ADP1755 Analog Devices, ADP1755 Datasheet - Page 14

• Manufacturer Part Number: ADP1755
• Description: 1.2A Low-Vin, Adjustable-Vout LDO Linear Regulator
• Manufacturer: Analog Devices

ADP1754/ADP1755
APPLICATIONS INFORMATION
CAPACITOR SELECTION
Output Capacitor
The ADP1754/ADP1755 are designed for operation with small,
space-saving ceramic capacitors, but they can function with most
commonly used capacitors as long as care is taken with the
effective series resistance (ESR) value. The ESR of the output
capacitor affects the stability of the LDO control loop. A mini-
mum of 3.3 μF capacitance with an ESR of 500 mΩ or less is
recommended to ensure the stability of the ADP1754/ADP1755.
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 ADP1754/ADP1755 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.
1
2
1
2
CH1
CH1
Figure 33. Output Transient Response, C
Figure 34. Output Transient Response, C
500mA
500mA
T
T
1mA TO 1.2A LOAD STEP, 2.5A/µs, 500mA/DIV
1mA TO 1.2A LOAD STEP, 2.5A/µs, 500mA/DIV
B
B
W
W
CH2 50mV
CH2 20mV
50mV/DIV
20mV/DIV
I
I
V
V
LOAD
LOAD
OUT
OUT
B
B
W
W
M1µs
M1µs
T
T
11.2%
11.0%
V
C
V
C
IN
IN
IN
IN
= 3.6V, V
= 3.6V, V
= C
= C
OUT
A CH1
OUT
A CH1
OUT
OUT
= 4.7µF
= 22µF
= 4.7 μF
= 22 μF
OUT
OUT
= 1.5V
= 1.5V
380mA
340mA
Rev. B | Page 14 of 20
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
ADP1754, as long as they meet the minimum capacitance and
maximum ESR requirements. Ceramic capacitors are manufac-
tured 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 recom-
mended. 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 about ±15% over the −40°C to
+85°C temperature range and is not a function of package size
or voltage rating.
Equation 3 can be used to determine the worst-case capacitance,
accounting for capacitor variation over temperature, component
tolerance, and voltage.
where:
C
TEMPCO is the worst-case capacitor temperature coefficient.
TOL is the worst-case component tolerance.
EFF
is the effective capacitance at the operating voltage.
C
EFF
5
4
3
2
1
0
0
= C
Figure 35. Capacitance vs. Voltage Bias Characteristics
OUT
× (1 − TEMPCO) × (1 − TOL)
2
MURATA P/N GRM219R61A475KE34
VOLTAGE BIAS (V)
4
6
8
10
(3)