AAT2612 Datasheet, PDF (11/18 Pages) Skyworks Solutions Inc. – Step-Down DC/DC Converter With Three High PSRR LDOs

DATA SHEET
AAT2612
Step-Down DC/DC Converter With Three High PSRR LDOs
ic capacitors. However, the design will allow for opera-
tion over a wide range of capacitor types.
The regulator comes with complete short circuit and ther-
mal protection. The combination of these two internal
protection circuits gives a comprehensive safety system
to guard against extreme adverse operating conditions.
Application Information
Step-down Converter
Input Capacitor
Select a 4.7uF to 10uF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor value
and size, determine the acceptable input ripple voltage
level (V_pp) and solve for C_IN. The calculated value varies
with input voltage and is a maximum when V_IN is double
the output voltage.
C_IN =
V_{O · 1 -} V_O
V_IN
V_IN
V_PP
I_O
- ESR
· f_S
D=
V_O
V_IN
C_IN(MIN) =
1
V_PP
I_O
- ESR
· 4 · f_S
Where C_IN is the input capacitance, V_IN is the input volt-
age, V_O is the output voltage, f_S is the switching fre-
quency, I_O is the output current, ESR is the equivalent
series resistor of output capacitor, and D is the duty
cycle.
The maximum input capacitor RMS current is:
I_RMS = I_O ·
V_{O · 1 -} V_O
V_IN
V_IN
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 =
I_O
2
The maximum input voltage ripple also appears at 50%
duty cycle.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2612. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function. To minimize parasitic inductances, 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 capacitors (C1, C2,
and C3) is shown in the evaluation board layout 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 can also result in the loop
phase and gain measurements. Since the inductance of
a short PCB trace feeding the input voltage is signifi-
cantly 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 capacitor 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 pro-
vides holdup during large load transitions. A typical
4.7μF 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 dom-
inated 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 relationship of the output volt-
age droop during the three switching cycles to the output
capacitance can be estimated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
11
202407B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013