AAT2514_08 Datasheet, PDF (9/14 Pages) Advanced Analogic Technologies – Dual Channel 600mA Step-Down Converter

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PRODUCT DATASHEET
AAT2514
Dual Channel 600mA Step-Down Converter
Applications Information
Setting the Output Voltage
Figure 1 shows the basic application circuit for the
AAT2514. Resistors R1 and R3 and R2 and R4 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 R1 and R3 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 R1 and R3 set to either
59kΩ for good noise immunity or 316kΩ for reduced no
load input current.
The adjustable feedback resistors, combined with a
external feed forward capacitors (C4 and C5 in Figure 1),
deliver enhanced transient response for extreme pulsed
load applications. The addition of the feed forward
capacitor typically requires a larger output capacitor C2
and C3 for stability. The external resistor sets the output
voltage according to the following equation:
V_OUT
=
0.6V
·
⎜⎛1
⎝
+
R2_⎟⎞
R1_⎠
or
⎡⎣ _{R2 =}
⎜⎝⎛^VVOREUFT
⎟⎞
⎠
-
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, R3 = 59kΩ
R2, R4 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R1, R3 = 316kΩ
R2, R4 (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 AAT2514 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=
V_OUT · (V_IN - V_OUT)
V_IN · ΔI_L · f_OSC
Where ΔI_L is inductor ripple current. Large value induc-
tors 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 ΔI_L = 210mA.
VIN 2.5V to 5.5V
V_OUT2 2.5V
C1
10μF
L2 2.2μH
EN1
EN2
POR
IN _AAT2514
LX2
LX1
C3
10μF
C5 22pF
R4
1MΩ
FB 2
R3
316kΩ
GND
FB1
R1
316kΩ
R5
100kΩ
RESET
L1 2.2μH
C4 22pF
R2
634kΩ
C2
10μF
VOUT1 1.8V
Figure 1: AAT2514 Typical Application Circuit.
2514.2008.02.1.1
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