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ZHCSOE3E August 2021 – September 2023 LMR38020

PRODUCTION DATA

8.3.6 Minimum On Time, Minimum Off Time, and Frequency Foldback

Minimum on time (T_{ON_MIN}) is the smallest duration of time that the high-side switch can be on. T_{ON_MIN} is typically 75 ns in the LMR38020. Minimum off time (T_{OFF_MIN}) is the smallest duration that the high-side switch can be off. T_{OFF_MIN} is typically 190 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:

Equation 3.

D_{M I N} = T_{O N - M I N} \times f_{S W}

The maximum duty cycle without frequency foldback allowed is:

Equation 4.

D_{M A X} = 1 - (T_{O F F - M I N} \times f_{S W})

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

Equation 5.

V_{I N_M A X} = \frac{V_{O U T}}{T_{O N - M I N} \times f_{S W}}

The minimum V_IN without frequency foldback can be calculated by:

Equation 6.

V_{I N_M I N} = \frac{V_{O U T}}{1 - {(T}_{O F F - M I N} \times f_{S W})}

In the LMR38020, a frequency foldback scheme is employed after T_{ON_MIN} or T_{OFF_MIN} is triggered, which can extend the maximum duty cycle or lower the minimum duty cycle.

The on time decreases while V_IN voltage increases. After 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 3.

The frequency foldback scheme also works after larger duty cycle is needed under low V_IN condition. The frequency decreases after the device hits its T_{OFF_MIN}, which extends the maximum duty cycle according to Equation 4. In such condition, the frequency can be as low as approximately 133 kHz minimum. Wide range of frequency foldback allows the LMR38020 output voltage stay in regulation with a much lower supply voltage V_IN, which leads to a lower effective dropout.

The fixed frequency operation at FPWM mode with switching frequency greater than 1.2 MHz, is maintained with minimum load current about 100 mA for wider input voltage range. And for very light load and switching frequency over 1.2 MHz, frequency drop can be expected during mintoff frequency foldback.

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

F_SW = 1.1 MHz

V_OUT = 5 V

I_OUT = 2 A

Figure 8-8 Typical Frequency Foldback at T_{ON_MIN}

F_SW = 1.1 MHz

V_OUT = 5 V

I_OUT = 2 A

Figure 8-9 Typical Frequency Foldback at T_{OFF_MIN}