Datasheet

ADP1740/ADP1741 Data Sheet
Rev. F | Page 14 of 20
APPLICATIONS INFORMATION
CAPACITOR SELECTION
Output Capacitor
The ADP1740/ADP1741 are designed for operation with small,
space-saving ceramic capacitors, but they function with most
commonly used capacitors as long as care is taken with regard
to the effective series resistance (ESR) value. The ESR of the
output capacitor affects the stability of the LDO control loop. A
minimum of 3.3 µF capacitance with an ESR of 100 mΩ or less is
recommended to ensure the stability of the ADP1740/ADP1741.
Transient response to changes in load current is also affected by
output capacitance. Using a larger value of output capacitance
improves the transient response of the ADP1740/ADP1741 to
large changes in load current. Figure 33 and Figure 34 show the
transient responses for output capacitance values of 4.7 µF and
22 µF, respectively.
2
CH1 1.0A Ω
B
W
CH2 50.0mV
B
W
M1.0µs A CH1 380mA
1
T 10.80%
T
07081-032
I
LOAD
1A/DIV
1mA TO 2A LOAD STEP, 2.5A/µs
V
OUT
50mV/DIV
V
IN
= 3.6V, V
OUT
= 1.5V
C
IN
= C
OUT
= 4.7µF
Figure 33. Output Transient Response, C
OUT
= 4.7 µF
2
CH1 1.0A Ω
B
W
CH2 50.0mV
B
W
M1.0µs A CH1 880mA
1
T 11.80%
T
07081-033
I
LOAD
1A/DIV
1mA TO 2A LOAD STEP, 2.5A/µs
V
OUT
50mV/DIV
V
IN
= 3.6V, V
OUT
= 1.5V
C
IN
= C
OUT
= 22µF
Figure 34. Output Transient Response, C
OUT
= 22 µF
Input Bypass Capacitor
Connecting a 4.7 µF capacitor from the VIN pin to GND
reduces the circuit sensitivity to printed circuit board (PCB)
layout, especially when long input traces or high source
impedance are encountered. If output capacitance greater than
4.7 µF is required, it is recommended that the input capacitor be
increased to match it.
Input and Output Capacitor Properties
Any good quality ceramic capacitors can be used with the
ADP1740/ADP1741, as long as they meet the minimum
capacitance and maximum ESR requirements. Ceramic
capacitors are manufactured with a variety of dielectrics, each
with different behavior over temperature and applied voltage.
Capacitors must have a dielectric adequate to ensure the
minimum capacitance over the necessary temperature range
and dc bias conditions. X5R or X7R dielectrics with a voltage
rating of 6.3 V or 10 V are recommended. Y5V and Z5U
dielectrics are not recommended, due to their poor temperature
and dc bias characteristics.
Figure 35 shows the capacitance vs. voltage bias characteristics
of an 0805 case, 4.7 μF, 10 V, X5R capacitor. The voltage stability
of a capacitor is strongly influenced by the capacitor size and
voltage rating. In general, a capacitor in a larger package or with
a higher voltage rating exhibits better stability. The temperature
variation of the X5R dielectric is approximately ±15% over the
−40°C to +85°C temperature range and is not a function of
package size or voltage rating.
5
4
3
2
1
0
0 2 4 6 8 10
CAPACITANCE (µF)
VOLTAGE BIAS (V)
MURATA P/N GRM219R61A475KE34
07081-133
Figure 35. Capacitance vs. Voltage Bias Characteristics
Use Equation 3 to determine the worst-case capacitance,
accounting for capacitor variation over temperature, com-
ponent tolerance, and voltage.
C
EFF
= C
OUT
× (1 − TEMPCO) × (1 − TOL) (3)
where:
C
EFF
is the effective capacitance at the operating voltage.
TEMPCO is the worst-case capacitor temperature coefficient.
TOL is the worst-case component tolerance.