AAT1106_07 Datasheet, PDF (11/18 Pages) Advanced Analogic Technologies – 600mA Step-Down Converter

Applications Information
Figure 1 shows the basic application circuit with
AAT1106 fixed output versions.
VIN
2.5V to 5.5V
C1
4.7µF
IN
LX
AAT1106-1.8
EN
OUT
GND
L1
2.2µH
VOUT
1.8V
C3
10µF
Figure 1: Basic Application Circuit with Fixed
Output Versions.
VIN
2.5V to 5.5V
C1
4.7µF
IN
LX
AAT1106-0.6
FB
EN
GND
L1
2.2µH
C2
22pF
R2
634K
R1
316K
VOUT
1.8V
C3
10µF
Figure 2: Basic Application Circuit with
Adjustable Output Version.
Setting the Output Voltage
For applications requiring an adjustable output volt-
age, the AAT1106-0.6 adjustable version can be
externally programmed. Resistors R1 and R2 of
Figure 2 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 main-
taining good noise immunity, the minimum sug-
gested value for R1 is 59kΩ. Although a larger
value will further reduce quiescent current, it will
also increase the impedance of the feedback node,
making it more sensitive to external noise and
interference. Table 1 summarizes the resistor val-
ues for various output voltages with R1 set to either
59kΩ for good noise immunity or 316kΩ for
reduced no load input current.
The adjustable version of the AAT1106, combined
with an external feed forward capacitor (C2 in
Figure 2), delivers enhanced transient response for
extreme pulsed load applications. The addition of
the feed forward capacitor typically requires a larg-
er output capacitor C3 for stability. The external
resistor sets the output voltage according to the fol-
lowing equation:
1106.2007.07.1.0
AAT1106
600mA Step-Down Converter
V_OUT = 0.6V · 1 + ^RR12
or
 _{R2 = 0V.}^O₆^U_V^T_{ - 1 · R1}
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
R1 = 59k
R2 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R1 = 316kΩ
R2 (kΩ)
105
158
210
261
316
365
422
475
634
655
732
1000
1430
Table 1: Resistor Selection for Output Voltage
Setting; Standard 1% Resistor Values
Substituted Closest to the Calculated Values.
Inductor Selection
For most designs, the AAT1106 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 follow-
ing equation:
L=
V_OUT · (V_IN - V_OUT)
V_IN · ∆I_L · f_OSC
Where ∆I_L is inductor ripple current. Large value
inductors lower ripple current and small value
inductors result in high ripple currents. Choose
inductor ripple current approximately 35% of the
maximum load current 600mA, or ∆I_L = 210mA.
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