ISL81601 datasheet(32/54 Pages) RENESAS

32 / 54 page

FN9299 Rev.3.1

Page 32 of 53

May 27, 2021

ISL81601

5. Functional Description

5.6

Buck-Boost Conversion Topology and Control Algorithm

The ISL81601 uses the Renesas proprietary buck-boost control algorithm to achieve optimized power conversion

performance. The buck-boost topology is shown in Figure 46. The ISL81601 controls the four power switches Q1,

Q2, Q3, and Q4 to work in either Buck or Boost mode. When VIN is far lower than VOUT, the converter works in

Boost mode. When VIN is far higher than VOUT, the converter works in Buck mode. When VIN is equal or close to

VOUT, the converter alternates between Buck and Boost mode as necessary to provide a regulated output voltage,

which is called Buck-Boost mode. Figure 47 shows the relationship between the operation modes and VOUT - VIN.

RS_IN is a current sense resistor to sense the inductor current during Q1 on-time. As shown in the “Block Diagram”

on page 7, the sensed signal is fed into the CS+ and CS- pins and used for peak or valley current-mode control, DE

mode control, input average current monitor, constant current control, and protections.

RS_OUT is a current sense resistor to sense the inductor current during Q4 on-time. As shown in the Block Diagram,

the sensed signal is fed into the ISEN+ and ISEN- pins and used for negative peak inductor current limit, output

average current monitor, constant current control, and protections.

5.6.1

Buck Mode Operation (VIN > VOUT)

In Buck mode, Q4 is always on and Q3 is always off unless boot refresh or inductor negative peak current limit

is tripped. Q1 and Q2 runs in a normal peak current controlled sync buck operation mode. Q1 turns on by the

clock. During Q1 on-time, op amp A1 senses the inductor current by the voltage on RS_IN. Q1 turns off when

the sensed signal combined with the slope compensation ramp is higher 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 48 on page 33.

Figure 46. Buck-Boost Topology

Figure 47. Operation Modes vs VOUT - VIN

VIN

VOUT

RS_IN

RS_OUT

UG1

LG1

UG2

LG2

PH1

PH2

Boost Mode

Buck/Boost Mode

Buck Mode

Q1 On, Q2 Off

Q3, Q4 PWM Switching

Q4 On, Q3 Off

Q1, Q2 PWM Switching

4-Switch PWM

Q3 Max Duty

Q3 Min Duty

Q2 Min Duty

Q2 Max Duty

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