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Teilenummer | ACT4012A |
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Beschreibung | Wide Input 1.5A Step Down Converter | |
Hersteller | Active-Semi | |
Logo | ||
Gesamt 10 Seiten 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
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
(Ω)
but limit RCOMP to 15kΩ maximum.
(9)
STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15kΩ, the equation
for CCOMP is:
C COMP
=
1.8 × 10 −5
R COMP
(F) (10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
CCOMP = 1.2 ×10−5VOUTCOUT
(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×O1UT0−6 ,0.012 ×VOUT ⎟⎟⎠⎞ (Ω) (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/30mΩ
470µF/6.3V/30mΩ
470µF/6.3V/30mΩ
RCOMP
12kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
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.
6 Page | ||
Seiten | Gesamt 10 Seiten | |
PDF Download | [ ACT4012A Schematic.PDF ] |
Teilenummer | Beschreibung | Hersteller |
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