Datasheet

The MAX5033 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 differ-
ence 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:
IN OUT
OUTMAX SW
(V V ) D
L
0.3 I f
−×
=
××
where: D = V
OUT
/V
IN
, I
OUTMAX
is the maximum output
current required, and f
SW
is the operating frequency of
125kHz. Use an inductor with a maximum saturation cur-
rent rating equal to at least the peak switch current limit
(I
LIM
). Use inductors with low DC resistance for higher
efficiency.
Selecting a Rectier
The MAX5033 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 efficiency. 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 capaci-
tor. The switching frequency, peak inductor current, and
the allowable peak-to-peak voltage ripple that reflects
back to the source dictate the capacitance requirement.
The MAX5033 high switching frequency allows the use of
smaller-value input capacitors.
The input ripple is comprised of ΔV
Q
(caused by the
capacitor discharge) and ΔV
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,
calculate the input capacitance and the ESR required for
a specified ripple using the following equations:
ESR
IN
L
OUT
OUT
IN
Q SW
IN OUT OUT
L
IN SW
OUT
V
ESR
I
I
2
I D(1 D)
C
Vf
where :
(V V ) V
I
Vf L
V
D
∆
=
∆

+


×−
=
∆×
−×
∆=
××
=
IN
V
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 and V
OUT
= 3.3V, the ESR and
input capacitance are calculated for the input peak-topeak
ripple of 100mV or less, yielding an ESR and capacitance
value of 130mΩ and 27μF, respectively.
Low-ESR, ceramic, multilayer chip capacitors are rec-
ommended 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 temperature. The maximum
capacitor RMS current occurs at about 50% duty cycle.
Table 1. Diode Selection
V
IN
(V) DIODE PART NUMBER MANUFACTURER
7.5 to
36
15MQ040N IR
B240A Diodes Incorporated
B240 Central Semiconductor
MBRS240, MBRS1540 ON Semiconductor
7.5 to
56
30BQ060 IR
B360A Diodes Incorporated
CMSH3-60 Central Semiconductor
MBRD360, MBR3060 ON Semiconductor
7.5 to
76
50SQ100, 50SQ80 IR
MBRM5100 Diodes Incorporated
MAX5033 500mA, 76V, High-Efciency, MAXPower
Step-Down DC-DC Converter
www.maximintegrated.com
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