ISL81601 datasheet(34/54 Pages) RENESAS

34 / 54 page

FN9299 Rev.3.1

Page 34 of 53

May 27, 2021

ISL81601

5. Functional Description

Figure 49. Boost Mode Equivalent Circuit and Operation Waveforms

In Boost mode, the Q3 duty cycle is given by:

DQ3 = (1 - VIN / VOUT) x 100%

As VIN increases close to VOUT, DQ3 decreases close to its minimum value decided by its minimum on-time.

When DQ3 reaches its minimum value, the converter moves to Buck-Boost mode.

When VIN is much lower than VOUT, DQ3 increases close to its maximum duty cycle decided by its minimum

off-time. To allow stable loop operation and avoid duty cycle jitter, Renesas recommends keeping the Q3

off-time always two to three times higher than the minimum off-time.

5.6.3

Buck-Boost Mode Operation (VIN ≈ VOUT)

In Buck-Boost mode, the converter runs in one cycle of Buck mode followed by one cycle of Boost mode

operation mode. It takes two clock cycles to finish a full buck-boost period.

When VIN is higher than VOUT, Q3 runs in minimum duty in the Boost mode cycle. Q1 duty cycle DQ1 is

modulated in the buck cycle to keep VOUT in regulation. As VIN increases, DQ1 decreases. When DQ1 decreases

to less than 66.7% of the clock period, the converter moves to Buck mode.

When VIN is lower than VOUT, Q1 runs in maximum duty in the Buck mode cycle. Q3 duty cycle DQ3 is

modulated in the Boost mode cycle to keep VOUT in regulation. As VIN decreases, DQ3 increases. When DQ3

increases to more than 33.3% of the clock period, the converter moves to Boost mode.

VIN

VOUT

RS_IN

RS_OUT

UG2

LG2

PH1

PH2

CLOCK

UG1

LG1

LG2

UG2

ISL81601 Scheda tecnica(PDF) 34 Page - Renesas Technology Corp