MAX5090ASLASHBSLASHC datasheet(11/17 Pages) MAXIM

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Thermal-overload protection is intended to protect the

MAX5090 in the event of a fault condition. For normal

circuit operation, do not exceed the absolute maximum

junction temperature rating of TJ = +150°C.

Setting the Output Voltage

The MAX5090A/MAX5090B have preset output voltages

of 3.3V and 5.0V, respectively. Connect FB to VOUT for

the preset output voltage (Figure 1).

The MAX5090C offers an adjustable output voltage. Set

the output voltage with a resistive divider connected from

the circuit’s output to ground (Figure 2). Connect the

center node of the divider to FB. Choose R4 less than

15kΩ, then calculate R3 as follows:

OUT

1.228)

1.228

−

The MAX5090 features internal compensation for

optimum closed-loop bandwidth and phase margin.

Because of the internal compensation, the output must

be sensed immediately after the primary LC.

Inductor Selection

The MAX5090 is a fixed-frequency converter with fixed

internal frequency compensation. The internal fixed

compensation assumes a 100µH inductor and 100µF

output capacitor with 50mΩ ESR. It relies on the location

of the double LC pole and the ESR zero frequency for

proper closed-loop bandwidth and the phase margin at the

closed-loop unity-gain frequency. See Table 2 for proper

component values. Usually, the choice of an inductor is

guided by the voltage difference between VIN and VOUT,

the required output current, and the operating frequency

of the circuit. However, use the recommended inductors

in Table 2 to ensure stable operation with optimum band-

width.

Use an inductor with a maximum saturation current rating

greater than or equal to the maximum peak current limit

(5A). Use inductors with low DC resistance for a higher

efficiency converter.

Selecting a Rectifier

The MAX5090 requires an external Schottky rectifier as

a freewheeling diode. Connect this rectifier close to the

device using short leads and short PCB traces. The recti-

fier diode must fully conduct the inductor current when the

power FET is off to have a full rectifier function. 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, VIN. 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-voltagedrop(VF)lessthan0.55Vand0.45Vat+25°C

and +125°C, respectively, and at maximum load current to

avoid forward biasing of the internal parasitic body diode

(LX to ground). See Figure 3 for forward-voltage drop vs.

temperature of the internal body diode of the MAX5090.

Internalparasiticbody-diodeconductionmaycauseimproper

operation, 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 reflecting back

to the source dictate the capacitance requirement. The

MAX5090 high-switching frequency allows the use of

smaller-value input capacitors.

The input ripple is comprised of ∆VQ (caused by the

capacitor discharge) and ∆VESR (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,

Table 1. Diode Selection

VIN (V)

DIODE PART

NUMBER

MANUFACTURER

6.5 to 36

B340LB

Diodes Inc.

RB051L-40

Central Semiconductor

MBRS340T3

ON Semiconductor

6.5 to 56

MBRM560

Diodes Inc.

RB095B-60

Central Semiconductor

MBRD360T4

ON Semiconductor

6.5 to 76

50SQ80

PDS5100H

Diodes Inc.

MAX5090A/B/C

2A, 76V, High-Efficiency MAXPower

Step-Down DC-DC Converters

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