AAT2500
1MHz Step-Down Converter/LDO Regulator
the exceptionally low output noise and optimum
Step-Down Converter
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.
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
AAT2500 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
Bypass Capacitor and Low Noise
Applications
This is the internal slope compensation for the
adjustable (0.6V) version or low-voltage fixed ver-
sions. When externally programming the 0.6V ver-
sion to 2.5V, the calculated inductance is 7.5µH.
A bypass capacitor pin is provided to enhance the
low noise characteristics of the LDO. The bypass
capacitor is not necessary for operation; however,
for best device performance, a small ceramic
capacitor in the range of 470pF to 10nF should be
placed between the bypass pin (BYP) and the
device ground pin (GND). To practically realize the
highest power supply ripple rejection and lowest
output noise performance, it is critical that the
capacitor connection between the BYP pin and
GND pin be direct and PCB traces should be as
short as possible.
0.75 ⋅ VO
0.75
⋅
VO
A
μsec
A
L =
=
≈
3
⋅ VO
m
0.24A
μsec
μsec
A
= 3
⋅ 2.5V = 7.5μH
In this case, a standard 10µH value is selected.
DC leakage on this pin can affect the LDO regula-
tor output noise and voltage regulation perform-
ance. For this reason, the use of a low leakage,
high quality ceramic (NPO or C0G type) or film
capacitor is highly recommended.
For high-voltage fixed versions (2.5V and above),
m = 0.48A/µsec. Table 1 displays inductor values
for the AAT2500 fixed and adjustable options.
Configuration
Output Voltage
0.6V to 2.0V
2.5V
Inductor
4.7µH
Slope Compensation
0.24A/µsec
0.6V Adjustable With
External Resistive Divider
10µH
0.24A/µsec
0.6V to 2.0V
2.5V to 3.3V
4.7µH
0.24A/µsec
Fixed Output
4.7µH
0.48A/µsec
Table 1: Inductor Values.
2500.2006.05.1.16
15
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
