UM1361S Datasheet, PDF (10/13 Pages) Union Semiconductor, Inc. – Hysteretic Buck High Brightness LED Driver with Internal Switch

VIN(6V-40V)
Ci
10μF
UM1361S
D1
Rs 0.13Ω
VIN
ISENSE
LED
3W
UM1361S
L1
68μH
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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http://www.union-ic.com Rev.04 Mar.2014
10/13