Datasheet

ADP2105/ADP2106/ADP2107 Data Sheet
Rev. D | Page 24 of 36
DESIGN EXAMPLE
Consider an application with the following specifications:
Input Voltage = 3.6 V to 4.2 V.
Output Voltage = 2 V.
Typical Output Current = 600 mA.
Maximum Output Current = 1.2 A.
Soft Start Time = 2 ms.
Overshoot ≤ 100 mV under all load transient conditions.
1. Choose the dc-to-dc converter that satisfies the maximum
output current requirement. Because the maximum output
current for this application is 1.2 A, the ADP2106 with a
maximum output current of 1.5 A is ideal for this
application.
2. See whether the output voltage desired is available as a
fixed output voltage option. Because 2 V is not one of the
fixed output voltage options available, choose the adjustable
version of ADP2106.
3. The first step in external component selection for an
adjustable version converter is to calculate the resistance of
the resistive voltage divider that sets the output voltage.
kΩ40
μA20
V8.0
===
STRING
FB
BOT
I
V
R
kΩ60
V8.0
V8.0V2
kΩ40 =








−
×=






−
=
FB
FB
OUT
BOTTOP
V
VV
RR
Calculate the minimum inductor value as follows:
For the ADP2106:
L > (0.83 μH/V) × V
OUT
⇒
L > 0.83 μH/V × 2 V
⇒
L > 1.66 μH
Next, calculate the ideal inductor value that sets the
inductor peak-to-peak current ripple (ΔI
L
) to 1/3 of the
maximum load current at the maximum input voltage as
follows:
=
×
−××
= μH
)(5.2
)(MAX
LOAD
IN
OUT
IN
OUT
IDEAL
IV
VVV
L
μH2.18μH
2.12.4
)22.4(25.2
=
×
−××
4. The closest standard inductor value is 2.2 μH. The maximum
rms current of the inductor is to be greater than 1.2 A, and
the saturation current of the inductor is to be greater than
2 A. One inductor that meets these criteria is the LPS4012-
2.2 μH from Coilcraft.
5. Choose the output capacitor based on the transient response
requirements. The worst-case load transient is 1.2 A, for
which the overshoot must be less than 100 mV, which is 5%
of the output voltage. For a 1 A load transient, the overshoot
must be less than 4% of the output voltage, then from
Figure 39:
Output Capacitor × Output Voltage = 60 μC
μF30
V0.2
μC60
≈=⇒ CapacitorOutput
Taking into account the loss of capacitance due to dc bias, as
shown in Figure 40, two 22 μF X5R MLCC capacitors from
Murata (GRM21BR60J226M) are sufficient for this
application.
6. Because the ADP2106 is being used in this application, the
input capacitors are 10 μF and 4.7 μF X5R Murata capacitors
(GRM21BR61A106K and GRM21BR61A475K).
7. The 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.
8. Choose a soft start capacitor of 2 nF to achieve a soft start
time of 2 ms.
9. Calculate the compensation resistor and capacitor as
follows:
















=
REF
OUTOUT
CS
m
CROSS
COMP
V
VC
GG
F
R
)π2(
8.0
=
kΩ215
V8.0
V2μF30
V/A8125.2V/μA50
kHz80)π2(
8.0 =








×








×
×
pF39
kΩ215kHz80π
2
π
2
=
××
==
COMPCROSS
COMP
RF
C