MAX5035DASA-T Maxim Integrated Products, MAX5035DASA-T Datasheet - Page 12

• Manufacturer Part Number: MAX5035DASA-T
• Description: DC/DC Switching Converters 1A 76V MAXPower Step-Down
• Manufacturer: Maxim Integrated Products

1A, 76V, High-Efficiency MAXPower
Step-Down DC-DC Converter
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 ΔV
capacitor discharge) and ΔV
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
12
where :
PRMS
Z
) occurs between 20kHz to 40kHz. Use the following
______________________________________________________________________________________
is the input switch RMS current, I
I
I
I
and D
PRMS
PK
AVGIN
=
f
I
Z
CRMS
I
OUT
=
=
=
=
V
2
V
V
(
OUT
I
+
O O UT
IN
×
PK
=
V
Δ
π
IN
2
2
I
I
PRMS
L
+
×
×
×
,
I
I
I
DC
OUT
DC
η
Output Filter Capacitor
C
OUT
2
2
1
OESR
=
−
Z
+
I
. Capacitors with 100mΩ
I
OUT
(
AVGIN
×
I
PK
ESR
(caused by the ESR
−
×
OQ
I
Δ
2 2
DC
OUT
2
I
L
(caused by the
)
)
AVGIN
×
D
3
is the
capacitance and the ESR required for a specified rip-
ple using the following equations:
The MAX5035 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 each from the output
capacitance discharge and the ESR drop, use the fol-
lowing equations to calculate the required ESR and
capacitance 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 typically.
Proper PCB layout is essential. Minimize ground noise
by connecting the anode of the Schottky rectifier, the
input bypass capacitor ground lead, and the output fil-
ter capacitor ground lead to a single point (“star”
STEP
C
is the load step and t
C
SS
OUT
OUT
RESPONSE
ESR
ESR
to avoid output overshoot. The output
=
PCB Layout Considerations
≈
OUT
OUT
I
2 2 .
STEP
=
×
=
) depends on the closed-
×
Δ
Δ
Δ
Δ
V
Δ
I
t
V
V
V
STEP
RESPONSE
Δ
OQ
I
OESR
OESR
OQ
L
I
L
×
f
SW
RESPONSE
SS
is the
) of