AAT2405 PWM Control Duty Cycle Capabilities and Limitations
●Purpose:
■To demonstrate the minimum and maximum PWM duty cycle capabilities of the AAT2405's constant current sinks.
●Background:
■LED string dimming is achieved by applying an external PWM signal to the AAT2405's PWMIN input pin. The AAT2405 has an internal clock that self-synchronizes to the PWM dimming input signal. This function enables a synchronized feed-through of the PWM dimming control to the PWMOUT output pin so a common PWM signal can be used to drive as many cascaded AAT2405 backlight controllers as needed for a system. The period of the PWM signal control is based on the period of the input signal with in a 50Hz to 20kHz range.
■The AAT2405 is capable of a wide PWM dimming signal duty cycle range. The maximum duty cycle is 100% PWM at which point the current sinks will be fully turned on to the current level set by the RSET resistor. The minimum duty cycle is bounded by the collective turn-on and turn-off slew rate of the PWM control signal along with the current sink and control circuit. The minimum slew rate boundary on/off time is typically 20μs, but this value can vary depending upon the gate charge specification of the external current sink MOSFET selected for use with the AAT2405. A typical 20μs on/off slew rate time will yield a minimum PWM duty cycle of 0.24% for a 120Hz control signal or 0.48% for a 240Hz control signal.
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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September 24, 2012 |
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202379A |
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1.2 MB |
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