AAT2138 Datasheet, PDF (14/18 Pages) Skyworks Solutions Inc. – High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch

DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Considering 20% capacitance tolerance, the minimum
capacitance should be 684μF. So select a 680μF tanta-
lum capacitor as C_REGIN, as well as an additional 10μF
ceramic capacitor to closely filter the input voltage V_REGIN
of the step-down converter on the PCB board.
Input Capacitor Selection
Select a 1μF X7R or X5R ceramic capacitor for the sys-
tem input. To estimate the required input capacitor size,
determine the acceptable input ripple level and solve for
C_IN. The calculated value varies with input voltage and is
a maximum when V_IN is twice the output voltage.
C_IN =
1 - ^VOUT
V_IN
·
V_OUT
V_IN
V_PP
I_OUT
- ESR · ^f_S
for V_IN = 2 · V_OUT,
and
1 - ^VOUT
V_IN
·
V_OUT
V_IN
1
=4
C_IN(MIN) =
1
V_PP
I_OUT
- ESR · ^{4 · f}_S
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2138. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function due to their low ESR and ESL. To minimize
stray parasitic inductance, place the capacitor as close
as possible to the IC. This keeps the high frequency
content of the input current localized, minimizing EMI
and input voltage ripple.
The proper placement of the input capacitor (C1) can be
seen in the evaluation board layout in the Layout section
of this datasheet (see Figure 4).
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the eval-
uation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
Feedback Resistor Selection
The output voltage on the AAT2138 is adjustable with
external resistors R1 and R2. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum sug-
gested value for R2 is 59kΩ. Although a larger value will
further reduce quiescent current, it will also increase the
impedance of the feedback node, making it more sensi-
tive to external noise and interference. The maximum
value of R1 should be below 1Mohm to keep reference
voltage normal and avoid noise coupling.
The external resistor R1, combined with an external
capacitor up to 22pF feed-forward capacitor (C4 in
Figure 3), delivers enhanced transient response for
extreme pulsed load applications and reduces ripple in
light load conditions. The addition of the feed forward
capacitor typically requires a larger output capacitor
C31-C32 for stability. The external resistors set the out-
put voltage according to the following equation:
R1 =
V_OUT
V_REF
-1
· R2 =
V_OUT
0.6V
-
1
· R2
Table 2 shows the standard 1% metal film resistor
examples for different step-down ouput voltages
V_OUT (V)
3.3
3.4
3.6
3.8
R2 = 59kΩ,
R1 (kΩ)
267
274
294
316
Table 2: Resistor Selections for Different Output
Voltage Settings.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
14
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013