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
MR16 and General Lighting
Automotive Lighting
Low Voltage Industrial Lighting
LED Back Lighting
Illuminated Signs
Features
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
Pin Configurations
Top View
M: Month Code
UM1350
SOT23-5
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UM1350
Ordering Information
Part Number
UM1350
Packaging Type
SOT23-5
Marking Code
5AS
Shipping Qty
3000pcs/7Inch
Tape & Reel
Pin Description
Pin
Number
1
2
Symbol
LX
GND
Drain of NDMOS switch.
Ground (0V).
Multi-function On/Off and brightness control pin:
• Leave floating for normal operation.(V
VSET
= 2V
REF
giving
nominal average output current I
OUTnom
=100mV/R
S
)
• Drive to voltage below 0.2V to turn off output current
• Drive with DC voltage (0.3V<V
VSET
<2.5V) to adjust output
current from 12% to 100% of I
OUTnom
• 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 R
S
from this pin to V
IN
to define nominal average
output current (I
OUTnom
=100mV/R
S
)
Input Voltage
Function
3
VSET
4
5
I
SENSE
V
IN
Absolute Maximum Ratings (Note 1)
Symbol
V
IN
V
SENSE
V
LX
V
VSET
I
LX
P
D
T
J
Input Voltage
I
SENSE
Voltage
LX Output Voltage
Adjust Pin Input Voltage
Switch Output Current
Power Dissipation
Operating Junction Temperature
Parameter
Value
-0.3 to +40
+0.3 to -5(Note2)
-0.3 to +40
-0.3 to +6
500
450
-40 to +120
Unit
V
V
V
V
mA
mW
°
C
T
STG
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 V
IN
.
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UM1350
Electrical Characteristics
(V
IN
=12V, T
A
=25℃, unless otherwise noted.)
Symbol
Parameter
Test Conditions
V
IN
Input Voltage Range
Internal Regulator
V
SU
V
IN
Rising
Start-up Threshold
I
INQoff
I
INQon
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
I
SENSE
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
VSET Pin Grounded
VSET Pin Floating
f=250kHz
Measured on I
SENSE
Pin
with respect to V
IN
V
VSET
=1.25V
Min
6
Typ
4.5
40
220
Max
40
Unit
V
V
μA
μA
V
SENSE
95
100
105
mV
V
SENSEHYS
I
SENSE
V
REF
±
15%
V
SENSE
= V
IN
– 0.1
Measured on VSET Pin
with Pin Floating
16
1.20
1.25
20
1.30
μA
V
V
VSET
0.3
2.5
V
V
VSEToff
V
VSET
falling
0.15
0.20
0.25
V
V
VSETon
V
VSET
rising
0.20
0.25
0.30
V
I
LXmean
R
LX
I
LX(leak)
0.37
0.95
1.8
1
A
Ω
μA
Note 3: 100% brightness corresponds to V
VSET
=2.5V. Driving the VSET pin above 2.5V
will not increase the output current proportionally.
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UM1350
Block Diagram
VIN
D1
LED
Rs
LX
VIN
ISENSE
L1
GM
C1
Regulator
BG
VSET
Ref
R2
Low Voltage
Detector
OTP
VSET
EN
Logic
&
Driver
Internal
Enable
GND
Ilimit
R1
Typical Application Circuit
D1
Rs
0.33Ω
V
IN
(12V-30V)
V
IN
I
SENSE
C1
1μF
Floating
UM1350
VSET
GND
LX
L1
100μH
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UM1350
Device description
The device, in conjunction with the coil (L1) and current sense resistor (R
S
), 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 (V
VSET
) appears directly at the (+) input of the comparator.
When input voltage V
IN
is first applied, the initial current in L1 and R
S
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 V
IN
to ground, via R
S
, L1 and the LED(s). The current rises at a rate determined by V
IN
and L1 to produce a voltage ramp (V
SENSE
) across R
S
. The supply referred voltage V
SENSE
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 V
VSET
.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to V
IN
. 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 V
VSET
±15%.
Switching thresholds
With V
VSET
=V
REF
, the ratios of R1, R2 and R3, define an average V
SENSE
switching threshold of
100mV (measured on the I
SENSE
pin with respect to V
IN
). The average output current I
OUTnom
is then
defined by this voltage and Rs according to:
I
OUTnom
=100mV/R
S
Nominal ripple current is ±
22mV/R
S
Adjusting output current
The VSET pin can be overdriven with either DC or pulse signals to change the V
SENSE
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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