AAT2500
1MHz Step-Down Converter/LDO Regulator
VIN
VIN
C3
C3
3
5
9
6
7
8
4
VP
VCC
EN
10μF
3
5
9
6
7
8
4
10
2
VP
VCC
EN
10μF
VLDO
ENLDO
OUT
BYP
VOUTBUCK
10
2
L1
VLDO
ENLDO
OUT
BYP
VOUTBUCK
LX
L1
VOUTLDO
LX
11
12
1
VOUTLDO
FB
R1
11
12
1
FB
SGND
PGND
SGND
PGND
C1
4.7μF
GND
C8
C4
4.7μF
GND
C4
4.7μF
100pF
C5
U1
AAT2500
C1
4.7μF
C5
U1
AAT2500
R2
59k
10nF
10nF
Figure 1: AAT2500 Fixed Output.
Figure 2: AAT2500 with Adjustable Step-Down
Output and Enhanced Transient Response.
Low Dropout Operation
Applications Information
For conditions where the input voltage drops to the
output voltage level, the converter duty cycle
increases to 100%. As 100% duty cycle is
approached, the minimum off-time initially forces
the high side on-time to exceed the 1MHz clock
cycle and reduce the effective switching frequency.
Once the input drops below the level where the out-
put can be regulated, the high side P-channel
MOSFET is turned on continuously for 100% duty
cycle. At 100% duty cycle, the output voltage tracks
the input voltage minus the IR drop of the high side
Linear Regulator
Input and Output Capacitors: An input capacitor
is not required for basic operation of the linear reg-
ulator. However, if the AAT2500 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.
P-channel MOSFET RDS(ON)
.
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.
Low Supply
The under-voltage lockout (UVLO) guarantees suf-
ficient VIN bias and proper operation of all internal
circuitry prior to activation.
Fault Protection
For overload conditions, the peak inductor current is
limited. Thermal protection disables switching when
the internal dissipation or ambient temperature
becomes excessive. The junction over-temperature
threshold is 140°C with 15°C of hysteresis.
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
14
2500.2006.05.1.16
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