PRODUCT DATASHEET
AAT2503178
SystemPowerTM
Adjustable 3-Channel Regulator
Ceramic Capacitor Materials
POK Output
Ceramic capacitors less than 0.1μF are typically made
from NPO or C0G materials. NPO and C0G materials gen-
erally have tight tolerance and are very stable over tem-
perature. Larger capacitor values are usually composed
of X7R, X5R, Z5U, or Y5V dielectric materials. These two
material types are not recommended for use with LDO
regulators 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 desir-
able. The temperature tolerance of X7R dielectric is better
than ±15%. Capacitor area is another contributor to
ESR. Capacitors which are physically large in size will
have a lower ESR when compared to a smaller sized
capacitor of an equivalent material and capacitance value.
These larger devices can improve circuit transient response
when compared to an equal value capacitor in a smaller
package size. Consult capacitor vendor datasheets care-
fully when selecting capacitors for LDO regulators.
LDOA of the AAT2503 features an integrated Power OK
comparator which can be used as an error flag. The POK
open drain output goes low when output voltage is 10%
(typ) below its nominal regulation voltage. Additionally,
any time LDOA is in shutdown, the POK output is pulled
low. Connect a pull-up resistor from POK to OUTA.
Enable Function
The AAT2503 features an LDO regulator enable/disable
function. Each LDO has its own dedicated enable pin.
These pins (ENA, ENB) are active high and are compatible
with CMOS logic. To assure the LDO regulators will switch
on, ENA/B must be greater than 1.4V. The LDO regulators
will shut down when the voltage on the ENA/B pins falls
below 0.6V. In shutdown, the LDO regulators will con-
sume less than 1.0μA of current. If the enable function is
not needed in a specific application, it may be tied to VIN
to keep the LDO regulator in a continuously on state.
Thermal Protection
Adjustable Output Resistor Selection
Each of the two LDOs of the AAT2503 has an internal ther-
mal protection circuit which will turn on when the device
die temperature exceeds 140°C. The LDO regulator out-
puts will remain in a shutdown state until the internal die
temperature falls back below the ~125°C trip point.
The output voltage on the linear regulator is programmed
with external resistors: R4 and R7 for LDOA and R5 and
R8 for LDOB. Table 3 summarizes the resistor values for
various output voltages with R4 and R5 set to either
59kΩ for good noise immunity or 221kΩ for reduced no
load input current.
No-Load Stability
The LDOs in the AAT2503 are designed to maintain out-
put voltage regulation and stability under operational
no-load conditions. This is an important characteristic for
applications where the output current may drop to zero.
LDO VOUT
(V)
R7, R8 = 59kΩ
R4, R5 (kΩ)
R7, R8 = 221kΩ
R4, R5 (kΩ)
0.6*
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
0
0
75
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
113
150
187
221
261
301
332
442
464
523
715
1000
Reverse Output-to-Input
Voltage Conditions and Protection
Under normal operating conditions, a parasitic diode
exists between the output and input of the LDO regula-
tor. The input voltage should always remain greater than
the output load voltage maintaining a reverse bias on
the internal parasitic diode. Conditions where VOUT might
exceed VIN should be avoided since this would forward
bias the internal parasitic diode and allow excessive cur-
rent flow into the VOUT pin, possibly damaging the LDO
regulator. In applications where there is a possibility of
Table 3: LDO Linear Regulators Resistor Values
for Various Output Voltages.
* For the 0.6V output, R7 and R8 are open.
w w w . a n a l o g i c t e c h . c o m
2503.2008.02.1.4
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