AAT2114A Datasheet, PDF (12/16 Pages) Skyworks Solutions Inc. – 2.5A Low-Noise, Fast Transient 3MHz Step-Down Regulator

DATA SHEET
AAT2114A
2.5A Low-Noise, Fast Transient 3MHz Step-Down Regulator
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
I_RMS(MAX) =
I_O
2
occurs when V_IN = 2 · V_O.
The calculated value varies with the input voltage and is
at a maximum when V_IN is twice the output voltage V_OUT.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2114A.
Low ESR/ESL X7R and X5R ceramic capacitors are ideal
for this function. To minimize stray inductance, the
capacitor should be placed as closely as possible to the
IC. This keeps the high frequency content of the input
current localized, minimizing EMI and input voltage rip-
ple.
The proper placement of the input capacitor can be seen
in the evaluation board layout shown in Figure 2.
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.
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
In applications where the input power source lead induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum electrolytic should be placed in parallel with the
low ESR/ESL bypass ceramic capacitor. This dampens
the high Q network and stabilizes the system.
Output Capacitor
The output capacitor limits the output ripple and main-
tains the output voltage during large load transitions. A
22μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL charac-
teristics necessary for low output ripple.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic output
capacitor. During a step increase in load current, the
ceramic output capacitor alone supplies the load current
until the loop responds. Within two or three switching
cycles, the loop responds and the inductor current
increases to match the load current demand. The rela-
tionship of the output voltage droop during the three
switching cycles to the output capacitance can be esti-
mated by:
C_OUT
=
3 · ΔI_O
V_DROOP · F_SW
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum value for the
output capacitor with respect to load transients.
The internal voltage loop compensation also limits the
minimum output capacitor value to 20μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
itance will reduce the crossover frequency with greater
phase margin.
Adjustable Feedback Network
The output voltage on the AAT2114A is programmed
with external resistors R₃ and R₄. To limit the bias cur-
rent required for the external feedback resistor string
while maintaining good noise immunity. Although a
larger value will further reduce quiescent current, it will
also increase the impedance of the feedback node, mak-
ing it more sensitive to external noise and interference.
Therefore, the recommended value range for R₄ is 59kΩ
for good noise immunity or 221kΩ for reduced no load
input current.
The external resistor R₃, combined with an external
100pF feed forward capacitor (C₅ in Figure 1), delivers
enhanced transient response for extreme pulsed load
applications and reduces ripple in light load conditions.
The external resistors set the output voltage according
to the following equation:
V_O = 0.6V ·
1 + ^R3
R₄
or solving for R₃:
R₃ =
V_O
0.6V
-1
· R₄
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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202004C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013