UM1350
350mA, LED Lighting Driver
UM1350 SOT23-5
General Description
The UM1350 is capable of driving single or multiple series connected LEDs efficiently from a
voltage source higher than the LED voltage. This step-down converter provides an externally
adjustable output current of up to 350mA from an input supply between 6V and 30V. It can even
reach 8 watts of output power, depending on supply voltage and external components. The
UM1350 consists of an output switch and a high-side output current sensing circuit that uses an
external resistor to set the nominal average output current. Through applying an external control
signal to the VSET pin, it can adjust the output current below the set value. The VSET pin will
accept either a DC voltage or a PWM waveform to provide a continuous or a gated output current.
The chip contains a PWM filter which provides a soft-start feature by controlling the rise of
input/output current. It can raise the soft-start by using an external capacitor from the VSET pin to
ground. 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 UM1350 is available in a low profile SOT23-5 package.
Applications
Features
MR16 and General Lighting
Automotive Lighting
Low Voltage Industrial Lighting
LED Back Lighting
Illuminated Signs
Pin Configurations
Built in Thermal and Over Current Shut down
Internal 40V NDMOS Switch
350mA Output Current
Single Pin on/off and Dimming Control Using
DC Voltage or PWM at VSET Pin.
Internal PWM Filter
Soft-Start
High Efficiency (up to 95%)
Wide Input Voltage Range: 6V to 40V
40V Transient Capability
Output Shutdown
Up to 1MHz Switching Frequency
Inherent Open-Circuit LED Protection
Typical 5% Output Current Accuracy
Top View
M: Month Code
UM1350
SOT23-5
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UM1350
Ordering Information
Part Number
Packaging Type
Marking Code
UM1350
SOT23-5
5AS
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.(V VSET= 2VREF giving
nominal average output current IOUTnom=100mV/RS)
• Drive to voltage below 0.2V to turn off output current
• Drive with DC voltage (0.3V<VVSET<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 I OUTnom for f<500Hz
• Connect a capacitor from this pin to ground to increase soft-start
time. (Default soft-start time=20us. Additional soft-start time is
approx.0.6ms/nF)
Connect resistor RS from this pin to VIN to define nominal average
output current (IOUTnom=100mV/RS)
Input Voltage
Absolute Maximum Ratings (Note 1)
Symbol
Parameter
Value
Unit
VIN
Input Voltage
-0.3 to +40
V
VSENSE
ISENSE Voltage
+0.3 to -5(Note2)
V
LX Output Voltage
-0.3 to +40
V
Adjust Pin Input Voltage
-0.3 to +6
V
VLX
VVSET
ILX
Switch Output Current
500
mA
PD
Power Dissipation
450
mW
TJ
Operating Junction Temperature
-40 to +120
°C
TSTG
Storage Temperature Range
-55 to +150
°C
Note 1: These are stress ratings only. Operation above the absolute maximum rating may cause
device failure. Operation at the absolute maximum ratings, for extended periods, may
reduce device reliability.
Note 2: This voltage is measured with respect to VIN.
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UM1350
Electrical Characteristics
(VIN=12V, TA=25℃, unless otherwise noted.)
Symbol
Parameter
Test Conditions
VIN
Input Voltage Range
Internal Regulator
VSU
VIN Rising
Start-up Threshold
IINQoff
IINQon
VSENSE
VSENSEHYS
ISENSE
VREF
VVSET
VVSEToff
VVSETon
ILXmean
RLX
ILX(leak)
Quiescent Supply
Current with Output off
Quiescent Supply
Current with Output
Switching
Mean Current Sense
Threshold Voltage
(define LED current
setting accuracy)
Sense Threshold
Hysteresis
ISENSE pin input current
Internal Reference
Voltage
External Control
Voltage Range on
VSET pin for DC
Brightness Control
(Note 3)
DC voltage on VSET
pin to switch device
from active (on) state to
quiescent (off) state
DC voltage on VSET
pin to switch device
from quiescent (off)
state to
active (on) state
Continuous LX switch
current
LX Switch ‘On’
resistance
LX switch leakage
current
Min
6
Typ
Max
40
Unit
V
4.5
V
VSET Pin Grounded
40
μA
VSET Pin Floating
f=250kHz
220
μA
Measured on ISENSE Pin
with respect to VIN
VVSET=1.25V
95
100
105
mV
16
20
μA
1.25
1.30
V
2.5
V
±15%
VSENSE = VIN – 0.1
Measured on VSET Pin
with Pin Floating
1.20
0.3
VVSET falling
0.15
0.20
0.25
V
VVSET rising
0.20
0.25
0.30
V
0.37
A
1.8
Ω
1
μA
0.95
Note 3: 100% brightness corresponds to VVSET=2.5V. Driving the VSET pin above 2.5V
will not increase the output current proportionally.
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UM1350
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 0.33Ω
VIN(12V-30V)
VIN
ISENSE
L1
UM1350
C1
100μH
1μF
Floating
VSET
LX
GND
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UM1350
Device description
The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a
self-oscillating continuous-mode buck converter.
Device operation (Refer to block diagram (page 4) and Figure 1 (page 6) - Operating waveforms )
Operation can be best understood by assuming that the VSET pin of the device is unconnected and
the voltage on this pin (VVSET) appears directly at the (+) input of the comparator.
When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by V IN
and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is forced
across internal resistor R1 by the current sense circuit and produces a proportional current in
internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of the
comparator. When this reaches the threshold voltage (V VSET), the comparator output switches low
and MN turns off. The comparator output also drives another NMOS switch, which bypasses
internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be
nominally 15% of VVSET.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The
current decays at a rate determined by the LED and diode forward voltages to produce a falling
voltage at the input of the comparator. When this voltage returns to V VSET, the comparator output
switches high again. This cycle of events repeats, with the comparator input ramping between
limits of VVSET ±15%.
Switching thresholds
With VVSET=VREF, the ratios of R1, R2 and R3, define an average VSENSE switching threshold of
100mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is then
defined by this voltage and Rs according to:
IOUTnom=100mV/RS
Nominal ripple current is ±22mV/RS
Adjusting output current
The VSET pin can be overdriven with either DC or pulse signals to change the VSENSE switching
threshold and adjust the output current.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The output of the low pass filter drives the shutdown circuit. When the input voltage to this
circuit falls below the threshold (0.2V nom), the internal regulator and the output switch are
turned off. The voltage reference remains powered during shutdown to provide the bias
current for the shutdown circuit. Quiescent supply current during shutdown is nominally 40μA
and switch leakage is below 1μA.
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UM1350
VVSET
VVSET
VVSET
Figure 1 Operating Waveforms
Typical Operating Waveforms
(Vin=12V, L=100μH, Rs=0.33Ω, 3LED)
Normal Operation: Output Current and LX Voltage
Io 100mA/div
LX 5V/div
0V
0mA
1μs/div
s/div
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UM1350
Typical Operating Characteristics
(VIN=12V, TA=25℃, unless otherwise noted)
Efficiency vs.Vin (Rs=0.33Ω,L=100uH)
100
90
90
Efficiency (%)
Efficiency (%)
Efficiency vs.Vin (Rs=0.33Ω,L=47uH)
100
80
70
1LED
2LED
3LED
80
70
4LED
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
60
60
5LED
6LED
7LED
8LED
50
50
0
5
10
15
20
25
0
30
5
10
15
25
30
Duty Cycle vs. Vin (Rs=0.33Ω,L=47uH)
Efficiency vs.Vin (Rs=0.33Ω,L=220uH)
100
100.0
90
80.0
Duty Cycle(%)
Efficiency (%)
20
Vin (V)
Vin (V)
80
60.0
40.0
70
1LED
2LED
3LED
4LED
20.0
60
5LED
6LED
7LED
8LED
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
0.0
50
0
5
10
15
20
25
5
30
10
15
20
25
30
Vin(V)
Vin (V)
Duty Cycle vs. Vin (Rs=0.33Ω,L=220uH)
Duty Cycle vs. Vin (Rs=0.33Ω,L=100uH)
100.0
100.0
80.0
Duty Cycle(%)
Duty Cycle(%)
80.0
60.0
60.0
40.0
40.0
20.0
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
20.0
5
5
10
15
Vin(V)
20
25
30
2LED
3LED
4LED
5LED
6LED
7LED
0.0
0.0
1LED
8LED
10
15
20
25
Vin(V)
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UM1350
Typical Operating Characteristics (Continued)
(VIN=12V, TA=25℃, unless otherwise noted)
Operating Frequency vs. Vin (Rs=0.33Ω,L=47uH)
Operating Frequency vs. Vin (Rs=0.33Ω,L=100uH)
900
1400
1300
1LED
2LED
1200
3LED
4LED
1100
5LED
6LED
7LED
8LED
700
900
Frequency(KHz)
Frequency(KHz)
1000
800
800
700
600
500
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
600
500
400
300
400
200
300
200
100
100
0
0
0
5
10
15
20
25
0
30
5
10
15
20
25
30
Vin(V)
Vin(V)
Operating Frequency vs. Vin (Rs=0.33Ω,L=220uH)
Output current variation vs. Vin (Rs=0.33Ω,L=47uH)
500
2LED
3LED
4LED
5LED
6LED
7LED
8LED
12
Deviation from nominal set current (%)
Frequency(KHz)
400
1LED
300
200
100
10
8
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
6
4
2
0
-2
0
0
5
10
15
20
25
-4
30
5
Vin(V)
15
20
25
30
Vin(V)
Output current variation vs. Vin (Rs=0.33Ω,L=220uH)
Output current variation vs. Vin (Rs=0.33Ω,L=100uH)
8
8
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
Deviation from nominal set current (%)
10
Deviation from nominal set current (%)
10
6
4
2
6
1LED
2LED
3LED
4LED
5LED
6LED
7LED
8LED
4
2
0
0
-2
-2
5
10
15
20
Vin(V)
25
30
5
10
15
20
25
Vin(V)
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UM1350
Typical Operating Characteristics (Continued)
(VIN=12V, TA=25℃, unless otherwise noted)
Supply Current vs. Vin (Operating)
Supply Current vs. Vin (Quiescent)
400
100
300
Iq (uA)
Iq (uA)
80
60
200
40
100
20
0
0
0
5
10
15
20
25
30
0
5
10
15
Vin(V)
700
Deviation from nominal set value
Rs=0.56ohm
Rs=1ohm
Iout mean (mA)
30
2
Rs=0.33ohm
500
400
300
200
100
1
0
-1
-2
-3
-4
-5
-6
0
-7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-50
-30
-10
10
VADJ(V)
30
50
70
90
110
130
150
130
150
Temperature (℃)
Output Current Change vs. Temperature
(Vin=30V, L=100uH, Rs=0.33Ω)
Output Current Change vs. Temperature
(Vin=12V, L=100uH, Rs=0.33Ω)
10
3
2
Deviation from nominal set value
Deviation from nominal set value
25
Output Current Change vs. Temperature
(Vin=7V, L=100uH, Rs=0.33Ω )
Output Current vs. VADJ (Vin=12V,L=100uH,3LED)
600
20
Vin(V)
1
0
-1
-2
-3
-4
8
6
4
2
0
-5
-6
-2
-50
-30
-10
10
30
50
70
Temperature (℃)
90
110
130
150
-50
-30
-10
10
30
50
70
90
110
Temperature (℃)
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UM1350
Typical Operating Characteristics (Continued)
(VIN=12V, TA=25℃, unless otherwise noted)
LX Switch 'On' Resistance vs. Temperature
2.2
2.0
Ohms
1.8
1.6
1.4
1.2
1.0
-50
-30
-10
10
30
50
70
90
110
130
150
Temperature (℃)
Applications Information
Setting nominal average output current with external resistor R S
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:
I OUTnom = 0.1/ RS (Rs ≥ 0.27Ω)
The table below gives values of nominal average output current for several preferred values of
current setting resistor (RS) in the typical application circuit shown on page 5:
RS ( Ω)
Nominal average output current (mA)
0.27
0.3
0.33
0.39
370
333
300
256
The above values assume that the VSET pin is floating and at a nominal voltage of V REF. Note
that RS=0.27Ω is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value.
Output current adjustment by external DC control voltage
The VSET pin can be driven by an external dc voltage, as shown, to adjust the output current
to a value below the nominal average value defined by Rs.
The average output current is given by:
IOUT = (0.1*VVSET/2.5)/RS [for 0.3V< VVSET <2.5V]
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UM1350
D1
Rs 0.33Ω
LED
1W
VIN(6V-30V)
VIN
ISENSE
L1
UM1350
C1
100μH
1μF
VSET
LX
GND
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 = (Vpulse *0.1*D)/（2.5*RS） (0≤D≤100%, 0.3V＜Vpulse＜2.5V)
D1
Rs 0.33Ω
LED
1W
VIN(6V-30V)
VIN
C1
ISENSE
L1
UM1350
100μH
1μF
VSET
LX
GND
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 of (Vpulse *0.1/ RS) 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
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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 1μF 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 UM1350 are in the range 47uH to 220uH. 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 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.
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 –ILED *( 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 0.3 × ILED }
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RLX is the switch resistance (Ω)
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.
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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 Rs as close as possible to the Isense and
VIN. For better noise immunity, a Kelvin connection is strongly recommended between Isense and
Rs.
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Package Information
UM1350 SOT23-5
Outline Drawing
D
b
θ
Symbol
5
E
E1
4
2
3
L
1
e
c
0.2
e1
A1
A
End View
A2
Top View
Side View
A
A1
A2
b
c
D
E
E1
e
e1
L
θ
DIMENSIONS
MILLIMETERS
Min
Max
1.050
1.250
0.000
0.100
1.050
1.150
0.300
0.500
0.100
0.200
2.820
3.020
1.500
1.700
2.650
2.950
0.950REF
1.800
2.000
0.300
0.600
0°
8°
INCHES
Min
Max
0.041 0.049
0.000 0.004
0.041 0.045
0.012 0.020
0.004 0.008
0.111 0.119
0.059 0.067
0.104 0.116
0.037REF
0.071 0.079
0.012 0.024
0°
8°
Land Pattern
2.40
0.90
0.60
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.
0.95
5AS
M
Tape and Reel Orientation
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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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