AAT2505
Dual Channel, Step-Down
Converter/Linear Regulator
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%.
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 CIN capacitor will be needed for
stable operation. Typically, a 1µF or larger capaci-
tor is recommended for CIN in most applications.
CIN should be located as closely to the device VIN
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 CIN. There is no specific capacitor ESR
requirement for CIN. However, for 300mA LDO reg-
ulator output operation, ceramic capacitors are rec-
ommended for CIN due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
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.
For proper load voltage regulation and operational
stability, a capacitor is required between OUT and
GND. The COUT capacitor connection to the LDO
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 COUT. In low output current applications,
where output load is less than 10mA, the minimum
value for COUT can be as low as 0.47µF.
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.
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.
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.24
A
m =
=
L
4.7μH
μsec
14
2505.2006.06.1.1
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