Datasheet
ADP2441 Data Sheet
Rev. A | Page 24 of 32
Output Capacitor Selection
Select the output capacitor by using Equation 12 and Equation 13:
)(8
)(
ESRIVf
I
C
LRIPPLE
SW
L
MINOUT
Equation 12 is based on the output voltage ripple (V
RIPPLE
),
which is 1% of the output voltage.
DROOPSW
STEPOUTMINOUT
Vf
IC
3
)()(
Equation 13 calculates the capacitor selection based on the
transient load performance requirement of 2%. Perform these
calculations, and then use the equation that yields the larger
capacitor size to select a capacitor.
In this example, the values listed in Table 12 are substituted for
the variables in Equation 12 and Equation 13.
Table 12. Requirements
Parameter Test Conditions/Comments Value
Ripple Current Fixed at 0.3 A for the ADP2441 0.3 A
Voltage Ripple 1% of V
OUT
50 mV
Voltage Droop Due
to Load Transient
2% of V
OUT
100 mV
ESR 5 mΩ
f
SW
700 kHz
The calculation based on the output voltage ripple (see
Equation 12) dictates that the minimum output capacitance is
F1.1
)m5A3.0mV50(kHz7008
A3.0
)(
MINOUT
C
whereas the calculation based on the transient load (see
Equation 13) dictates that the minimum output capacitance is
F22
V1.0kHz700
3
5.0
)(
MINOUT
C
To meet both requirements, use the value determined by the
latter equation. As shown in Figure 57, capacitance degrades
with dc bias; therefore, choose a capacitor that is 1.5 times the
calculated value.
C
OUT
= 1.5 × 22 µF = 32 µF
Compensation Selection
Calculate the compensation component values for the feedback
loop by using the following equations:
REF
OUTOUT
CS
m
CROSSOVER
COMP
V
VC
Gg
f
R
2
9.0
COMP
ZERO
COMP
Rf
C
2
1
Selecting the crossover frequency to be 1/12 of the switching
frequency and placing the zero frequency at 1/8 of the crossover
frequency ensures that there is enough phase margin in the system.
Table 13. Calculated Parameter Value
Parameter Test Conditions/Comments Value
f
CROSSOVER
1/12 of f
SW
58.3 kHz
f
ZERO
1/8 of f
CROSSOVER
7.3 kHz
V
REF
Fixed reference 0.6 V
g
m
Transconductance of error
amplifier
250 µA/V
G
CS
Current sense gain 2 A/V
C
OUT
Output capacitor 22 µF
V
OUT
Output voltage 5 V
Based on the values listed in Table 13, calculate the compen-
sation value:
k121
6.0
522
2250
3.582
9.0
COMP
R
The closest standard resistor value is 118 kΩ. Therefore,
pF180pF185
1183.72
1
COMP
C
SYSTEM CONFIGURATION
Configure the system as follows:
1.
Connect a capacitor of 1 µF between the VCC and PGND
pins and another capacitor of 1 µF between the VCC and
AGND pins. For best performance, use ceramic X5R or
X7R capacitors with a 25 V voltage rating.
2.
Connect a ceramic capacitor of 10 nF with a 50 V voltage
rating between the BST and SW pins.
3.
Connect a resistor between the FREQ and AGND pins as
close as possible to the IC.
4.
If using the power-good feature, connect a pull-up resistor
of 50 kΩ to an external supply of 5 V.
5.
Connect a capacitor of 10 nF between the SS and AGND pins.
If the tracking feature is needed, connect a resistor divider
between the TRK pin and another supply, as shown in
Figure 50.
See Figure 60 for a schematic of this design example and Table 14
for the calculated component values.