max5033ausat Maxim Integrated Products, Inc., max5033ausat Datasheet - Page 11

• Manufacturer Part Number: max5033ausat
• Description: Max5033 500ma, 76v, High-efficiency, Maxpower Step-down Dc-dc Converter
• Manufacturer: Maxim Integrated Products, Inc.

Ensure that the ripple specification of the input capaci-
tor exceeds the worst-case capacitor RMS ripple cur-
rent. Use the following equations to calculate the input
capacitor RMS current:
I
input average current, and η is the converter efficiency.
The ESR of aluminum electrolytic capacitors increases
significantly at cold temperatures. Use a 1µF or greater
value ceramic capacitor in parallel with the aluminum
electrolytic input capacitor, especially for input voltages
below 8V.
The worst-case peak-to-peak and RMS capacitor ripple
current, allowable peak-to-peak output ripple voltage,
and the maximum deviation of the output voltage dur-
ing load steps determine the capacitance and the ESR
requirements for the output capacitors.
The output capacitance and its ESR form a zero, which
improves the closed-loop stability of the buck regulator.
Choose the output capacitor so the ESR zero frequency
(f
equation to verify the value of f
to 250mΩ ESR are recommended to ensure the closed-
loop stability while keeping the output ripple low.
The output ripple is comprised of ∆V
capacitor discharge) and ∆V
of the capacitor). Use low-ESR tantalum or aluminum
electrolytic capacitors at the output. Assuming that the
contributions from the ESR and capacitor discharge
equal 80% and 20%, respectively, calculate the output
PRMS
where
Z
) occurs between 20kHz to 40kHz. Use the following
I
I
I
and D
PRMS
AVGIN
PK
is the input switch RMS current, I
=
I
OUT
=
=
=
I
CRMS
f
Z
V
V


OUT
OUT
V
=
I
+
PK
IN
V
2 π
______________________________________________________________________________________
∆
500mA, 76V, High-Efficiency, MAXPower
IN
2
× ×
2
I
L
×
=
×
,
+
I
OUT
η
I
DC
C
I
I
DC
PRMS
Output Filter Capacitor
OUT
=
2
1
OESR
+
Z
I
2
×
OUT
. Capacitors with 100mΩ
ESR
(
−
I
PK
(caused by the ESR
I
−
AVGIN
OUT
OQ
×
∆
2
I
L
I
(caused by the
DC
2
AVGIN
)

 ×
Step-Down DC-DC Converter
D
3
is the
capacitance and the ESR required for a specified rip-
ple using the following equations:
The MAX5033 has an internal soft-start time (t
400µs. It is important to keep the output rise time at
startup below t
rise time is directly proportional to the output capacitor.
Use 68µF or lower capacitance at the output to control
the overshoot below 5%.
In a dynamic load application, the allowable deviation
of the output voltage during the fast-transient load dic-
tates the output capacitance value and the ESR. The
output capacitors supply the step load current until the
controller responds with a greater duty cycle. The
response time (t
loop bandwidth of the converter. The resistive drop
across the capacitor ESR and capacitor discharge
cause a voltage droop during a step load. Use a com-
bination of low-ESR tantalum and ceramic capacitors
for better transient load and ripple/noise performance.
Keep the maximum output-voltage deviation above the
tolerable limits of the electronics being powered.
Assuming a 50% contribution from the output capaci-
tance discharge and the ESR drop, use the following
equations to calculate the required ESR and capaci-
tance value:
where I
response time of the controller. Controller response
time is approximately one-third of the reciprocal of the
closed-loop unity-gain bandwidth, 20kHz (typ).
Proper PC board layout is essential. Minimize ground
noise by connecting the anode of the Schottky rectifier,
the input bypass-capacitor ground lead, and the output
filter-capacitor ground lead to a single point (star-
STEP
C
C
PC Board Layout Considerations
is the load step and t
OUT
SS
OUT
ESR
ESR
RESPONSE
to avoid output overshoot. The output
=
≈
OUT
OUT
I
STEP
2 2 .
×
=
=
∆
×
∆
∆
) depends on the closed-
∆
V
∆
t
V
V
I
OQ
RESPONSE
V
STEP
∆
I
OESR
OQ
L
OESR
I
L
×
f
SW
RESPONSE
SS
is the
) of
11