Copyright
ANPEC Electronics Corp.
Rev. A.4 - May., 2005
APL5913
www.anpec.com.tw
13
Application Information (Cont.)
Output Capacitor (Cont.)
The APL5913 is designed with a programmable
feedback compensation adjusted by an external
feedback network for the use of wide ranges of ESR
and capacitance in all applications. Ultra-low-ESR
capacitors (such as ceramic chip capacitors), low-ESR
bulk capacitors (such as solid Tantalum, POSCap, and
Aluminum electrolytic capacitors) all can be used as
an output capacitor. The value of the output capacitors
can be increased without limit.
During load transients, the output capacitors,
depending on the stepping amplitude and slew rate of
load current, are used to reduce the slew rate of the
current seen by the APL5913 and help the device to
minimize the variations of output voltage for good
transient response. For the applications with large
stepping load current, the low-ESR bulk capacitors
are normally recommended.
Decoupling ceramic capacitors must be placed at the
load and ground pins as close as possible and the
impedance of the layout must be minimized.
Input Capacitor
The APL5913 requires proper input capacitors to sup-
ply current surge during stepping load transients to
prevent the input rail from dropping . Because the
parasitic inductor from the voltage sources or other
bulk capacitors to the VIN pin limit the slew rate of the
surge currents. More parasitic inductance needs more
input capacitance.
Ultra-low-ESR capacitors, such as ceramic chip
capacitors, are very good for the input capacitors An
aluminum electrolytic capacitor (>100mF, ESR
<300mW) is recommended as the input capacitor. It
is not necessary to use low-ESR capacitors. More
capacitance reduce the variations of the input voltage
of VIN pin.
Feedback Network
Figure 1 shows the feedback network between VOUT,
GND and FB pins. It works with the internal error
amplifier to provide proper frequency response for the
linear regulator. The ESR is the equivalent series
resistance of the output capacitor. The C
OUT
is ideal
capacitance in the output capacitor. The V
OUT
is the
setting of the output voltage.
V
ERR
V
FB
R1
R2
C1
V
OUT
FB
VOUT
V
REF
EAMP
APL5913
C
OUT
ESR
Figure 1
The feedback network selection depends on the values
of the ESR and C
OUT
, which has been classified into
three conditions:
Condition 1 : Large ESR (
≥
18m
)
- Select the R1 in the range of 400
~ 2.4k
- Calculate the R2 as the following :
- Calculate the C1 as the following :
Condition 2 : Middle ESR
- Calculate the R1 as the following:
(1)
..........
0.8
-
V
0.8
R1
R2
(V)
OUT(V)
(V)
)
(k
)
(k
=
(2)
......
R1
V
40
C1
≤
R1
V
10
)
(k
OUT(V)
(nF)
)
(k
OUT(V)
≤
(3)
.........
15
V
37.5
ESR
2157
R1
OUT(V)
)
(m
)
(k
+
=