AME5259A datasheet(9/21 Pages) AME

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AME

AME5259A

Rev.A.02

1.2A, 1.5MHz Synchronous

Step-Down Converter

Detailed Description

Main Control Loop

AME5259A uses a constant frequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET)

switches are intermal. During normal operation, the in-

ternal top power MOSFET is turned on each cycle when

the oscillator sets the RS latch, and turned off when the

current comparator resets the RS latch. While the top

MOSFET is off, the bottom MOSFET is turned on until

either the inductor current starts to reverse as indicated

by the current reversal comparator IRCMP.

Pulse Skipping Mode Operation

At light loads, the inductor current may reach zero or

reverse on each pulse. The bottom MOSFET is turned

off by the current reversal comparator, IRCMP, and the

switch voltage will ring. This is discontinuous mode op-

eration, and is normal behavior for the switching regula-

tor.

Short-Circuit Protection

When the output is shorted to ground, the frequency of

the oscillator is reduced to about 180KHz. This frequency

foldback ensures that the inductor current hsa more time

do decay, thereby preventing runaway. The oscillator’ s

frequency will progressively increase to 1.5MHz when V

or V

OUT rises above 0V.

Dropout Operation

As the input supply voltage decreases to a value ap-

proaching the output voltage, the duty cycle increases

toward the maximum on-time. Further reduction of the

supply voltage forces the main switch to remain on for

more than one cycle until it reaches 100% duty cycle.

The output voltage will then be determined by the input

voltage minus the voltage drop across the P-channel

MOSFET and the inductor.

The basic AME5259A application circuit is shown in

Typical Application Circuit. External component selec-

tion is determined by the maximum load current and be-

gins with the selection of the inductor value and followed

by C

IN and COUT.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current DI

L increases with higher VIN and decreases

with higher inductance.

A reasonable starting point for setting ripple current is

∆I

L=0.4(lmax).

The DC current rating of the inductor

should be at least equal to the maximum load current

plus half the ripple current to prevent core saturation. For

better efficiency, choose a low DC-resistance inductor.

IN and COUT Selection

The input capacitance, C

IN is needed to filter the trap-

ezoidal current at the source of the top MOSFET. To

prevent large voltage transients, a low ESR input

capacitorsized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

This formula has a maximum at V

IN=2VOUT, where

RMS=IOUT/2.

This simple worst-case condition is com-

monly used for design because even significant devia-

tions do not offer much relief. Note that the capacitor

manufacturer ripple current ratings are often based on 2000

hours of life. This makes it advisable to further derate the

capacitor, or choose a capacitor rated at a higher tem-

perature than required.

The selection of C

OUT is determined by the effective series

resistance(ESR) that is required to minimize voltage ripple

and load step transients. The output ripple, V

OUT, is de-

termined by:

Application Information

)

(

OUT

−

∆

)

(

−

OUT

MAX

OUT

RMS

∆

≅

∆

OUT

ESR

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