AAT3183
300mA Inductorless Step-Down Converter
A conventional LDO regulator maintains input cur-
rent which is equal to the output current. Operation
efficiency (η) of an LDO regulator is as follows:
old is 150mV greater than the UVLO turn-off thresh-
old. UVLO hysteresis minimizes spurious under-
voltage detection and eliminates output glitches.
Shutdown and Soft-Start
PIN
POUT
η =
The AAT3183 offers an enable pin (EN). When VEN
is below 0.4V (maximum), the device is in standby
(shutdown) mode and draws less than 1μA of input
current. The output will remain at 0V when EN volt-
age is low (VEN ≤ 0.4V). When EN is connected to
a voltage greater than 1.4V (minimum), the
AAT3183 will initiate soft-start and resume normal
operation.
(VOUT · IOUT
(VIN · IIN)
)
=
IIN = IOUT
(VOUT · IOUT
(VIN · IOUT
)
η =
)
The product features built-in soft-start circuitry to
reduce inrush current and eliminate output voltage
overshoot. The soft-start circuitry is enabled when
input UVLO conditions are satisfied and the EN volt-
age is high (VEN ≥ 1.4V). If EN is tied to IN, the soft
start is initiated when UVLO conditions are satisfied.
The soft-start circuitry ramps up the output voltage
in a controlled manner and minimizes output over-
shoot. Start-up time from EN positive transitioning
(VEN: ≤0.4V to ≥1.4V) to output (VOUT) in regulation
is 100μs (typical).
VOUT
VIN
=
Therefore, the AAT3183 provides a 100% efficien-
cy improvement over conventional LDO regulators;
as illustrated in Figure 2.
Efficiency vs. Output Current
(VIN = 3.6V; VOUT = 1.5V)
90
80
Thermal and Short-Circuit Protection
AAT3183
70
High device temperature may result at elevated
ambient temperatures or in cases where high output
current causes self heating of the device. The device
will disable all switching of the charge pump when the
internal junction temperature exceeds 150°C (typi-
cal). The device will restart and enable the soft-start
sequence when the temperature is reduced 15°C.
This hysteresis ensures that the absolute device
temperature is maintained below the over-tempera-
ture threshold and protects the device from damage.
60
50
40
LDO
30
20
10
0
50
100
150
200
250
300
Output Current (mA)
Figure 2: AAT3183 Efficiency Comparison
vs. LDO.
In the event of a short circuit, an internal current
limit is activated and limits the output current to 1A
(typical). This current is maintained until the output
fault condition is removed or device over-tempera-
ture is reached. Under sustained short-circuit con-
ditions, the device will typically reach over-temper-
ature and latch off. The device will cool down after
a short period and continue to oscillate between
active and over-temperature protection states until
the fault is removed. Under these worst case con-
ditions, the device average junction temperature
will be less than 150°C.
Under-Voltage Lockout
Under-voltage lockout (UVLO) circuitry monitors the
input voltage (VIN) and ensures that the device will
remain in standby (VOUT = 0V) until a valid VIN is
present. When VIN is less than 1.5V (typical), the
input current is less than 1μA and the output voltage
(VOUT) remains at 0V, regardless of the status of the
enable pin (EN). Typically, the UVLO turn-on thresh-
10
3183.2007.07.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
