UM1351_16 Datasheet, PDF (9/14 Pages) Union Semiconductor, Inc. – Hysteretic Buck High Brightness LED Driver with Internal Switch

UM1351
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 (R_S) connected between VIN and ISENSE and is given by:
I_OUTnom=0.1/R_S [for R_S≥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, 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 (V_EN), as shown, to adjust the output
current to a value below the nominal average value defined by Rs.
D1
VIN(6V-35V)
C1
10μF
Rs 0.13Ω
VIN
ISENSE
LED
3W
UM1351
L1
68μH
VSET
LX
The average output current is given by:
I_OUT=(0.1*V_EN)/(2.5*Rs) [for 0.3V<V_EN<2.5V]
Note that 100% brightness setting corresponds to:
(2.5V≤V_EN≤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:
I_OUT=(0.1*D)/R_S (0≤D≤100%, 2.5V<V_pulse<5V)
I_OUT=(V_pulse*0.1*D)/(2.5*R_S) (0≤D≤100%, 0.5V<V_pulse<2.5V)
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http://www.union-ic.com Rev.04 Apr.2016
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