Datasheet

ADP2105/ADP2106/ADP2107 Data Sheet
Rev. D | Page 18 of 36
Table 8. Minimum Inductor Value for Common Output
Voltage Options for the ADP2107 (2 A)
V
OUT
V
IN
2.7 V 3.6 V 4.2 V 5.5 V
1.2 V 0.83 µH 1.00 µH 1.07 µH 1.17 µH
1.5 V 0.99 µH 1.09 µH 1.21 µH 1.36 µH
1.8 V 1.19 µH 1.19 µH 1.29 µH 1.51 µH
2.5 V 1.65 µH 1.65 µH 1.65 µH 1.70 µH
3.3 V 2.18 µH 2.18 µH 2.18 µH 2.18 µH
Table 9. Inductor Recommendations for the ADP2105/
ADP2106/ADP2107
Vendor
Small-Sized Inductors
(< 5 mm × 5 mm)
Large-Sized Inductors
(> 5 mm × 5 mm)
Sumida CDRH2D14, 3D16,
3D28
CDRH4D18, 4D22,
4D28, 5D18, 6D12
Toko 1069AS-DB3018,
1098AS-DE2812,
1070AS-DB3020
D52LC, D518LC,
D62LCB
Coilcraft LPS3015, LPS4012,
DO3314
DO1605T
Cooper
Bussmann
SD3110, SD3112,
SD3114, SD3118,
SD3812, SD3814
SD10, SD12, SD14, SD52
OUTPUT CAPACITOR SELECTION
The output capacitor selection affects both the output voltage ripple
and the loop dynamics of the converter. For a given loop crossover
frequency (the frequency at which the loop gain drops to 0 dB), the
maximum voltage transient excursion (overshoot) is inversely
proportional to the value of the output capacitor. Therefore, larger
output capacitors result in improved load transient response. To
minimize the effects of the dc-to-dc converter switching, the cross-
over frequency of the compensation loop should be less than 1/10
of the switching frequency. Higher crossover frequency leads to
faster settling time for a load transient response, but it can also
cause ringing due to poor phase margin. Lower crossover
frequency helps to provide stable operation but needs large output
capacitors to achieve competitive overshoot specifications.
Therefore, the optimal crossover frequency for the control loop of
ADP2105/ADP2106/ADP2107 is 80 kHz, 1/15 of the switching
frequency. For a crossover frequency of 80 kHz, Figure 39 shows
the maximum output voltage excursion during a 1 A load transient,
as the product of the output voltage and the output capacitor is
varied. Choose the output capacitor based on the desired load
transient response and target output voltage.
18
0
06079-070
15 70
OUTPUT CAPACITOR × OUTPUT VOLTAGE (μC)
OVERSHOOT OF OUTPUT VOLTAGE (%)
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
20 25 30 35 40 45 50 55 60 65
Figure 39. Percentage Overshoot for a 1 A Load Transient Response vs.
Output Capacitor × Output Voltage
For example, if the desired 1 A load transient response (overshoot)
is 5% for an output voltage of 2.5 V, t h e n f rom Figure 39
Output Capacitor × Output Voltage = 50 μC
Fμ20
5
.2
Cμ50
≈=⇒ CapacitorOutput
The ADP2105/ADP2106/ADP2107 have been designed for
operation with small ceramic output capacitors that have low
ESR and ESL. Therefore, they are comfortably able to meet tight
output voltage ripple specifications. 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. Table 10 shows a list of recommended
MLCC capacitors from Murata and Taiyo Yuden.
When choosing output capacitors, it is also important to
account for the loss of capacitance due to output voltage dc bias.
Figure 40 shows the loss of capacitance due to output voltage dc
bias for three X5R MLCC capacitors from Murata.
20
–100
06079-060
VOLTAGE (V
DC
)
CAPACITANCE CHANGE (%)
0
–20
–40
–60
–80
0
2 4 6
1
3
2
1
4.7µF 0805 X5R MURATA GRM21BR61A475K
2
10µF 0805 X5R MURATA GRM21BR61A106K
3
22µF 0805 X5R MURATA GRM21BR60J226M
Figure 40. Percentage Drop-In Capacitance vs. DC Bias for Ceramic
Capacitors (Information Provided by Murata Corporation)
For example, to get 20 µF output capacitance at an output voltage
of 2.5 V, based on Figure 40, as well as to give some margin for
temperature variance, a 22 μF and a 10 μF capacitor are to be