The LM5143-Q1 oscillator is programmed by a resistor between RT and AGND to set an oscillator frequency between 100 kHz to 2.2 MHz. CLK1 is the clock for channel 1 and CLK2 is for channel 2. CLK1 and CLK2 are 180° out of phase. Use Equation 1 to calculate the RT resistance for a given switching frequency.

Equation 1.

Under low V_IN conditions when either of the on-time of the high-side MOSFETs exceeds the programmed oscillator period, the LM5143-Q1 extends the switching period of that channel until the PWM latch is reset by the current sense ramp exceeding the controller compensation voltage. In such an event, the oscillators (CLK1 and CLK2) operate independently and asynchronously until both channels can maintain output regulation at the programmed frequency.

The approximate input voltage level where this occurs is given by Equation 2.

Equation 2.

where

• where t_SW is the switching period
• t_OFF(min) is the minimum off-time of 60 ns