PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
junction over-temperature threshold is 140°C with 15°C
of hysteresis. Once an over-temperature or over-current
fault condition is removed, the output voltage automati-
cally recovers.
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 AAT2158 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
Peak current mode control and optimized internal com-
pensation provide high loop bandwidth and excellent
response to input voltage and fast load transient events.
Soft start eliminates output voltage overshoot when the
enable or the input voltage is applied. Under-voltage
lockout prevents spurious start-up events.
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.
Control Loop
The AAT2158 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.
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.
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. The
reference voltage is internally set to program the con-
verter output voltage greater than or equal to 0.6V.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VCC
input. Under-voltage lockout (UVLO) guarantees suffi-
cient VIN bias and proper operation of all internal cir-
cuitry prior to activation.
LX
Enable
C8
U1
VOUT+
VIN+
AAT2158
12
11
10
4
VP
FB
VOUT(V)
R3 (kΩ)
R3
15
14
13
16
3
R1
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
VP
LX
LX
100
L1
VP
3.0μH
R2
7
EN
LX
100K
9
6
8
5
C1
10μF
VCC
N/C
N/C
N/C
R4
59.0k
C2
0.1μF
PGND
PGND
137
C3
22μF
2
187
267
1
SGND PGND
GND
GND
C1 Murata 10μF 6.3V X5R GRM42-6X5R106K6.3
C3 Murata 22μF 6.3V GRM21BR60J226ME39L X5R 0805
L1 Sumida CDRH5D28-3R0NC (see Table 2)
R1 and C2 are an optional noise filter for internal VCC
R6, C4, C5-C7 are not populated
C8 100pF feed forward
.
Figure 1: AAT2158 Evaluation Schematic.
w w w . a n a l o g i c t e c h . c o m
10
2158.2008.02.1.2
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