Advanced Analog Technology, Inc.
AAT1501/A
where:
V
O
is the Monitored Output Voltage (AAT1501:5V, AAT1501A:4.5V).
V
F
is the LED Forward Voltage.
Dimming Control
1. Dimming Using PWM at SHDN Pin
A PWM signal applied to SHDN can control the LED brightness in direct dependence on the duty cycle.
The recommended PWM signal frequency is about 100Hz to ensure a good match between the input signal
duty cycle and the LED average current.
2. Dimming Using PWM by a NMOS
The additional NMOS can control the LED brightness in direct dependence on the duty cycle as shown in
application circuit for backlight dimming, The PWM signal frequency can be up to kHz level.
Thermal Shutdown
The AAT1501/A includes a thermal-limit circuit that shuts down the IC at approximately +160°C. The
device turns on after the IC cools by approximately 20°C.
Capacitor Selection
For LED applications, input ripple is more important than output ripple. Input ripple depends on the source
supply’s impedance. Adding a lowpass filter to the input further reduces input ripple. A charge pump flying
capacitor of value between 0.1 F to 1 F, should be connected from C+ to C–. The recommended
capacitor’s type is X5R or X7R ceramic capacitor.
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Advanced Analog Technology, Inc
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