A6986H datasheet(39/71 Pages) STMICROELECTRONICS

Part #	A6986H

Description	Automotive 38 V, 2 A synchronous step-down switching regulator with 30 μA

PDF	71 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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39 / 71 page

fPHF= 1

2⋅π⋅R0⋅(C0+Cp)

(26)

whereas the zero is defined as:

fZ= 1

2⋅π⋅Rc⋅Cc

(27)

6.3

Voltage divider

The contribution of the internal voltage divider for fixed output voltage devices is:

GDIV(s)= R2R1+R2= VFBVOUT=0.853.3=0.2575 A6986H3V

GDIV(s)= R2R1+R2= VFBVOUT=0.855=0.17 A698HF5V

(28)

while for the adjustable output part number A6986H is:

GDIV(s)= R2R1+R2 A6986H

(29)

A small signal capacitor in parallel to the upper resistor (see Figure 37. Leading network example) of the voltage

divider implements a leading network (fzero < fpole), sometimes necessary to improve the system phase margin:

Figure 37. Leading network example

A6986H

Power GND

signal GND

Cr1

uC RST

RST

VCC

SS/INH

SYNCH

FSW

MLF

COMP

DELAY

SGND

PGND PGND

VIN

VBIAS

GND

VIN

VOUT

The Laplace transformer of the leading network is:

GDIV(s)= R2R1+R2⋅ (1+s+R1⋅CR1)

(1+s⋅ R1⋅R2

R1+R2⋅CR1)

(30)

where

fZ= 1

2⋅π⋅R1⋅CR1

fp= 1

2⋅π⋅ R1⋅R2

R1+R2⋅CR1

fZ<fp

(31)

A6986H

Voltage divider

DS12861 - Rev 3

page 39/71

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