ISL81601 datasheet(29/54 Pages) RENESAS

29 / 54 page

FN9299 Rev.3.1

Page 29 of 53

May 27, 2021

ISL81601

5. Functional Description

5.5.1

Output Voltage Regulation Loop

The ISL81601 provides a precision 0.8V internal reference voltage to set the output voltage. Based on this

internal reference, the output voltage can be set from 0.8V up to a level determined by the feedback voltage

divider, as shown in Figure 43.

A resistive divider from the output to ground sets the output voltage. Connect the center point of the divider to

the FB_OUT pin. The output voltage value is determined by Equation 5.

where RFBO1 is the top resistor of the feedback divider network and RFBO2 is the bottom resistor connected

from FB_OUT to ground, shown in Figure 43.

Figure 43. Output Voltage Regulator

As shown in Figure 43, the RCOMP, CCOMP1, and CCOMP2 network connected on the Gm1 regulator output

COMP pin is needed to compensate the loop for stable operation. The loop stability can be affected by many

different factors such as VIN, VOUT, load current, switching frequency, inductor value, output capacitance and

the compensation network on COMP pin. For most applications 22nF is a good value for CCOMP1. A larger

CCOMP1 makes the loop more stable by giving a larger phase margin, but the loop bandwidth is lower. CCOMP2

is typically 1/10th to 1/30th of CCOMP1 to filter high frequency noise. A good starting value for RCOMP is 10k.

Lower RCOMP improves stability but slows the loop response. Optimize the final compensation network with a

bench test.

5.5.2

Input Voltage Regulation Loop

As shown in Figure 44 on page 30, the input voltage VIN can be sensed by the FB_IN pin using a resistor

divider RFBIN1/RFBIN2 and regulated by Gm2. When the FB_IN pin voltage falls below the 0.8V reference

voltage, the COMP pin voltage is pulled low to reduce the PWM duty cycle and thus the input current. For

applications with a high input source impedance, such as a solar panel, the input voltage regulation loop can

prevent the input voltage from being pulled too low in high output load conditions. For applications with a low

input source impedance, such as batteries, the VIN feedback loop can prevent the battery from being over-

discharged. For applications with loads on the VIN supply, such as a DC back up system, the input voltage

regulation loop can reduce the input current to negative area to reverse power conversion direction to discharge

the backup battery or super capacitor to supply a regulated VIN for the loads. The regulated input voltage value

is determined by Equation 6.

VOUT

0.8V

RFBO1 RFBO2

RFBO2

---------------------------------------------







(EQ. 5)

COMP

FB_OUT

0.8V

REF

GM1

VOUT

RFBO1

RFBO2

RCOMP

CCOMP1

CCOMP2

VIN

0.8V

RFBIN1 RFBIN2

RFBIN2

------------------------------------------------







(EQ. 6)

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