AAT2505 Analogic Corporation, AAT2505 Datasheet - Page 14

• Manufacturer Part Number: AAT2505
• Manufacturer: Analogic Corporation

Applications Information
Linear Regulator
Input and Output Capacitors: An input capacitor
is not required for basic operation of the linear reg-
ulator. However, if the AAT2505 is physically locat-
ed more than three centimeters from an input
power source, a C
stable operation. Typically, a 1µF or larger capaci-
tor is recommended for C
C
pin as practically possible.
An input capacitor greater than 1µF will offer supe-
rior input line transient response and maximize
power supply ripple rejection. Ceramic, tantalum,
or aluminum electrolytic capacitors may be select-
ed for C
requirement for C
ulator output operation, ceramic capacitors are rec-
ommended for C
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
For proper load voltage regulation and operational
stability, a capacitor is required between OUT and
GND. The C
regulator ground pin should be made as directly as
practically possible for maximum device perform-
ance. Since the regulator has been designed to
function with very low ESR capacitors, ceramic
capacitors in the 1.0µF to 10µF range are recom-
mended for best performance. Applications utilizing
the exceptionally low output noise and optimum
power supply ripple rejection should use 2.2µF or
greater for C
where output load is less than 10mA, the minimum
value for C
Equivalent Series Resistance: ESR is a very
important characteristic to consider when selecting a
capacitor. ESR is the internal series resistance asso-
ciated with a capacitor that includes lead resistance,
internal connections, size and area, material compo-
sition, and ambient temperature. Typically, capacitor
ESR is measured in milliohms for ceramic capaci-
tors and can range to more than several ohms for
tantalum or aluminum electrolytic capacitors.
14
IN
should be located as closely to the device V
IN
. There is no specific capacitor ESR
OUT
OUT
OUT
can be as low as 0.47µF.
. In low output current applications,
IN
capacitor connection to the LDO
IN
. However, for 300mA LDO reg-
IN
due to their inherent capability
capacitor will be needed for
IN
in most applications.
IN
Ceramic Capacitor Materials: Ceramic capacitors
less than 0.1µF are typically made from NPO or
C0G materials. NPO and C0G materials generally
have tight tolerance and are very stable over tem-
perature. Larger capacitor values are usually com-
posed of X7R, X5R, Z5U, or Y5V dielectric materi-
als. Large ceramic capacitors (i.e., greater than
2.2µF) are often available in low-cost Y5V and Z5U
dielectrics. These two material types are not rec-
ommended for use with the regulator, since the
capacitor tolerance can vary more than ±50% over
the operating temperature range of the device. A
2.2µF Y5V capacitor could be reduced to 1µF over
temperature; this could cause problems for circuit
operation. X7R and X5R dielectrics are much more
desirable. The temperature tolerance of X7R dielec-
tric is better than ±15%.
Capacitor area is another contributor to ESR.
Capacitors that are physically large in size will have
a lower ESR when compared to a smaller sized
capacitor of an equivalent material and capaci-
tance value. These larger devices can improve cir-
cuit transient response when compared to an equal
value capacitor in a smaller package size. Consult
capacitor vendor datasheets carefully when select-
ing capacitors for LDO regulators.
Step-Down Converter
Inductor Selection: The step-down converter
uses peak current mode control with slope com-
pensation to maintain stability for duty cycles
greater than 50%. The output inductor value must
be selected so the inductor current down slope
meets the internal slope compensation require-
ments. The internal slope compensation for the
adjustable and low-voltage fixed versions of the
AAT2505 is 0.24A/µsec. This equates to a slope
compensation that is 75% of the inductor current
down slope for a 1.5V output and 4.7µH inductor.
m =
Converter/Linear Regulator
0.75 ⋅ V
Dual Channel, Step-Down
L
O
=
0.75 ⋅ 1.5V
4.7μH
= 0.24
2505.2006.06.1.1
μsec
A