Datasheet

calculate the input capacitance and the ESR required for
a specified ripple using the following equations:
ESR
IN
L
OUT
OUT
IN
Q SW
V
ESR
I
I
2
I D(1 D)
C
Vf
∆
=
∆

+


×−
=
∆×
where:
IN OUT OUT
L
IN SW
OUT
IN
(V V ) V
I
Vf L
V
D
V
−×
∆=
××
=
I
OUT
is the maximum output current of the converter and
f
SW
is the oscillator switching frequency (127kHz). For
example, at V
IN
= 48V, V
OUT
= 3.3V, the ESR and input
capacitance are calculated for the input peak-to-peak
ripple of 100mV or less, yielding an ESR and capacitance
value of 40mΩ and 100µF, respectively.
Low-ESR ceramic multilayer chip capacitors are
recommended for size-optimized application. For ceramic
capacitors, assume the contribution from ESR and capac-
itor discharge is equal to 10% and 90%, respectively.
The input capacitor must handle the RMS ripple current
without significant rise in the temperature. The maximum
capacitor RMS current occurs at approximately 50% duty
cycle. Ensure that the ripple specification of the input
capacitor exceeds the worst-case capacitor RMS ripple
current. Use the following equations to calculate the input
capacitor RMS current:
2
2
CRMS PRMS
AVGin
I II= −
where:
2
2
PK DC
PRMS PK
DC I I
OUT OUT
AVGin
IN
L
PK OUT
L
DC OUT
OUT
IN
D
I (I I )
3
V I
I
V
I
II
2
I
II
2
V
D
V
+×
=+×
×
=
×η
∆
= +
∆
= −
=
I
PRMS
is the input switch RMS current, I
AVGin
is the input
average current, and h is the converter efficiency.
The ESR of the aluminum electrolytic capacitor 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.
Output Filter Capacitor
The output capacitor (C
OUT
) forms double pole with
the inductor and a zero with its ESR. The MAX5090’s
internal fixed compensation is designed for a 100µF
capacitor, and the ESR must be from 20mΩ to 100mΩ. The
use of an aluminum or tantalum electrolytic capacitor is
recommended. See Table 2 to choose an output capacitor
for stable operation.
The output ripple is comprised of ∆V
OQ
(caused by the
capacitor discharge), and ∆V
OESR
(caused by the ESR
of the capacitor). Use low-ESR tantalum or aluminum
electrolytic capacitors at the output. Use the following
equations to calculate the contribution of output capaci-
tance and its ESR on the peak-to-peak output rip voltage:
OESR L
L
OQ
OUT SW
V I ESR
I
V
8C f
∆ =∆×
∆
∆≈
××
The MAX5090 has a programmable soft-start time (t
SS
).
The output rise time is directly proportional to the output
capacitor, output voltage, and the load. The output rise time
also depends on the inductor value and the current-limit
threshold. It is important to keep the output rise time at
startup the same as the soft-start time (t
SS
) to avoid output
Figure 3. Forward-Voltage Drop vs. Temperature of the Internal
Body Diode of MAX5090
0
100
200
300
400
500
600
700
800
-40 10025 125 150
TEMPERATURE (°C)
V
F_D1
(mV)
MAX5090A/B/C 2A, 76V, High-Efciency MAXPower
Step-Down DC-DC Converters
www.maximintegrated.com
Maxim Integrated │ 12