ISL81601 datasheet(35/54 Pages) RENESAS

35 / 54 page

FN9299 Rev.3.1

Page 35 of 53

May 27, 2021

ISL81601

5. Functional Description

Figure 50. Buck-Boost Mode Equivalent Circuit and Operation Waveforms

5.7

Light-Load Efficiency Enhancement

The ISL81601 can be set to DE and Burst mode to improve light load efficiency. The dual functional

LG1/PWM_MODE pin sets the DE or PWM mode operation in the initiation stage before soft-start. During the

initiation stage, a typical 10µA current source IMODELG1 from the LG1/PWM_MODE pin creates a voltage drop

on the resistor RLG1 connected between the LG1/PWM_MODE pin and GND. When the voltage is lower than

typical 0.3V, PWM mode is set. Otherwise, DE mode is set.

To set for DE mode operation, select RLG1 to meet:

When DE mode is set, the buck sync FET driven by LG1 and the boost sync FET driven by UG2 are all running in

DE mode. The inductor current is not allowed to reverse (discontinuous operation) depending on the zero cross

detection reference level VCROSS1 for buck sync FET and VCROSS2 for boost sync FET. At light load conditions,

the converter goes into diode emulation. When the load current is less than the level set by VIMONOUTBSTEN

typical 0.85V on the IMON_OUT pin, the part enters Burst mode. Equation 12 sets the Burst mode operation enter

condition.

where (refer to Figure 45 on page 31):

ISENOFFSET is the output current sense op amp internal offset current, typical 20µA

GmISEN is the output current sense op amp Gm, typical 200µS.

The part exits Burst mode when the output current increases to higher than the level set by VIMONOUTBSTEX

typical 0.88V on the IMON_OUT pin. Equation 13 sets the Burst mode operation exit condition.

When the part enters Burst mode, the BSTEN pin goes low. To fully avoid any enter/exit chattering, a 4-10MΩ

resistor can be added between the BSTEN and IMON_OUT pins to further expand the hysteresis.

CLOCK

UG1

LG1

LG2

UG2

VIN

VOUT

RS_IN

RS_OUT

UG1

LG1

UG2

LG2

PH1

PH2

RLG1xIMODELG1 0.34V



(EQ. 11)

RIM_OUTxISENOFFSET IOUTxRS_OUTxGmISEN

 V

IMONOUTBSTEN



(EQ. 12)

RIM_OUTxISENOFFSET IOUTxRS_OUTxGmISEN

 v

IMONOUTBSTEX



(EQ. 13)

ISL81601 Datasheet(PDF) 35 Page - Renesas Technology Corp

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