A6902D View Datasheet(PDF) - STMicroelectronics

Part Name

Description

MFG CO.

A6902D

Up to 1 A step down switching regulator with adjustable current limit for automotive applications

STMicroelectronics 

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A6902D

LC filter

7.2.3

GPWR_CM s

=

s

∙

LIND

+

RDC

+

RSENSE

RSENSE + 1

RLOAD

+

1

RES

+

1

s∙

1

COUT

The previous equation can be rewritten and simplified, assuming RES and RDC negligible compared to RLOAD:

GPWR_CM s

=

1+

1

+

2π

∙

s

FZ_ESR_CM

2π

∙

s

Q∙

FLC

+

s

2π ∙ FLC

2

∙ RSENSE

FZ_ESR_CM =

2πCOUT ∙

1

RLOAD + RES

≈

1

2πCOUT ∙

RLOAD

FP_LC =

2π LIND ∙ COUT ∙

1

RLOAD + RES

≈

2π

1

LIND ∙ COUT

RLOAD + RDC + RSENSE

Q≈

LIND ∙ COUT

LIND

RLOAD

+

COUT

∙

RSENSE

As a consequence, the frequency of the zero in current loop operation depends on the loading conditions,

whereas the current and voltage power loop transfer functions have the same denominator.

In terms of principle it is possible to compensate the current loop with ceramic capacitor at the output, placing

fZ_ESR_CM inside the system bandwidth to guarantee enough phase margin for a given loading range.

Negligible ESR means fZ_ESR_VM >> fP_LC and so the voltage loop results stable only in discontinuous conduction

mode / no load operation but unstable working in continuous conduction mode.

An application that fits this operation is when the voltage loop implements just an OVP protection for load

disconnection or end of charge detection in battery charging application.

PWM comparator

In voltage mode control loop the PWM comparator in Figure 2. Block diagram generates a PWM control signal for

the driving circuitry comparing the internal saw tooth waveform and the output of the error amplifier.

The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to the switching net (OUT

pin) results in an almost constant gain, due to the voltage feed-forward which generates the saw tooth signal with

amplitude proportional to the input voltage:

(11)

GPWM s

=

VIN

∆ VOSC

=

VIN

K ∙ VIN

=

1

0.076

This means that even if the input voltage changes, the error amplifier does not change its value to keep the loop

in regulation, thus ensuring a better line regulation and faster line transient response.

In summary, the open loop gain can be expressed as:

(12)

GTOT s

=

GPWM s

GPWM s

∙ GPWR_VM s

∙ GPWR_CM s

∙ A0 s

∙ A0 s

For constant current or constant voltage operation, respectively.

DS5503 - Rev 6

page 15/33

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