米6体育平台手机版

ZHCSUE4 August 2024 LMR51606-Q1 , LMR51610-Q1

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- MPDS026Q

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7.3.4 Minimum ON Time, Minimum OFF Time, and Frequency Foldback

The minimum ON time (t_{ON_MIN}) is the shortest duration of time that the high-side switch can be turned on. t_{ON_MIN} is typically 80ns for the LMR516xx-Q1. The minimum OFF time (t_{OFF_MIN}) is the shortest duration of time that the high-side switch can be off. t_{OFF_MIN} is typically 200ns. 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 2.

D_{M I N} = t_{O N_M I N} \times f_{S W}

The maximum duty cycle without frequency foldback allowed is:

Equation 3.

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 4.

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

The minimum V_IN without frequency foldback can be calculated by:

Equation 5.

V_{I N_M I N} = \frac{V_{O U T}}{{1 - f}_{S W} \times t_{O F F_M I N}}

In the LMR516xx-Q1, a frequency foldback scheme is employed after the 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 4.

The frequency foldback scheme also works after larger duty cycle is needed under a low V_IN condition. The frequency decreases after the device hits t_{OFF_MIN}, which extends the maximum duty cycle according to Equation 5. In such condition, the frequency can be as low as approximately 200kHz. A wide range of frequency foldback allows for the LMR516xx-Q1 output voltage to stay in regulation with a much lower supply voltage V_IN, which leads to a lower effective dropout.

With frequency foldback while maintaining a regulated output voltage, V_{IN_MAX} is raised and V_{IN_MIN} is lowered by decreased f_SW.