Efficiency
Wide Input Voltage Range: 6V to 40V
± 5% LED Current Accuracy
Adjustable Constant LED Current
Analog or PWM Control Signal for PWM
Dimming
Over Temperature, Open Circuit LED
Protection
Up to 1MHz Switching Frequency
Pb-Free SOT23-5 Package
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UM1361S
Pin Configurations
Top View
5
LX
1
5
4
VIN
EQXYW
GND
2
VSET
3
4
ISENSE
1
2
3
X: Internal Code; Y: Year Code; W: Week Code
UM1361S
SOT23-5
Ordering Information
Part Number
Packaging Type
Marking Code
UM1361S
SOT23-5
EQX
Shipping Qty
3000pcs/7Inch
Tape & Reel
Pin Description
Pin
Number
1
Symbol
LX
2
GND
3
VSET
4
ISENSE
5
VIN
Function
Drain of NDMOS switch.
Ground (0V).
Multi-function On/Off and brightness control pin.
Leave floating for normal operation.
Drive to voltage below 0.2V to turn off output current.
Drive with DC voltage (0.3V＜VSET＜2.5V) to adjust output current from
12% to 100% of IOUTnom.
Drive with PWM signal from open-collector or open-drain transistor, to
adjust output current. Adjustment range 1% to 100% of IOUTnom for f＜
500Hz and 2% to 100% of IOUTnom for f>20kHz.
Connect resistor RS from this pin to VIN to define nominal average
output current IOUTnom = 0.1/RS.
Input voltage (6V to 40V). Decouple to ground with 10μF or higher X7R
ceramic capacitor close to device.
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UM1361S
Absolute Maximum Ratings
Over operating free-air temperature (unless otherwise noted) (Note 1)
Symbol
VIN
VLX,
VISENSE
VSET
Parameter
Value
Unit
Input Voltage Range
-0.3 to +45
V
Voltages on LX, ISENSE
-0.3 to +45
V
VSET Pin Voltage
-0.3 to +6
V
θjA
Thermal Resistance (Junction to Ambient)
250
°C/W
θjC
Thermal Resistance (Junction to Case)
130
°C/W
TJ
Maximum Junction Temperature
+170
°C
-65 to +170
°C
+300
°C
TSTG
TL
Storage Temperature Range
Maximum Lead Temperature for Soldering 5
Seconds
Note 1: These are stress ratings only and functional operation is not implied. Exposure to absolute
maximum ratings for prolonged time periods may affect device reliability. All voltage
values are with respect to network ground terminal.
Recommended Operating Conditions
Symbol
VIN
TA
TJ
Parameter
Input Voltage Range
Operating Ambient Temperature
Operating Junction Temperature
Min
6.0
-40
-40
Typ
Max
40
85
150
Unit
V
°C
°C
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UM1361S
Electrical Characteristics
(VIN=16V, TA=25°C, unless otherwise noted)
Symbol
VIN
Parameter
Input Voltage Range
ILED
Output Current
IQ
ISD
VSENSE
VSENSE_HYS
ISENSE
Ven
Venon
Venoff
RLX
ILX(leak)
FLX
FLXmax
Ton_rec
DLX
TPD
(Note 2)
TOTP
TOTP_Hys
IXLmax
Quiescent Current
without Switching
Shutdown Current
Mean Current Sense
Threshold Voltage
Sense Threshold
Hysteresis
ISENSE Pin Input
Current
VSET Range on VSET
Pin
DC Voltage on VSET
Pin to Enable
DC Voltage on VSET
Pin to Disable
LX Switch on
Resistance
LX Switch Leakage
Current
Operating Frequency
Recommended
Maximum Switch
Frequency
Recommended
Minimum Switch ON
Time
Max Duty Cycle
Recommended Duty
Cycle Range
Internal Comparator
Propagation Delay
Over Temperature
Protection
Temperature
Protection Hysteresis
Current Limit
Test Conditions
RS=0.3Ω
RS=0.1Ω
VSET Pin Floating,
VIN=16V
VSET Pin Grounded
Measured on ISENSE
Pin with Respect to
VIN
Min
6
95
VSENSE=VIN-0.1
For DC Dimming
Typ
Max
40
333
1
Unit
V
mA
A
0.6
mA
37
50
μA
100
105
mV
±13
%
16
μA
0.3
2.5
V
Ven Rising
0.25
V
Ven Falling
0.2
V
ILX=100mA
0.5
Ω
5
VIN=16V, VOUT=9.6V
(3LEDs), L=47μH,
△I=0.25A (ILED=1A)
233
kHz
1.0
For 4% Accuracy
1.5
MHz
500
ns
98
%
25
Peak Inductor Current
μA
75
%
45
ns
155
°C
30
°C
A
Note 2: Parameters are not tested at production, but guaranteed by design.
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UM1361S
Function Block Diagram
D1
VIN
LED
L1
Rs
LX
VIN
ISENSE
GM
Low Voltage
Detector
Regulator
C1
OTP
BG
VSET
Logic
&
Driver
Ref
VSET
EN
R2
Internal
Enable
R1
Ilimit
GND
Typical Application Circuit
D1
Rs 0.3Ω
VIN(6V-40V)
VIN
ISENSE
L1
UM1361S
Ci
100μH
10μF
Floating
VSET
LX
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UM1361S
Typical Operating Characteristics
Efficiency vs.Vin (Rs=0.15Ω,L=47uH)
Efficiency vs.Vin (Rs=0.33Ω,L=100uH)
90
90
Efficiency (%)
100
Efficiency (%)
100
80
70
60
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
80
70
60
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
50
50
0
5
10
15
20
25
30
0
5
10
Vin (V)
600
1LED
4LED
7LED
10LED
500
Frequency (KHz)
Efficiency (%)
90
80
70
1LED
4LED
7LED
2LED
5LED
8LED
3LED
6LED
10
15
Vin (V)
20
25
30
Operating Frequency vs. Vin (Rs=0.15Ω, L=47μH)
Efficiency vs.Vin (Rs=0.104Ω,L=33uH)
100
60
15
Vin (V)
2LED
5LED
8LED
3LED
6LED
9LED
400
300
200
100
50
0
0
5
20
25
30
0
20
Vin (V)
30
40
30
40
Shutdown Current vs. Vin
Quiescent Current vs. Vin
600
10
30
25
Shutdown current (uA)
Quiescent current (uA)
550
500
450
400
20
15
10
5
0
350
0
10
20
Vin (V)
30
40
0
10
20
Vin (V)
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UM1361S
Typical Operating Characteristics (Continued)
LED Current vs. VSET
(Vin=16V,3LEDs,100uH,Rs=0.33Ω)
LED Current vs. VSET Duty Cycle
(Vin=16V,3LEDs,100uH,Rs=0.33Ω)
350
300
300
250
250
ILED (mA)
ILED (mA)
350
200
150
200
150
100
100
50
50
PWM=100Hz
0
0
0
10
20
30
40
50
60
70
80
90
100
Duty Cycle (% )
0
0.5
1
1.5
2
2.5
VSET (V)
Feedback Voltage vs. Vin
Sense throshold voltage (mV)
120
100
80
60
40
20
H
L
0
0
5
10
15
20
25
30
35
40
45
Vin (V)
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UM1361S
Typical Operating Characteristics (Continued)
Steady State Waveforms
(3LEDs, 100μH, Vin=16V, Rs=0.33Ω)
Start up Waveforms
10V/div
SW
2V/div
VSET
ILED
500mA/div
100mA/div
ILED
2μs/div
10μs/div
Dimming Waveforms
(PWM=50%, 3LEDs, 100μH, Vin=16V,Rs=0.33Ω)
Pulse Skip Mode
(3LEDs, 100μH, Vin=10V, Rs=0.33Ω)
2V/div
10V/div
SW
PWM
ILED
100mA/div
ILED
100mA/div
500μs/div
20μs/div
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UM1361S
Applications Information
Setting Nominal Average Output Current with External Resistor RS
The nominal average output current in the LED(s) is determined by the value of the external
current sense resistor (RS) connected between VIN and ISENSE and is given by:
IOUTnom = 0.1/RS (Rs≥0.1Ω)
This equation is valid when VSET pin is float or applied with a voltage higher than 2.5V
(must be less than 5V).Actually, R S sets the maximum average current which can be adjusted
to a less one by dimming.
Output Current Adjustment by External DC Control Voltage
The VSET pin can be driven by an external dc voltage (VDIM), as shown, to adjust the output
current to a value below the nominal average value defined by Rs.
D1
Rs 0.13Ω
LED
3W
VIN(6V-40V)
VIN
Ci
ISENSE
L1
UM1361S
68μH
10μF
VSET
LX
The average output current is given by:
IOUT = (0.1*VDIM)/(2.5*Rs) [for 0.3V<VDIM<2.5V]
Note that 100% brightness setting corresponds to:
(2.5V≤VDIM≤5V)
Output Current Adjustment by PWM Control
A Pulse Width Modulated (PWM) signal with duty cycle PWM can be applied to the VSET
pin, as shown below, to adjust the output current to a value below the nominal average value
set by resistor RS:
IOUT = (0.1*D)/RS (0≤D≤100%, 2.5V＜Vpulse＜5V)
IOUT = (Vpulse*0.1*D)/(2.5*RS) (0≤D≤100%, 0.5V＜Vpulse＜2.5V)
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UM1361S
D1
Rs 0.13Ω
LED
3W
VIN(6V-40V)
VIN
Ci
ISENSE
L1
UM1361S
68μH
10μF
VSET
LX
PWM dimming provides reduced brightness by modulating the LED’s forward current
between 0% and 100%. The LED brightness is controlled by adjusting the relative ratios of
the on time to the off time. A 25% brightness level is achieved by turning the LED on at full
current for 25% of one cycle. To ensure this switching process between on and off state is
invisible by human eyes, the switching frequency must be greater than 100 Hz. Above 100 Hz,
the human eyes average the on and off times, seeing only an effective brightness that is
proportional to the LED’s on-time duty cycle. The advantage of PWM dimming is that the
forward current is always constant, therefore the LED color does not vary with brightness as it
does with analog dimming. Pulsing the current provides precise brightness control while
preserving the color purity.
Capacitor Selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor
appears in series with the supply source impedance and lowers overall efficiency. This
capacitor has to supply the relatively high peak current to the coil and smooth the current
ripple on the input supply. A minimum value of 4.7μF is acceptable if the input source is close
to the device, but higher values will improve performance at lower input voltages, especially
when the source impedance is high. The input capacitor should be placed as close as possible
to the IC.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better
dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in
this application and should NOT be used.
Inductor Selection
Recommended inductor values for the UM1361S are in the range 27uH to 100uH. Higher
values of inductance are recommended at lower output current in order to minimize errors due
to switching delays, which result in increased ripple and lower efficiency. Higher values of
inductance also result in a smaller change in output current over the supply voltage range.
The inductor should be mounted as close to the device as possible with low resistance
connections to the SW and VIN pins. The chosen coil should have a saturation current higher
than the peak output current and a continuous current rating above the required mean output
current. Following table gives the guideline on inductor selection:
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UM1361S
Load Current
Inductor
Saturation Current
IOUT>1A
27-47μH 1.3-1.5 Times of Load Current
0.8A<IOUT≤1A
33-82μH
0.4A<IOUT≤0.8A 47-100μH
IOUT≤0.4A
68-220μH
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off'
times within the specified limits over the supply voltage and load current range. The following
equations can be used as a guide.
SW Switch 'On' time
TON = (L*ΔI)/(VIN-VLED-I LED*(RS+RL+RLX))
SW Switch 'Off' time
TOFF = (L*ΔI)/(VLED+VD+ILED*(RS+RL))
Where:
L is the coil inductance (H)
RL is the coil resistance (Ω)
RS is the current sense resistance (Ω)
ILED is the required LED current (A)
ΔI is the coil peak-peak ripple current (A) {Internally set to 0.25×ILED}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RLX is the switch resistance (Ω) {=0.3Ω nominal}
VD is the diode forward voltage at the required load current (V)
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance
Schottky diode with low reverse leakage at the maximum operating voltage and temperature.
They also provide better efficiency than silicon diodes, due to a combination of lower forward
voltage and reduced recovery time.
It is important to select parts with a peak current rating above the peak coil current and a
continuous current rating higher than the maximum output load current. It is very important to
consider the reverse leakage of the diode when operating above 85°C. Excess leakage will
increase the power dissipation in the device and if close to the load may create a thermal
runaway condition.
The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will
increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to
ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed
the specified maximum value.
PCB Layout Guidelines
Careful PCB layout is critical to achieve low switching losses and stable operation. Minimize
ground noise by connecting high current ground returns, the input bypass capacitor ground lead,
and the output filter ground lead to a single point. Place Rsense as close as possible to the Isense
and VIN. For better noise immunity, a Kelvin connection is strongly recommended between
Isense and Rsense.
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UM1361S
Package Information
UM1361S SOT23-5
Outline Drawing
θ
D
b
L
Symbol
4
E
E1
5
1
2
3
e1
c
Top View
End View
A1
A2
A
e
Side View
A
A1
A2
b
c
D
E
E1
e
e1
L
θ
DIMENSIONS
MILLIMETERS
INCHES
Min Typ Max Min
Typ
Max
1.013 1.15 1.40 0.040 0.045 0.055
0.00 0.05 0.10 0.000 0.002 0.004
1.00 1.10 1.30 0.039 0.043 0.051
0.30
0.50 0.012
0.020
0.10 0.15 0.20 0.004 0.006 0.008
2.82
3.10 0.111
0.122
1.50 1.60 1.70 0.059 0.063 0.067
2.60 2.80 3.00 0.102 0.110 0.118
0.95REF
0.037REF
1.90REF
0.075REF
0.30
0.60 0.012
0.024
0°
8°
0°
8°
Land Pattern
1.20
2.35
0.56
0.95
0.95
NOTES:
1. Compound dimension: 2.92×1.60;
2. Unit: mm;
3. General tolerance ±0.05mm unless otherwise
specified;
4. The layout is just for reference.
Tape and Reel Orientation
EQXYW
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UM1361S
GREEN COMPLIANCE
Union Semiconductor is committed to environmental excellence in all aspects of its
operations including meeting or exceeding regulatory requirements with respect to the use
of hazardous substances. Numerous successful programs have been implemented to
reduce the use of hazardous substances and/or emissions.
All Union components are compliant with the RoHS directive, which helps to support
customers in their compliance with environmental directives. For more green compliance
information, please visit:
http://www.union-ic.com/index.aspx?cat_code=RoHSDeclaration
IMPORTANT NOTICE
The information in this document has been carefully reviewed and is believed to be
accurate. Nonetheless, this document is subject to change without notice. Union assumes
no responsibility for any inaccuracies that may be contained in this document, and makes
no commitment to update or to keep current the contained information, or to notify a
person or organization of any update. Union reserves the right to make changes, at any
time, in order to improve reliability, function or design and to attempt to supply the best
product possible.
Union Semiconductor, Inc
Add: Unit 606, No.570 Shengxia Road, Shanghai 201210
Tel: 021-51093966
Fax: 021-51026018
Website: www.union-ic.com
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