AAT2505
Dual Channel, Step-Down
Converter/Linear 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.
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
measurements can also result.
VOBUCK
VIN
⎛
· 1
⎝
VOBUCK
VIN
⎞
⎠
1
2
-
=
D
· (1 - D) = 0.52 =
for VIN = 2 x VOBUCK
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.
IOBUCK
IRMS(MAX)
=
2
VOBUCK
VIN
⎛
⎝
VOBUCK
⎞
VIN
·
1 -
The term
appears in both the
⎠
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 should be placed
in parallel with the low ESR, ESL bypass ceramic.
This dampens the high Q network and stabilizes
the system.
input voltage ripple and input capacitor RMS cur-
rent equations and is a maximum when VOBUCK is
twice VIN. This is why the input voltage ripple and
the input capacitor RMS current ripple are a maxi-
mum at 50% duty cycle.
The input capacitor provides a low impedance loop
for the edges of pulsed current drawn by the
AAT2505. 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.
Output Capacitor
The output capacitor limits the output ripple and
provides holdup during large load transitions. A
4.7µF to 10µF X5R or X7R ceramic capacitor pro-
vides sufficient bulk capacitance to stabilize the
output during large load transitions and has the
ESR and ESL characteristics necessary for low
output ripple.
The proper placement of the input capacitor (C2)
can be seen in the evaluation board layout in
Figure 3.
Figure 3: AAT2505 Evaluation Board Top Side.
Figure 4: AAT2505 Evaluation Board
Bottom Side.
16
2505.2006.06.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
