AAT2146 Datasheet, PDF (10/19 Pages) Advanced Analogic Technologies – Low Noise, Fast Transient 600mA Step-Down Converter

SwitchReg^TM
PRODUCT DATASHEET
AAT2146
Low Noise, Fast Transient 600mA Step-Down Converter
VIN
VOUT
C1
10µF
GND
1
2
3
Enable
CFF
R1
118k
L1
4.7µH
R2
59k
U1
AAT2146
1 _{EN PGND} 8
2 _{OUT PGND} 7
3 _{VIN PGND} 6
4 _{LX AGND} 5
C2
4.7µF
GND2
LX
U1 AAT2146 SC70JW-8
L1 CDRH3D16-4R7
C1 10µF 10V 0805 X5R
C2 4.7µF 10V 0805 X5R
C_FF 100pF 0402 X5R
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT2146 is a peak current mode step-down con-
verter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short
circuit and overload protection. A fixed slope compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak cur-
rent mode loop appears as a voltage-programmed cur-
rent source in parallel with the output capacitor.
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. For
fixed voltage versions, the error amplifier reference volt-
age is internally set to program the converter output
voltage. For the adjustable output, the error amplifier
reference is fixed at 0.6V.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2146 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles.
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 140°C with 15°C of hys-
teresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the V_IN input.
Under-voltage lockout (UVLO) guarantees sufficient V_IN
bias and proper operation of all internal circuitry prior to
activation.
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2146.2008.04.1.1