RT4539A

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is a registered trademark of Richtek Technology Corporation

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DS4539A-01

December 2022

16

PWM Mode

The ON/OFF of the current source is synchronized to the PWM signal. The frequency of LED current is equal to the

PWM input signal.

0

time

PWMI

time

ILED

ILED Max

PWMO

Duty

Figure 1. PWM Dimming

DC Mode

The LED current will have two-cycle delay in this mode, while the delay cycles are for average current calculation.

time

0

time

PWMI

ILED

ILED Max

50%

25%

75%

25%

75%

50%

50%

10%

Figure 2. DC Dimming

Mixed Mode

The ILED will delay for two periods to response. The First cycle delay is required for period, while the second cycle

delay is for the duty rate calculation.



When 25% ≤ PWM duty ≤ 100%, the current source outputs are DC dimming, and the PWM duty cycle modulates

the amplitude of the currents in 100% dimming.



PWM duty < 25%, the DC dimming will translate to PWM dimming, controlling the PWM duty instead by

amplitude. The LED current is fixed on quarter of LED current setting.