8.3.4 Minimum ON-Time, Minimum OFF-Time and Frequency Foldback

Minimum ON-time, T_{ON_MIN}, is the smallest duration of time that the HS switch can be on. T_{ON_MIN} is typically 60 ns in the TPS560430. Minimum OFF-time, T_{OFF_MIN}, is the smallest duration that the HS switch can be off. T_{OFF_MIN} is typically 100 ns. In CCM operation, T_{ON_MIN} and T_{OFF_MIN} limit the voltage conversion range without switching frequency foldback.

The minimum duty cycle without frequency foldback allowed is

The maximum duty cycle without frequency foldback allowed is

Given a required output voltage, the maximum V_IN without frequency foldback can be found by

Equation 4.

The minimum V_IN without frequency foldback can be calculated by

Equation 5.

In the TPS560430, a frequency foldback scheme is employed once the T_{ON_MIN} or T_{OFF_MIN} is triggered, which may extend the maximum duty cycle or lower the minimum duty cycle.

The on-time decreases while V_IN voltage increases. Once the on-time decreases to T_{ON_MIN}, the switching frequency starts to decrease while V_IN continues to go up, which lowers the duty cycle further to keep V_OUT in regulation according to Equation 2.

The frequency foldback scheme also works once larger duty cycle is needed under low V_IN condition. The frequency decreases once the device hits its T_{OFF_MIN}, which extends the maximum duty cycle according to Equation 3. In such condition, the frequency can be as low as about 133 kHz minimum. Wide range of frequency foldback allows the TPS560430 output voltage stay in regulation with a much lower supply voltage V_IN, which leads to a lower effective drop-out.

With frequency foldback, V_{IN_MAX} is raised, and V_{IN_MIN} is lowered by decreased f_SW.

1.

VOUT = 1 V

fSW = 1.1 MHz

Figure 16. Frequency Foldback at T_{ON_MIN}

1.

VOUT = 5 V

fSW = 1.1 MHz

Figure 17. Frequency Foldback at T_{OFF_MIN}