AAT1145_08 Datasheet, PDF (10/17 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter

SwitchReg^TM
Applications Information
VIN 2.5V-5.5V
C1
10μF
1
EN
LX ⁸
2 _IN
LX ⁷
3 _{AIN AAT1145-0.6 FB} 5
6 _AGND
4 _AGND
PGND ¹⁰
PGND ⁹
L1
2.2μH
C3
22pF
VOUT
1.8V,1.5A
R1
634kΩ ^C2
22μF
R2
316kΩ
Figure 1: Basic Application Circuit for the
Adjustable Output Version.
VIN 2.5V-5.5V
C1
10μF
1
EN
LX ⁸
2 _IN
LX ⁷
3 _{AIN AAT1145-1.8 OUT} 5
6 _AGND
4 _AGND
PGND ¹⁰
PGND ⁹
L1
2.2μH
VOUT
1.8V,1.5A
C2
22μF
Figure 2: Basic Application Circuit for the
Fixed Output Versions.
Setting the Output Voltage
Figure 1 shows the basic application circuit with the
AAT1145 adjustable output version while Figure 2 shows
the application circuit with the AAT1145 fixed output
version. For applications requiring an adjustable output
voltage, the AAT1145-0.6 adjustable version can be
externally programmed. Resistors R1 and R2 in Figure 1
program the output to regulate at a voltage higher than
0.6V. To limit the bias current required for the external
feedback resistor string while maintaining good noise
immunity, the minimum suggested value for R2 is 59kΩ.
Although a larger value will further reduce quiescent
current, it will also increase the impedance of the feed-
back node, making it more sensitive to external noise
and interference. Table 1 summarizes the resistor values
for various output voltages with R2 set to either 59kΩ
for good noise immunity or 316kΩ for reduced no load
input current.
The adjustable version of the AAT1145, combined with
an external feed forward capacitor (C3 in Figure 1),
PRODUCT DATASHEET
AAT1145
1.5A Step-Down Converter
delivers enhanced transient response for extreme pulsed
load applications. The addition of the feed forward
capacitor typically requires a larger output capacitor C2
for stability. The external resistor sets the output voltage
according to the following equation:
V_OUT = 0.6V ·
⎛⎝_{1 +}
R1 ⎞
R2 ⎠
R1 =
⎛
⎝
V_OUT
0.6V
-
1 ^⎞⎠
· R2
Table 1 shows the resistor selection for different output
voltage settings.
V_OUT (V)
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
R2 = 59kΩ
R1 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R2 = 316kΩ
R1 (kΩ)
105
158
210
261
316
365
422
475
634
655
732
1000
1430
Table 1: Resistor Selections for Different Output
Voltage Settings (Standard 1% Resistors
Substituted For Calculated Values).
Inductor Selection
For most designs, the AAT1145 operates with inductor
values of 1μH to 4.7μH. Low inductance values are
physically smaller but require faster switching, which
results in some efficiency loss. The inductor value can
be derived from the following equation:
L = ^{VOUT · (VIN - VOUT)}
V_IN · ΔI_L · f_OSC
10
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1145.2008.01.1.2