SNVSAH2E December 2015 – August 2020 LMR23630
PRODUCTION DATA
Refer to the PDF data sheet for device specific package drawings
Minimum ON-time, TON_MIN, is the smallest duration of time that the HS switch can be on. TON_MIN is typically 60 ns in the LMR23630. Minimum OFF-time, TOFF_MIN, is the smallest duration that the HS switch can be off. TOFF_MIN is typically 100 ns in the LMR23630. In CCM operation, TON_MIN and TOFF_MIN limit the voltage conversion range given a selected switching frequency.
The minimum duty cycle allowed is:
And the maximum duty cycle allowed is:
Given fixed TON_MIN and TOFF_MIN, the higher the switching frequency the narrower the range of the allowed duty cycle. In the LMR23630, a frequency foldback scheme is employed to extend the maximum duty cycle when TOFF_MIN is reached. The switching frequency decreases once longer duty cycle is needed under low VIN conditions. Wide range of frequency foldback allows the LMR23630 output voltage stay in regulation with a much lower supply voltage VIN. This leads to a lower effective dropout voltage.
Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size and efficiency. The maximum operation supply voltage can be found by:
At lower supply voltage, the switching frequency decreases once TOFF_MIN is tripped. The minimum VIN without frequency foldback can be approximated by:
Taking considerations of power losses in the system with heavy load operation, VIN_MAX is higher than the result calculated in Equation 5. With frequency foldback, VIN_MIN is lowered by decreased fSW.