AAT2500M Datasheet, PDF (14/22 Pages) Advanced Analogic Technologies – 400mA Step-Down Converter and 300mA LDO

AAT2500M
400mA Step-Down Converter and 300mA LDO
Input Capacitor
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 (V_PP) and solve for C2. The calculated
value varies with input voltage and is a maximum
when V_{IN_BUCK} is double the output voltage (V_O).
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
C_IN =
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_OSC
V_{O ·}
V_IN
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
1
- ESR^⎞⎠ · 4 · F_OSC
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:
I_RMS = I_O ·
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
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 (output)
current.
V_O
V_IN
· ^⎛⎝1 -
V_O
V_IN
⎞
⎠
=
D · (1 - D) =
0.5² =
1
2
for V_IN = 2 · V_O
I = _RMS(MAX)
I_O
2
The term
V_{O ·}
V_IN
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
appears in both the input volt-
age ripple and input capacitor RMS current equa-
tions and is a maximum when V_{IN_BUCK} is twice
V_{OUT_BUCK}. 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
AAT2500M. 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.
The proper placement of the input capacitor (C2)
can be seen in the evaluation board layout in
Figure 3.
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.
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.
14
2500M.2007.06.1.0