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
Ramp
Compensator
OSC
OSCN
–
ICOMP
+
4
V
IN
V
IN
C
IN
10µF
Frequency
Shift
5
V
OUT
/V
FB
R
1
FB
R
2
0.6V
+
Error
Amp
–
–
OVDET
AS1324
PMOS
Digital
Logic
Anti-
Shoot
Through
4.7µH
0.6V +
ΔV
OVL
+
–
3
SW
NMOS
V
OUT
C
OUT
10µF
1
EN
0.6V
Reference
0.6V -
ΔV
OVL
+
+
IRCMP
–
Shutdown
2
GND
Not applicable to AS1324
AS1324-12: R
1
+ R
2
= 600kΩ
AS1324-15: R
1
+ R
2
= 750kΩ
AS1324-18: R
1
+ R
2
= 900kΩ
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.
The peak inductor current (I
PK
) is controlled by the error amplifier. When I
OUT
increases, V
FB
decreases slightly relative
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.
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