MAX8553 Maxim, MAX8553 Datasheet - Page 19

• Manufacturer Part Number: MAX8553
• Description: 4.5V to 28V Input / Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
• Manufacturer: Maxim

peak inductor current at the maximum-defined load
current (I
The key selection parameters for the output capacitor
are the actual capacitance value, the ESR, the equiva-
lent series inductance (ESL), and the voltage-rating
requirements, which affect the overall stability, output
ripple voltage, and transient response.
The worst-case output ripple has three components:
variations in the charge stored in the output capacitor,
the voltage drop across the capacitor’s ESR, and ESL
caused by the current into and out of the capacitor.
This can be approximated by:
The output voltage ripple due to the ESR is:
The output voltage ripple due to the output capacitance is:
The output voltage ripple due to the ESL of the output
capacitor is:
I
After a load transient, the output voltage instantly
changes by ESR x ∆I
trollers respond within 100ns and try to regulate back to
the nominal output value.
Solid polymer or OSCON electrolytic capacitors are
recommended due to their low ESR and ESL at the
switching frequency. Higher output-current applications
require multiple output capacitors connected in parallel
to meet the output ripple-voltage requirements. Do not
exceed the capacitor’s voltage or ripple-current ratings.
4.5V to 28V Input, Synchronous PWM Buck Controllers
P-P
V
RIPPLE
is the peak-to-peak inductor current:
I
PEAK
for DDR Termination and Point-of-Load Applications
LOAD(MAX)
V
=
RIPPLE (ESL)
V
V
I
=
P P
RIPPLE ESR
V
RIPPLE C
RIPPLE ESR
-
I
LOAD MAX
=
______________________________________________________________________________________
):
Output-Capacitor Selection
(
( )
V
(
(
LOAD
IN
= (V
f
S
)
- V
=
×
)
)
+
OUT
IN
=
8
L
+ ESL x di/dt and the con-
+
V
×
x ESL) / (L+ESL)
RIPPLE C
I



P P
C
LIR
-
×
I
OUT
2
P P
V
-
×
OUT
V

 ×

IN
( )
×
ESR
I
LOAD MAX
f
S
+
V
RIPPLE ESL
(
)
(
)
Stability is determined by the value of the ESR zero rel-
ative to the switching frequency. To ensure stability, the
following condition must be met:
where f
For a typical 300kHz application, the ESR zero frequen-
cy must be well below 95kHz, preferably below 50kHz.
Do not put high-value ceramic capacitors directly
across the feedback sense point without taking precau-
tions to ensure stability. Large ceramic capacitors can
have a high-ESR zero frequency and cause erratic,
unstable operation. However, it is easy to add enough
series resistance by placing the capacitors a couple of
inches downstream from the feedback sense point,
which should be as close as possible to the inductor.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully
observe the output voltage-ripple envelope for over-
shoot and ringing. It can help to simultaneously monitor
the inductor current with an AC current probe. Do not
allow more than one cycle of ringing after the initial
step-response under- or overshoot.
The input capacitor (C
drawn from the input supply and reduces noise injec-
tion. The source impedance to the input supply largely
determines the value of C
requires high input capacitance. The input capacitor
must meet the ripple current requirement (I
imposed by the switching currents. The RMS input rip-
ple current is given by:
I
when V
For optimal circuit reliability, choose a capacitor that
has less than 10°C temperature rise at the peak ripple
current.
RMS
has a maximum value of 1/2 I
S
IN
I
RMS
is the switching frequency and:
is twice V
Output-Capacitor Stability Consideration
=
f
I
ESR
LOAD
OUT
=
×
f
ESR
Input-Capacitor Selection
2π
.
IN
×
V
) reduces the current peaks
R
IN
OUT
<
ESR
. High source impedance
f
S
π
1
×
×
C
V
(
V
IN
LOAD
OUT
IN
-
, which occurs
V
OUT
)
RMS
19
)