Datasheet

1A, 76V, High-Efficiency MAXPower
Step-Down DC-DC Converter
The MAX5035 features internal compensation for opti-
mum closed-loop bandwidth and phase margin. With
the preset compensation, it is strongly advised to sense
the output immediately after the primary LC.
Inductor Selection
The choice of an inductor is guided by the voltage dif-
ference between V
IN
and V
OUT
, the required output
current, and the operating frequency of the circuit. Use
an inductor with a minimum value given by:
where:
I
OUTMAX
is the maximum output current required, and
f
SW
is the operating frequency of 125kHz. Use an induc-
tor with a maximum saturation current rating equal to at
least the peak switch current limit (I
LIM
). Use inductors
with low DC resistance for higher efficiency.
Selecting a Rectifier
The MAX5035 requires an external Schottky rectifier as
a freewheeling diode. Connect this rectifier close to the
device using short leads and short PC board traces.
Choose a rectifier with a continuous current rating
greater than the highest expected output current. Use a
rectifier with a voltage rating greater than the maximum
expected input voltage, V
IN
. Use a low forward-voltage
Schottky rectifier for proper operation and high efficien-
cy. Avoid higher than necessary reverse-voltage
Schottky rectifiers that have higher forward-voltage
drops. Use a Schottky rectifier with forward-voltage
drop (V
FB
) less than 0.45V at +25°C and maximum load
current to avoid forward biasing of the internal body
diode (LX to ground). Internal body diode conduction
may cause excessive junction temperature rise and
thermal shutdown. Use Table 1 to choose the proper
rectifier at different input voltages and output current.
Input Bypass Capacitor
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input
capacitor. The switching frequency, peak inductor cur-
rent, and the allowable peak-to-peak voltage ripple that
reflects back to the source dictate the capacitance
requirement. The MAX5035 high switching frequency
allows the use of smaller-value input capacitors.
The input ripple is comprised of ∆V
Q
(caused by the
capacitor discharge) and ∆V
ESR
(caused by the ESR of
the capacitor). Use low-ESR aluminum electrolytic
capacitors with high ripple-current capability at the input.
Assuming that the contribution from the ESR and capaci-
tor discharge is equal to 90% and 10%, respectively, cal-
culate the input capacitance and the ESR required for a
specified ripple using the following equations:
I
OUT
is the maximum output current of the converter
and f
SW
is the oscillator switching frequency (125kHz).
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 80mΩ and 51µF, respectively.
Low-ESR, ceramic, multilayer chip capacitors are recom-
mended for size-optimized application. For ceramic
capacitors, assume the contribution from ESR and capaci-
tor discharge is equal to 10% and 90%, respectively.
The input capacitor must handle the RMS ripple current
without significant rise in temperature. The maximum
capacitor RMS current occurs at about 50% duty cycle.
ESR
V
I
I
IN
ESR
OUT
=
+
∆
∆
LL
IN
OUT
C
I
2
⎛
⎝
⎜
⎞
⎠
⎟
=
×
DDD
Vf
where
QSW
()
:
1−
×∆
()
,∆I
VV V
Vf L
L
IN OUT OUT
IN SW
=
−×
××
D
V
V
OUT
IN
=
D
V
V
OUT
IN
=
L
VV D
If
IN OUT
OUTMAX SW
=
−×
××
()
.03
V
IN
(V) DIODE PART NUMBER MANUFACTURER
15MQ040N IR
B240A Diodes, Inc.
B240 Central Semiconductor
7.5 to 36
MBRS240, MBRS1540 ON Semiconductor
30BQ060 IR
B360A Diodes, Inc.
CMSH3-60 Central Semiconductor
7.5 to 56
MBRD360, MBR3060 ON Semiconductor
50SQ100, 50SQ80 IR
7.5 to 76
MBRM5100 Diodes, Inc.
Table 1. Diode Selection
MAX5035
Maxim Integrated
11