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AS1324

Data Sheet - D e t a i l e d D e s c r i p t i o n

8 Detailed Description

The AS1324 is a high-efficiency buck converter that uses a constant-frequency current-mode architecture. The device

contains two internal MOSFET switches and is available in adjustable- and fixed-output voltage versions.

Figure 21. Block Diagram

Main Control Loop

During PWM operation the converters use a 1.5MHz fixed-frequency, current-mode control scheme. Basis of the cur-

rent-mode PWM controller is an open-loop, multiple input comparator that compares the error-amp voltage feedback

signal against the sum of the amplified current-sense signal and the slope-compensation ramp. At the beginning of

each clock cycle, the internal high-side PMOS turns on until the PWM comparator trips. During this time the current in

the inductor ramps up, sourcing current to the output and storing energy in the inductor’s magnetic field. When the

PMOS turns off, the internal low-side NMOS turns on. Now the inductor releases the stored energy while the current

ramps down, still providing current to the output. The output capacitor stores charge when the inductor current

exceeds the load and discharges when the inductor current is lower than the load. Under overload conditions, when

the inductor current exceeds the current limit, the high-side PMOS is turned off and the low-side NMOS remains on

until the next clock cycle.

When the PMOS is off, the NMOS is turned on until the inductor current starts to reverse (as indicated by the current

reversal comparator (IRCMP)), or the next clock cycle begins. The IRCMP detects the zero crossing.

to the internal 0.6V reference, causing the error amplifier’s output voltage to increase until the average inductor current

matches the new load current.

The over-voltage detection comparator (OVDET) guards against transient overshoots by turning the main switch off

and keeping it off until the transient is removed.

AS1324

OSC

Frequency

Shift

0.6V

Reference

+

–

Error

Amp

Shutdown

Ramp

Compensator

0.6V

FB

OSCN

Digital

Logic

0.6V +

0.6V -

–

OVDET

+

–

+

–

ICOMP

+

–

IRCMP

+

Anti-

Shoot

Through

Not applicable to AS1324

Ω

Ω

Ω

PMOS

1

EN

5

4

3

SW

2

GND

NMOS

10µF

4.7µH

10µF