Datasheet

Data Sheet ADP2105/ADP2106/ADP2107
Rev. D | Page 19 of 36
used in parallel to ensure that the output capacitance is sufficient
under all conditions for stable behavior.
Table 10. Recommended Input and Output Capacitor
Selection for the ADP2105/ADP2106/ADP2107
Capacitor
Vendor
Murata Taiyo Yuden
4.7 µF, 10 V
X5R 0805
GRM21BR61A475K LMK212BJ475KG
10 μF, 10 V
X5R 0805
GRM21BR61A106K LMK212BJ106KG
22 μF, 6.3 V
X5R 0805
GRM21BR60J226M JMK212BJ226MG
INPUT CAPACITOR SELECTION
The input capacitor reduces input voltage ripple caused by the
switch currents on the PWIN pins. Place the input capacitors as
close as possible to the PWIN pins. Select an input capacitor
capable of withstanding the rms input current for the maximum
load current in your application.
For the ADP2105, it is recommended that each PWIN pin be
bypassed with a 4.7 μF or larger input capacitor. For the ADP2106,
bypass each PWIN pin with a 10 μF and a 4.7 μF capacitor, and
for the ADP2107, bypass each PWIN pin with a 10 μF capacitor.
As with the output capacitor, a low ESR ceramic capacitor is
recommended to minimize input voltage ripple. X5R or X7R
dielectrics are recommended, with a voltage rating of 6.3 V or
10 V. Y5V and Z5U dielectrics are not recommended due to
their poor temperature and dc bias characteristics. Refer to
Table 10 for input capacitor recommendations.
INPUT FILTER
The IN pin is the power source for the ADP2105/ADP2106/
ADP2107 internal circuitry, including the voltage reference and
current sense amplifier that are sensitive to power supply noise.
To prevent high frequency switching noise on the PWIN pins from
corrupting the internal circuitry of the ADP2105/ADP2106/
ADP2107, a low-pass RC filter should be placed between the IN
pin and the PWIN1 pin. The suggested input filter consists of
a small 0.1 μF ceramic capacitor placed between IN and AGND
and a 10 Ω resistor placed between IN and PWIN1. This forms a
150 kHz low-pass filter between PWIN1 and IN that prevents any
high frequency noise on PWIN1 from coupling into the IN pin.
SOFT START PERIOD
To set the soft start period, connect a soft start capacitor (C
SS
) from
SS to AGND. The soft start period varies linearly with the size
of the soft start capacitor, as shown in the following equation:
T
SS
= C
SS
× 10
9
ms
For a soft start period of 1 ms, a 1 nF capacitor must be
connected between SS and AGND.
LOOP COMPENSATION
The ADP2105/ADP2106/ADP2107 utilize a transconductance
error amplifier to compensate the external voltage loop. The
open loop transfer function at angular frequency (s) is given by
















=
OUT
REF
OUT
COMP
CS
m
V
V
sC
sZ
GGsH
)(
)(
where:
V
REF
is the internal reference voltage (0.8 V).
V
OUT
is the nominal output voltage.
Z
COMP
(s) is the impedance of the compensation network at the
angular frequency.
C
OUT
is the output capacitor.
g
m
is the transconductance of the error amplifier (50 μA/V
nominal).
G
CS
is the effective transconductance of the current loop.
G
CS
= 1.875 A/V for the ADP2105.
G
CS
= 2.8125 A/V for the ADP2106.
G
CS
= 3.625 A/V for the ADP2107.
The transconductance error amplifier drives the compensation
network that consists of a resistor (R
COMP
) and capacitor (C
COMP
)
connected in series to form a pole and a zero, as shown in the
following equation:








+
=








+=
COMP
COMPCOMP
COMP
COMPCOMP
sC
CsR
sC
RsZ
1
1
)(
At the crossover frequency, the gain of the open loop transfer
function is unity. For the compensation network impedance at
the crossover frequency, this yields the following equation:
















=
REF
OUTOUT
CS
m
CROSS
CROSSCOMP
V
VC
GG
F
FZ
)π2(
)(
where:
F
CROSS
= 80 kHz, the crossover frequency of the loop.
C
OUT
V
OUT
is determined from the Output Capacitor Selection
section.
To ensure that there is sufficient phase margin at the crossover
frequency, place the compensator zero at 1/4 of the crossover
frequency, as shown in the following equation:
1
4
)π2( =






COMPCOMP
CROSS
CR
F
Solving the three equations in this section simultaneously yields
the value for the compensation resistor and compensation
capacitor, as shown in the following equation:
















=
REF
OUTOUT
CS
m
CROSS
COMP
V
VC
GG
F
R
)π2(
8.0
COMPCROSS
COMP
RF
C
π
2
=