AAT2500M
400mA Step-Down Converter and 300mA LDO
cuit, which accelerates the power-up behavior of
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.
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.
When the regulator is in shutdown mode, an internal
1.5kΩ resistor is connected between OUT and GND.
This is intended to discharge COUT when the LDO
regulator is disabled. The internal 1.5KΩ resistor
has no adverse impact on device turn-on time.
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.
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 500ꢀA
load.
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 VIN_LDO to keep the LDO regulator in
a continuously on state.
AAT2500M
AAT2500M
L1
4. 7µH
L1
4. 7uH
12
7
1
12
7
1
VIN
VP_BUCK
LX
FB_BUCK
AGND
VOUT_BUCK
VIN
VP_BUCK
IN_LDO
LX
FB_BUCK
AGND
VOUT_BUCK
R1
C1
10µF
5
C1
5
IN_LDO
10µF
3
11
10
9
3
11
10
9
EN_BUCK
EN_LDO
OUT_LDO
PGND
EN_BUCK
EN_LDO
OUT_LDO
PGND
4
C8
4
AGND
AGND
100pF
R2
59k
6
C1
4.7µF
6
C1
4. 7µF
VOUT_LDO
AGND
VOUT_LDO
AGND
2
8
2
8
AGND
AGND
C4
4. 7µF
C4
4. 7µF
Figure 1: AAT2500M Fixed Output.
Figure 2: AAT2500M with Adjustable Step-Down
Output and Enhanced Transient Response.
2500M.2007.06.1.0
11
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
