AAT2500M Datasheet, PDF (11/22 Pages) Advanced Analogic Technologies – 400mA Step-Down Converter and 300mA LDO

AAT2500M
400mA Step-Down Converter and 300mA LDO
cuit, which accelerates the power-up behavior of
fundamental control and feedback circuits within
the LDO regulator. Fast turn-off time response is
achieved by an active output pull-down circuit,
which is enabled when the LDO regulator is
placed in shutdown mode. This active fast shut-
down circuit has no adverse effect on normal
device operation. The LDO regulator output has
been specifically optimized to function with low-
cost, low-ESR ceramic capacitors; however, the
design will allow for operation over a wide range
of capacitor types.
The regulator comes with complete short-circuit
and thermal protection. The combination of these
two internal protection circuits gives a comprehen-
sive safety system to guard against extreme
adverse operating conditions.
The regulator features an enable/disable function.
This pin (EN_LDO) is active high and is compatible
with CMOS logic. To assure the LDO regulator will
switch on, the EN_LDO turn-on control level must
be greater than 1.5V. The LDO regulator will go into
the disable shutdown mode when the voltage on
the EN_LDO pin falls below 0.6V. If the enable
function is not needed in a specific application, it
may be tied to V_{IN_LDO} to keep the LDO regulator in
a continuously on state.
The IN_LDO input powers the internal reference,
oscillator, and bias control blocks. For this reason,
the IN_LDO input must be connected to the input
power source to provide power to both the LDO
and step-down converter functions.
When the regulator is in shutdown mode, an internal
1.5kΩ resistor is connected between OUT and GND.
This is intended to discharge C_OUT when the LDO
regulator is disabled. The internal 1.5KΩ resistor
has no adverse impact on device turn-on time.
Step-Down Converter
The AAT2500M buck is a constant frequency peak
current mode PWM converter with internal com-
pensation. It is designed to operate with an input
voltage range of 2.7V to 5.5V. The output voltage
ranges from 0.6V to the input voltage. The 0.6V
fixed model shown in Figure 1 is also the
adjustable version and is externally programmable
with a resistive divider, as shown in Figure 2. The
converter MOSFET power stage is sized for
400mA load capability with up to 92% efficiency.
Light load efficiency is close to 80% at a 500µA
load.
VIN
VOUT_LDO
C1
10µF
C4
4. 7µF
AAT2500M
12
VP _BUCK
1
LX
7
IN_LDO
5
FB _BUCK
3
EN_BUCK
11
AGND
4
EN_LDO
10
AGND
6
OUT_LDO
9
AGND
2
PGND
8
AGND
L1
4. 7µH
VOUT _BUCK
C1
4.7µF
VIN
VOUT_LDO
C1
10µF
C4
4. 7µF
AAT2500M
12
VP _BUCK
1
LX
7
IN_LDO
5
FB _BUCK
3
EN_BUCK
11
AGND
4
EN_LDO
10
AGND
6
OUT_LDO
9
AGND
2
PGND
8
AGND
L1
4. 7uH
R1
VOUT_BUCK
C8
100pF
R2
59k
C1
4. 7µF
Figure 1: AAT2500M Fixed Output.
Figure 2: AAT2500M with Adjustable Step-Down
Output and Enhanced Transient Response.
2500M.2007.06.1.0
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