MDT2520-CN AD [Analog Devices], MDT2520-CN Datasheet - Page 23

• Manufacturer Part Number: MDT2520-CN
• Description: Dual 3 MHz, 800 mA Buck
• Manufacturer: AD [Analog Devices]

Data Sheet
LDO EXTERNAL COMPONENT SELECTION
Feedback Resistors
For the adjustable model, the maximum value of Rb is not to
exceed 200 kΩ (see Figure 48).
Output Capacitor
The
saving ceramic capacitors, but functions with most commonly
used capacitors as long as care is taken with the ESR value. The
ESR of the output capacitor affects stability of the LDO control
loop. A minimum of 0.70 µF capacitance with an ESR of 1 Ω or
less is recommended to ensure stability of the ADP5023. 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
current.
Input Bypass Capacitor
Connecting a 1 µF capacitor from VIN3 to ground reduces
the circuit sensitivity to printed circuit board (PCB) layout,
especially when long input traces or high source impedance
are encountered. If greater than 1 µF of output capacitance is
required, increase the input capacitor to match it.
Input and Output Capacitor Properties
Use any good quality ceramic capacitors with the
long as they meet the minimum capacitance and maximum ESR
requirements. Ceramic capacitors are manufactured with a variety
of dielectrics, each with a different behavior over temperature
and applied voltage. Capacitors must have a dielectric adequate
to ensure the minimum capacitance over the necessary tempera-
ture range and dc bias conditions. X5R or X7R dielectrics with a
voltage rating of 6.3 V or 10 V are recommended for best perfor-
mance. Y5V and Z5U dielectrics are not recommended for use
with any LDO because of their poor temperature and dc bias
characteristics.
Figure 51 depicts the capacitance vs. voltage bias characteristic
of a 0402 1 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 higher voltage
rating exhibits better stability. The temperature variation of the
ADP5023
LDO is designed for operation with small, space-
ADP5023
to large changes in load
ADP5023
as
Rev. A | Page 23 of 28
X5R dielectric is about ±15% over the −40°C to +85°C tempera-
ture range and is not a function of package or voltage rating.
Use the following equation to determine the worst-case capa-
citance 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.
In this example, the worst-case temperature coefficient
(TEMPCO) over −40°C to +85°C is assumed to be 15% for an
X5R dielectric. The tolerance of the capacitor (TOL) is assumed
to be 10% and C
Substituting these values into the following equation:
Therefore, the capacitor chosen in this example meets the
minimum capacitance requirement of the LDO over
temperature and tolerance at the chosen output voltage.
To guarantee the performance of the ADP5023, it is imperative
that the effects of dc bias, temperature, and tolerances on the
behavior of the capacitors are evaluated for each application.
BIAS
C
C
is the effective capacitance at the operating voltage.
1.2
1.0
0.8
0.6
0.4
0.2
EFF
EFF
0
0
= C
= 0.85 μF × (1 − 0.15) × (1 − 0.1) = 0.65 μF
Figure 51. Capacitance vs. Voltage Characteristic
BIAS
× (1 − TEMPCO) × (1 − TOL)
BIAS
1
is 0.85 μF at 1.8 V as shown in Figure 51.
DC BIAS VOLTAGE (V)
2
3
4
5
ADP5023
6