Thermally enhanced Low V
FB
Step-Down LED Driver
LED PWM Dimming
ADT6780
The LED brightness can be controlled by applying a pulse-width modulation(PWM) signal to the EN pin.
PWM frequency is limited by turn-on and turn-off time of the LED current. So, Using a PWM dimming
application, soft-start time control capacitor, C4 is not used for higher PWM dimming frequency. PWM
frequency is recommended in range of 100Hz to 1kHz to get a good dimming linearity.
APPLICATION INFORMATION
Figure 1 is the typical ADT6780 application circuit. And Figure 2 is the functional block diagram of the
ADT6780. For the application information, refer to the Figure 1 & 2 unless otherwise noted.
LED Current Resistor Selection
The LED current is set with a current sense resistor R1 between FB and GND. It is recommended to use 1%
tolerance or better resistor. LED current is calculated by the below equation.
I
LED
=
For 1A LED current, choose R1 = 0.2Ω
Inductor
0.2V
R1
The inductor required to supply constant current to the output load when it is driven by a switching voltage.
For given input and output voltage, inductance and switching frequency together decide the inductor ripple
current, that is:
ΔI
L
=
The peak inductor current is:
V
OUT
⎛
V
OUT
⎞
⎟
× ⎜
1
−
F
SW
×
L
⎜
V
IN
⎟
⎠
⎝
I
L.peak
=
I
OUT
+
ΔI
L
2
Higher inductance gives low inductor ripple current but requires larger size inductor to avoid saturation.
Low ripple current reduces inductor core losses. Also it reduces RMS current through inductor and switches,
which results in less conduction loss. Usually, peak to peak ripple current on inductor is designed to be 20%
to 30% of the output current limit. Make sure it is capable to handle the peak current without saturation.
Surface mount inductors in different shape and styles are available from TDK, TOKO and Murata. Shielded
inductors are small and radiate less EMI noise. But they cost more than unshielded inductors. The choice
depends on EMI requirement, price and size.
Output Freewheeling Diode
When the high side switch is off, freewheeling diode supplies the current to the inductor. The forward
voltage and reverse recovery times of the freewheeling diode are the key loss factors, so schottky diode is
mostly used for the freewheeling diode. Choose a diode whose maximum reverse voltage rating is greater
than the maximum input voltage, and whose current rating is greater than the maximum load current.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
8/13
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