AAT2552
Total Power Solution for Portable Applications
capacitor in this application will minimize switching
or power transient effects when the power supply is
"hot plugged" in.
IO
2
IRMS(MAX)
=
VO
VIN
⎛
⎝
VO
VIN
⎞
⎠
·
1 -
Step-Down Converter Input Capacitor (C6)
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (VPP) and solve for CIN. The calculated
value varies with input voltage and is a maximum
when VIN is double the output voltage.
The term
appears in both the input
voltage ripple and input capacitor RMS current
equations and is a maximum when VO is twice VIN.
This is why the input voltage ripple and the input
capacitor RMS current ripple are a maximum at
50% duty cycle.
The input capacitor provides a low impedance loop
for the edges of pulsed current drawn by the step-
down converter. 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 ripple.
VO
VIN
⎛
VO ⎞
VIN ⎠
· 1 -
⎝
CIN =
⎛ VPP
⎝ IO
⎞
⎠
- ESR ·FS
VO
VIN
⎛
VO ⎞
VIN ⎠
1
· 1 -
=
for VIN = 2 · VO
⎝
4
The proper placement of the input capacitor (C6)
can be seen in the evaluation board layout in
Figure 7.
1
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
A laboratory test set-up typically consists of two
long wires running from the bench power supply to
the evaluation board input voltage pins. The induc-
tance of these wires, along with the low-ESR
ceramic input capacitor, can create a high Q net-
work that may affect converter performance. This
problem often becomes apparent in the form of
excessive ringing in the output voltage during load
transients. Errors in the loop phase and gain meas-
urements can also result.
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
The maximum input capacitor RMS current is:
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.
VO
VIN
⎛
VO ⎞
VIN ⎠
IRMS = IO ·
· 1 -
⎝
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.
In applications where the input power source lead
inductance cannot be reduced to a level that does
not affect the converter performance, a high ESR
tantalum or aluminum electrolytic capacitor should
be placed in parallel with the low ESR, ESL bypass
ceramic capacitor. This dampens the high Q net-
work and stabilizes the system. The linear regula-
tor and the step-down convertor share the same
input capacitor on the evaluation board.
VO
VIN
⎛
VO ⎞
VIN ⎠
1
2
· 1 -
⎝
=
D · (1 - D) = 0.52 =
for VIN = 2 · VO
22
2552.2007.04.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
