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ACT4012A Schematic ( PDF Datasheet ) - Active-Semi

Teilenummer ACT4012A
Beschreibung Wide Input 1.5A Step Down Converter
Hersteller Active-Semi
Logo Active-Semi Logo 




Gesamt 10 Seiten
ACT4012A Datasheet, Funktion
ACT4012A
Rev0, 16-May-08
Wide Input 1.5A Step Down Converter
FEATURES
1.5A Output Current
Up to 94% Efficiency
Up to 20V Input Range
10µA Shutdown Supply Current
420kHz Switching Frequency
Adjustable Output Voltage
Cycle-by-Cycle Current Limit Protection
Thermal Shutdown Protection
Frequency Foldback at Short Circuit
Stability with Wide Range of Capacitors,
Including Low ESR Ceramic Capacitors
SOP-8 Package
APPLICATIONS
TFT LCD Monitors
Portable DVDs
Car-Powered or Battery-Powered Equipments
S-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
Termination Supplies
GENERAL DESCRIPTION
The ACT4012A is a current-mode step-down
DC/DC converter that generates up to 1.5A output
420kHz switching frequency. The device utilizes
Active-Semi’s proprietary ISOBCD20 process for
operation with input voltages up to 20V.
Consuming only 10µA in shutdown mode, the
ACT4012A is highly efficient with peak efficiency at
94% when in operation. Protection features include
cycle-by-cycle current limit, thermal shutdown, and
frequency foldback at short circuit.
The ACT4012A is available in a SOP-8 package
and requires very few external devices for opera-
tion.
TYPICAL APPLICATION CIRCUIT
12V
VIN
ENABLE
BS
IN SW
ACT4012A
EN
G
FB
COMP
+
5V/1.5A
Innovative PowerTM
- 1 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.






ACT4012A Datasheet, Funktion
ACT4012A
Rev0, 16-May-08
STABILITY COMPENSATION
Figure 2:
Stability Compensation
COMP
ACT4012A
CCOMP
RCOMP
CCOMP21
c: CCOMP2 is needed only for high ESR output capacitor
The feedback system of the IC is stabilized by the
components at the COMP pin, as shown in Figure 2.
The DC loop gain of the system is determined by
the following equation:
AVDC
1.3V
= IOUT
AVEA GCOMP
(4)
The dominant pole P1 is due to CCOMP:
fP1
=
G EA
2πAVEA C COMP
(5)
The second pole P2 is the output pole:
fP 2
=
I OUT
2πVOUT COUT
(6)
The first zero Z1 is due to RCOMP and CCOMP:
fZ1
=
1
2πRCOMPCCOMP2
(7)
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
fP3
=
1
2πRCOMP CCOMP2
(8)
Follow the following steps to compensate the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via RCOMP:
RCOMP
=
2πVOUT COUT fSW
10GEAGCOMP ×1.3V
=
2
× 10
V8
OUT
C OUT
()
but limit RCOMP to 15kmaximum.
(9)
STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15k, the equation
for CCOMP is:
C COMP
=
1.8 × 10 5
R COMP
(F) (10)
If RCOMP is limited to 15k, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
CCOMP = 1.2 ×105VOUTCOUT
(F) (11)
STEP 3. If the output capacitor’s ESR is high
enough to cause a zero at lower than 4 times the
cross over frequency, an additional compensation
capacitor CCOMP2 is required. The condition for using
CCOMP2 is:
RESRCOUT Min⎜⎜⎝⎛1.1C×O1UT06 ,0.012 ×VOUT ⎟⎟⎠⎞ () (12)
And the proper value for CCOMP2 is:
CCOMP2
=
C ROUT ESRCOUT
RCOMP
(13)
Though CCOMP2 is unnecessary when the output ca-
pacitor has sufficiently low ESR, a small value
CCOMP2 such as 100pF may improve stability against
PCB layout parasitic effects.
Table 2 shows some calculated results based on
the compensation method above.
Table 2:
Typical Compensation for Different Output
Voltages and Output Capacitors
VOUT
2.5V
3.3V
5V
2.5V
3.3V
5V
2.5V
3.3V
5V
COUT
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP CAP
47µF SP CAP
47µF SP CAP
470µF/6.3V/30m
470µF/6.3V/30m
470µF/6.3V/30m
RCOMP
12k
15k
15k
15k
15k
15k
15k
15k
15k
CCOMP
1.5nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
CCOMP2c
None
None
None
None
None
None
1nF
1nF
None
c: CCOMP2 is needed for high ESR output capacitor.
Figure 3 shows an example ACT4012A application circuit gener-
ating a 5V/1.5A output.
Innovative PowerTM
- 6 - www.active-semi.com
Copyright © 2008 Active-Semi, Inc.

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