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ACT4070 Schematic ( Datenblatt PDF ) - Active-Semi

Teilenummer ACT4070
Beschreibung Wide Input 3A Step Down Converter
Hersteller Active-Semi
Logo Active-Semi Logo 



Gesamt 11 Seiten
		
ACT4070 Datasheet, Funktion
ACT4070
Rev 2, 16-Sep-11
Wide Input 3A Step Down Converter
FEATURES
3A Output Current
Up to 95% Efficiency
4.5V to 30V Input Range
6µA Shutdown Supply Current
400kHz Switching Frequency
Adjustable Output Voltage
Cycle-by-Cycle Current Limit Protection
Thermal Shutdown Protection
Internal Soft Start Function
Frequency Fold Back at Short Circuit
Stability with Wide Range of Capacitors,
Including Low ESR Ceramic Capacitors
SOP-8/EP (Exposed Pad) Package
APPLICATIONS
TFT LCD Monitors or Televisions and HDTV
Portable DVD Players
Car-Powered or Battery-Powered Equipment
Set-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
GENERAL DESCRIPTION
The ACT4070 is a current-mode step-down DC/DC
converter that generates up to 3A output current at
400kHz switching frequency. The device utilizes
Active-Semi’s proprietary ISOBCD30 process for
operation with input voltage up to 30V.
Consuming only 6μA in shutdown mode, the
ACT4070 is highly efficient with peak efficiency at
95% when in operation. Protection features include
cycle-by-cycle current limit, thermal shutdown, and
frequency fold back at short circuit. The device also
includes an internal soft start function to prevent
overshoot.
The ACT4070 is available in SOP-8/EP exposed
pad package and requires very few external de-
vices for operation.
NOTE:
Refer to the last page (Page11) for the End of
Life Notice of the Part Number.
TYPICAL APPLICATION CIRCUIT
Innovative PowerTM
- 1 - www.active-semi.com
Copyright © 2011 Active-Semi, Inc.






ACT4070 Datasheet, Funktion
ACT4070
Rev 2, 16-Sep-11
Stability compensation
Figure 2:
Stability Compensation
c: CCOMP2 is needed only for high ESR output capacitor
The feedback system of the IC is stabilized by the
components at COMP pin, as shown in Figure 2.
The DC loop gain of the system is determined by
the following equation:
AVDC
= 1.222V
IOUT
AVEAGCOMP
(4)
The dominant pole P1 is due to CCOMP:
fP1
=
G EA
2 π AVEA C COMP
(5)
The second pole P2 is the output pole:
fP 2
=
IOUT
2 πVOUT COUT
(6)
The first zero Z1 is due to RCOMP and CCOMP:
fZ1
=
1
2πRCOMP CCOMP
(7)
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
fP 3
=
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 1.222V
= 1.25 x10 8VOUT COUT
()
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:
CCOMP
= 1.6 x10 5
RCOMP
(F) (10)
If RCOMP is limited to 15k, then the actual cross
over frequency is 4.8/(VOUTCOUT). Therefore:
CCOMP
=
8.8
x10
V6
OUT
COUT
(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 us-
ing CCOMP2 is:
RESROUT
Min⎜⎜⎝⎛
1.1x10
COUT
6
,0.012VOUT
⎟⎟⎠⎞
And the proper value for CCOMP2 is:
()
(12)
CCOMP
= COUT RESROUT
RCOMP
(13)
Though CCOMP2 is unnecessary when the output
capacitor has sufficiently low ESR, a small value
CCOMP2 such as 220pF 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
1.8V
COUT
22μF Ceramic
RCOMP
4k
CCOMP CCOMP2c
3.3nF 220pF
2.5V 22μF Ceramic 5.6k3.3nF 220pF
5V 22μF Ceramic 12k1.5nF 220pF
1.8V 100μF SP CAP 15k1.5nF 220pF
2.5V 100μF SP CAP 15k2.2nF 220pF
5V 100μF SP CAP 15k4.7nF 220pF
c: CCOMP2 is needed for board parasitic and high ESR output
capacitor.
Figure 3 shows a sample ACT4070 application
circuit generating a 2.5V/3A output.
Innovative PowerTM
- 6 - www.active-semi.com
Copyright © 2011 Active-Semi, Inc.

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