AAT2786 Datasheet, PDF (16/27 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter and 150mA LDO

SystemPower^TM
PRODUCT DATASHEET
AAT2786
1.5A Step-Down Converter and 150mA LDO
LDO Functional Description
The LDO regulator in the AAT2786 is intended for applica-
tions with output current load requirements from no load
to 150mA. The advanced circuit design of the AAT2786
LDO regulator has been specifically optimized for very
fast start-up and shutdown timing. This proprietary
CMOS LDO has also been tailored for superior transient
response characteristics. These traits are particularly
important for applications that require fast power supply
timing, such as GSM cellular telephone handsets.
The high-speed turn-on capability of the LDO regulator
is enabled through the implementation of a fast start
control circuit, which accelerates the turn-on behavior of
fundamental control and feedback circuits. Fast turn-off
response time is achieved by an active output pull-down
circuit, which is enabled when the LDO regulator is
placed in shutdown mode. This active fast shutdown cir-
cuit has no adverse effect on normal device operation.
The AAT2786 LDO regulator has very fast transient
response characteristics, which is an important feature for
applications in which fast line and load transient response
are required. This rapid transient response behavior is
accomplished through the implementation of an active
error amplifier feedback control. This proprietary circuit
design is unique to this MicroPower LDO regulator.
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.
A bypass pin has been provided to allow the addition of
an optional voltage reference bypass capacitor to reduce
output self noise and increase power supply ripple rejec-
tion. Device self noise and PSRR will be improved by the
addition of a small ceramic capacitor to this pin. However,
increased CBYPASS values may slow down the LDO
regulator turn-on time.
Enable Function
The AAT2786 features an LDO regulator enable/ disable
function. This pin (EN) is active high and is compatible
with CMOS logic. To assure the LDO regulator will switch
on, the EN turn-on control level must be greater than
1.5V. The LDO regulator will go into the disable shut-
down mode when the voltage on the EN pin falls below
0.6V. If the enable function is not needed in a specific
application, it may be tied to V_IN to keep the LDO regula-
tor in a continuously on state.
When the LDO regulator is in shutdown mode, an inter-
nal 1.5kΩ resistor is connected between VOUT and GND.
This is intended to discharge C_OUT when the LDO regula-
tor is disabled. The internal 1.5kΩ has no adverse effect
on device turn-on time.
Short-Circuit Protection
The AAT2786 contains an internal short-circuit protection
circuit that will trigger when the output load current
exceeds the internal threshold limit. Under short-circuit
conditions, the output of the LDO regulator will be cur-
rent limited until the short-circuit condition is removed
from the output or LDO regulator package power dissi-
pation exceeds the device thermal limit.
Thermal Protection
The AAT2786 has an internal thermal protection circuit
which will turn on when the device die temperature
exceeds 150°C. The internal thermal protection circuit
will actively turn off the LDO regulator output pass
device to prevent the possibility of over temperature
damage. The LDO regulator output will remain in a shut-
down state until the internal die temperature falls back
below the 150°C trip point.
The combination and interaction between the short cir-
cuit and thermal protection systems allows the LDO
regulator to withstand indefinite short-circuit conditions
without sustaining permanent damage.
No-Load Stability
The AAT2786 is designed to maintain output voltage
regulation and stability under operational no load condi-
tions. This is an important characteristic for applications
where the output current may drop to zero.
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 exces-
sive current flow into the VOUT pin, possibly damaging
the LDO regulator.
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