UM1351
Hysteretic Buck High Brightness LED Driver with Internal Switch
UM1351Y SOT89-5
UM1351S SOT23-5
General Description
The UM1351 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 UM1351 can drive up to 8 series connected LEDs. The device operates from an input
supply between 6V and 35V and employs hysteretic control with a high side current sense resistor
to set the constant output current up to 700mA. 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 24 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 UM1351 comes in small SOT89-5 and SOT23-5 packages. 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 35V 0.5Ω NDMOS
Up to 98% Efficiency
Wide Input Voltage Range: 6V to 35V
± 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 SOT89-5 and SOT23-5 Packages
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UM1351
Pin Configurations
Top View
4
5
LX 1
5 VIN
GND 2
GND
VSET 3
UM1351Y
XX
2
1
4 ISENSE
3
XX: Week Code
UM1351Y
SOT89-5
5
1
5
ISENSE
5FA
GND
2
1
LX
M
VSET
4
3
4
2
3
VIN
M: Month Code
UM1351S
SOT23-5
Ordering Information
Part Number
Packaging Type
Marking Code
UM1351Y
SOT89-5
UM1351Y
UM1351S
SOT23-5
5FA
Shipping Qty
1000pcs/7Inch
Tape & Reel
3000pcs/7Inch
Tape & Reel
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UM1351
Pin Description
Pin Number
UM1351Y UM1351S
Symbol
1
3
LX
2
2
GND
3
1
VSET
4
5
ISENSE
5
4
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<100kHz.
Connect a capacitor from this pin to ground to
increase soft-start time.
(Default soft-start time=10μs. Additional soft-start
time is approximately 700μs/nF.)
Connect resistor RS from this pin to VIN to define nominal
average output current IOUTnom=0.1/RS.
Input voltage (6V to 35V). Decouple to ground with 10μF
or higher X7R ceramic capacitor close to device.
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 +35
V
Voltages on LX, ISENSE
-0.3 to +35
V
VSET Pin Voltage
-0.3 to +6
V
θJA
Thermal Resistance (Junction to SOT89-5
Ambient)
SOT23-5
100
TJ
Maximum Junction Temperature
+150
°C
-55 to +150
°C
+300
°C
TSTG
TL
Storage Temperature Range
Maximum Lead Temperature for Soldering 5
Seconds
°C/W
230
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
35
85
150
Unit
V
°C
°C
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UM1351
Electrical Characteristics
(VIN=16V, TA=25°C, unless otherwise noted)
Symbol
VIN
Parameter
Input Voltage Range
ILED
Output Current
IQ
Quiescent Current
without Switching
ISD
TSS
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
Soft Start Time
FLX
Operating Frequency
VSENSE
VSENSE_HYS
ISENSE
VEN
VENON
VENOFF
RLX
ILX(leak)
Test Conditions
Min
6
Typ
Max
35
Unit
V
RS=0.33Ω
RS=0.15Ω
VSET Pin Floating,
VIN=16V
303
666
mA
430
μA
VSET=GND, VIN=16V
Measured on ISENSE Pin
with Respect to VIN
30
60
μA
100
105
mV
95
VSENSE=VIN-0.1
For DC Dimming
±15
%
20
μA
0.3
2.5
V
VEN Rising
0.25
V
VEN Falling
0.2
V
ILX=700mA
0.5
Ω
5
VIN=16V, CEN=1nF
VIN=16V, L=47μH,
VOUT=9.6V (3 LEDs),
ILED=700mA
μA
700
μs
350
kHz
Recommended
Maximum Switch
1.0
Frequency
Recommended
Ton_rec
Minimum
For 4% Accuracy
500
(Note 2)
Switch ON Time
Max Duty Cycle
100
Recommended Duty
DLX
25
Cycle Range
TPD
Internal Comparator
45
(Note 2)
Propagation Delay
Over Temperature
TOTP
Protection
Temperature
TOTP_HYS
Protection Hysteresis
ILXmax
Peak Switch Current
0.75
1.0
Note 2: Parameters are not tested at production, but guaranteed by design.
FLXmax
MHz
ns
%
75
%
ns
150
°C
40
°C
A
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UM1351
Function Block Diagram
D1
VIN
LED
L1
Rs
VIN
LX
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 0.33Ω
VIN(6V-35V)
VIN
ISENSE
L1
UM1351
C1
100μH
10μF
Floating
VSET
LX
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UM1351
Typical Operating Characteristics
Efficiency
vs. V
Efficiency vs. VinIN
(Rs=0.15Ω,
L1=47µH)
(Rs=0.15Ω, L=47μH)
100
100
95
95
90
90
Ef f iciency (%)
Ef f iciency (%)
Efficiency
vs. V
Efficiency vs. Vin IN
(Rs=0.33Ω,
(Rs=0.33Ω,L1=100µH)
L=100μH)
85
80
85
80
75
1LED
1LED
2LED
2LED
3LED
3LED
4LED
4LED
5LED
5LED
6LED
6LED
7LED
7LED
8LED
8LED
75
70
1LED
1LED
2LED
2LED
3LED
3LED
4LED
4LED
5LED
5LED
6LED
6LED
7LED
7LED
8LED
8LED
70
0
5
10
15
20
25
30
0
5
10
VIN (V)
Operating Frequency vs. VIN
Operating Frequency vs. Vin
(Rs=0.15Ω,
L1=47µH)
(Rs=0.15Ω, L=47μH)
20
25
30
Quiescent Current vs. VIN
Quiescent Current vs. Vin
510
800
700
1LED
1LED
2LED
2LED
3LED
3LED
4LED
4LED
5LED
5LED
6LED
6LED
7LED
8LED
7LED
600
490
8LED
Quiescent Current (μA)
Operating Frequency (kHz)
15
VIN (V)
500
400
300
470
450
430
410
200
390
100
370
350
0
0
5
10
15
20
VIN (V)
25
30
0
35
5
10
15
VIN (V)
20
25
30
35
LED LED
Current
vs. VSETDuty
Duty
Cycle
Current vs. VSET
Cycle
(VIN=16V,
3 LEDs,
Rs=0.33Ω)
(Vin=16V,
3 LEDs,100µH,
100μH, Rs=0.33Ω)
Shutdown
Current
vs. VIN
Shutdown Current
vs. Vin
40
350
35
300
250
25
ILED (mA)
Shutdown Current (μA)
30
20
200
150
15
100
10
50
5
PWM=100Hz
PWM=100Hz
0
0
0
5
10
15
20
VIN (V)
25
30
35
0
10
20
30
40
50
60
Duty Cycle (%)
70
80
90
100
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UM1351
Typical Operating Characteristics (Continued)
FeedbackVoltage
Voltage vs.vs.
Vin V
Feedback
IN
(Vin=16V, 3 LEDs, 100μH, Rs=0.33Ω)
350
140
300
120
250
100
Feedback Voltage (mV)
ILED (mA)
LED Current vs. VSET
LED Current vs. VSET
(VIN=16V,
3 LEDs,
Rs=0.33Ω)
(Vin=16V,
3 LEDs, 100µH,
100μH, Rs=0.33Ω)
200
150
100
80
60
40
H
H
20
50
0
0
0
0.5
1
1.5
VSET (V)
2
2.5
0
3
5
10
15
20
25
30
VIN (V)
LED Current Change vs. Temperature
LED Current Change vs. Temperature
(L1=47µH,
Rs=0.33Ω)
(L= 47μH, Rs=0.33
Ω)
LEDLED
Current
vs. Temperature
Current vs. Temperature
(L1=47µH,
Rs=0.33Ω)
(L=47μH, Rs=0.33Ω)
20
350
Deviation f rom Nominal Set Value (%)
340
330
320
ILED (mA)
LL
310
300
290
280
15
10
5
0
-5
-10
270
260
VIN=16V, 3 LEDs
250
-15
VINVin=16V,
=16V, 3 3LEDs
LEDs
-20
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130
Temperature (°C)
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130
Temperature (°C)
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UM1351
Typical Operating Characteristics (Continued)
Steady State Waveforms
(3 LEDs, 100μH, VIN=16V, Rs=0.33Ω)
Start up Waveforms
5V/div
VSET
LX
2V/div
ILED
ILED
100mA/div
1μs/div
Dimming Waveforms
(PWM=50%, 3 LEDs, 100μH,
VIN=16V, Rs=0.33Ω)
200mA/div
5μs/div
100% Duty Cycle Mode
(3 LEDs, 100μH, VIN=10V, Rs=0.33Ω)
5V/div
PWM
5V/div
LX
100mA/div
ILED
ILED
500ns/div
100mA/div
1ms/div
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UM1351
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 [for RS≥0.1Ω]
This equation is valid when VSET pin is floating or applied with a voltage higher than 2.5V
(must be less than 5V). Actually, RS 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 (VEN), 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-35V)
VIN
ISENSE
L1
UM1351
C1
68μH
10μF
VSET
LX
The average output current is given by:
IOUT=(0.1*VEN)/(2.5*Rs) [for 0.3V<VEN<2.5V]
Note that 100% brightness setting corresponds to:
(2.5V≤VEN≤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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UM1351
D1
Rs 0.13Ω
LED
3W
VIN(6V-35V)
VIN
C1
ISENSE
L1
UM1351
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 100Hz. Above 100Hz,
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 UM1351 are in the range 47μH to 220μH. Higher
values of inductance are recommended at higher supply voltages 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 LX 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. Suitable coils for the UM1351 are listed in the table below:
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UM1351
Load Current
0.4A<IOUT≤0.7A
IOUT≤0.4A
Inductor
47-100μH
68-220μH
Saturation Current
1.3-1.5 times of load current
Order Code
L(μH)
Size
DCR (mΩ)
Saturation
Current (mA)
Manufacturer
74408943470
47
4838
305
1100
Würth Elektronik
www.we-online.com
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.
LX Switch 'On' time
TON=(L1*ΔI)/(VIN-VLED-I LED*(RS+RL+RLX))
LX Switch 'Off' time
TOFF=(L1*ΔI)/(VLED+VD+ILED*(RS+RL))
Where:
L1 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 0.3×ILED}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RLX is the switch resistance (Ω) {=0.4Ω 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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UM1351
Package Information
UM1351Y SOT89-5
0.
20
0
Outline Drawing
DIMENSIONS
c
L
R
D1
E
Symbol
L
Pin #1 ID
b
b1
e
Top View
End View
A
8°(4X)
MILLIMETERS
A
Typ
1.50
Max
1.60
Min
0.055
Typ
0.059
Max
0.063
b
0.32
-
0.54
0.013
-
0.021
b1
0.38
-
0.62
0.015
-
0.024
c
0.35
-
0.44
0.014
-
0.017
D
4.40
4.50
4.60
0.173
0.177
0.181
D1
1.40
-
1.83
0.055
-
0.072
E
2.30
2.50
2.60
0.091
0.098
0.102
1.20
0.026
e
D
L
Side View
INCHES
Min
1.40
1.50TYP
0.65
-
0.059TYP
-
0.047
Land Pattern
4.50
1.60
1.60
4.55
5.35
3.10
1.80
0.75
0.60
1.50
NOTES:
1. Compound dimension: 4.50×2.50;
2. Unit: mm;
3. General tolerance ±0.05mm unless otherwise
specified;
4. The layout is just for reference.
Tape and Reel Orientation
UM1351Y
XX
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UM1351
UM1351S 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
2.35
0.56
1.20
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.
0.95
0.95
5FA
M
Tape and Reel Orientation
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UM1351
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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