1A, 76V, High-Efficiency MAXPower

Step-Down DC-DC Converter

______________________________________________________________________________________

11

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-

current, and the operating frequency of the circuit. Use

an inductor with a minimum value given by:

where:

tor with a maximum saturation current rating equal to at

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

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

°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

capacitor discharge) and

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:

input capacitance are calculated for the input peak-to-

peak ripple of 100mV or less yielding an ESR and

capacitance value of 80m

Ω 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

=

+

∆

IN

OUT

C

I

2

⎛

⎝⎜

⎞

⎠⎟

=

× D

DD

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

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