ISL81601 datasheet(33/54 Pages) RENESAS

33 / 54 page

FN9299 Rev.3.1

Page 33 of 53

May 27, 2021

ISL81601

5. Functional Description

In Buck mode, the Q1 duty cycle is given by:

DQ1 = VOUT / VIN x 100%

As VIN decreases to close to VOUT, DQ1 increases to close to its maximum value decided by its minimum

off-time. When DQ1 reaches its maximum value, the converter moves to Buck-Boost mode.

When VIN is much higher than VOUT, DQ1 decreases to close to its minimum duty cycle decided by its

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

Q1 on-time always two to three times higher than the minimum on-time.

5.6.2

Boost Mode Operation (VIN < VOUT)

In Boost mode, the converter Q1 is always on and Q2 is always off unless boot refresh or inductor negative

peak current limit is tripped. Q3 and Q4 run in a normal valley current controlled sync boost operation mode.

Q3 turns off by the clock. During Q3 off-time, op amp A1 senses the inductor current by the voltage on RS_IN.

Q3 turns on when the sensed signal combined with the slope compensation ramp is lower than the COMP pin

voltage which is the error signal from the upper voltage or current regulator. The equivalent circuit and

operation waveforms are shown in Figure 49 on page 34.

Figure 48. Buck Mode Equivalent Circuit and Operation Waveforms

CLOCK

UG1

LG1

LG2

UG2

VIN

VOUT

RS_IN

RS_OUT

UG1

LG1

PH1

PH2

ISL81601 Datenblatt(PDF) 33 Page - Renesas Technology Corp