ISL81601 datasheet(30/54 Pages) RENESAS

30 / 54 page

FN9299 Rev.3.1

Page 30 of 53

May 27, 2021

ISL81601

5. Functional Description

Figure 44. VIN Feedback Loop

FB_IN is a dual-function pin. It also sets the phase angle of the clock output signal on the CLKOUT/DITHER

pin, shown in Table 2 on page 38. The VIN feedback loop is disabled when the FB_IN pin voltage is below 0.3V

or above 4.7V. The VIN feedback loop is also disabled in DE mode and during soft-start.

5.5.3

Input and Output Average Current Monitoring and Regulation Loops

As shown in Figure 45 on page 31, the ISL81601 has two current sense operational amplifiers (op amps), A1

and A2, which monitor both input and output current. The voltage signals on the input and output current sense

resistor RS_IN and RS_OUT are sent to the differential inputs of CS+/CS- and ISEN+/ISEN-, respectively, after

the RC filters RS_IN1/CS_IN1, RS_IN2/CS_IN2, RS_OUT1/CS_OUT1, and RS_OUT2/CS_OUT2. Renesas recommends

using a 1Ω value for RS_IN1, RS_IN2, RS_OUT1, and RS_OUT2, and a 10nF value for CS_IN1, CS_IN2, CS_OUT1,

and CS_OUT2 to effectively damp the switching noise without delaying the current signal too much and

introducing too much error by the op amp bias current. The Gm op amps A1 and A2 then transfer the current

sense voltage signals to current signals ICS and IISEN.

where

•IIN is the input current in Q1 drain

•VCSOFFSET is the A1 input offset voltage

•GmCS is the gain of A1, typical 200µS

•VCSOFFSET GmCS = ICSOFFSET.

The typical value of ICSOFFSET is 19.5µA.

where

•IOUT is the output current in Q4 drain

•VISENOFFSET is the A2 input offset voltage

•GmISEN is the gain of A2, typical 200µS

•VISENOFFSET GmISEN = IISENOFFSET.

The typical value of IISENOFFSET is 20µA.

COMP

FB_IN

0.8V

REF

GM2

VIN

RFBIN1

RFBIN2

ICS

IIN

R

S_IN

VCSOFFSET

Gm

(EQ. 7)

IISEN

IOUT

R

S_OUT

VISENOFFSET

Gm

ISEN

(EQ. 8)

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