1A, 76V, High-Efficiency MAXPower

Step-Down DC-DC Converter

10

______________________________________________________________________________________

Detailed Description

The MAX5035 step-down DC-DC converter operates

from a 7.5V to 76V input voltage range. A unique volt-

age-mode control scheme with voltage feed-forward

and an internal switching DMOS FET provides high effi-

ciency over a wide input voltage range. This pulse-

width modulated converter operates at a fixed 125kHz

switching frequency. The device also features automat-

ic pulse-skipping mode to provide low quiescent cur-

rent and high efficiency at light loads. Under no load,

the MAX5035 consumes only 270µA, and in shutdown

mode, consumes only 10µA. The MAX5035 also fea-

tures undervoltage lockout, hiccup mode output short-

circuit protection, and thermal shutdown.

Shutdown Mode

Drive ON/

OFF to ground to shut down the MAX5035.

Shutdown forces the internal power MOSFET off, turns

rent to 10µA (typ). The ON/

OFF rising threshold is

1.69V (typ). Before any operation begins, the voltage at

ON/

OFF must exceed 1.69V (typ). The ON/OFF input

has 100mV hysteresis.

Undervoltage Lockout (UVLO)

Use the ON/

OFF function to program the UVLO thresh-

old at the input. Connect a resistive voltage-divider

shown in Figure 1. Calculate the threshold value by

using the following formula:

and 13V for the output voltages of 3.3V, 5V, and 12V,

respectively. The recommended value for R2 is less

than 1M

Ω.

If the external UVLO threshold-setting divider is not

used, an internal undervoltage-lockout feature monitors

ON/

OFF.

Boost High-Side Gate Drive (BST)

Connect a flying bootstrap capacitor between LX and

BST to provide the gate-drive voltage to the high-side

N-channel DMOS switch. The capacitor is alternately

charged from the internally regulated output voltage VD

and placed across the high-side DMOS driver. Use a

0.1µF, 16V ceramic capacitor located as close to the

device as possible.

On startup, an internal low-side switch connects LX to

ground and charges the BST capacitor to VD. Once the

BST capacitor is charged, the internal low-side switch

is turned off and the BST capacitor voltage provides

the necessary enhancement voltage to turn on the

high-side switch.

Thermal-Overload Protection

The MAX5035 features integrated thermal overload pro-

tection. Thermal overload protection limits total power

dissipation in the device, and protects the device in the

event of a fault condition. When the die temperature

exceeds +160°C, an internal thermal sensor signals the

shutdown logic, turning off the internal power MOSFET

and allowing the IC to cool. The thermal sensor turns the

internal power MOSFET back on after the IC’s die tem-

perature cools down to +140°C, resulting in a pulsed

output under continuous thermal overload conditions.

Applications Information

Setting the Output Voltage

The MAX5035A/B/C have preset output voltages of 3.3V,

5.0V, and 12V, respectively. Connect FB to the preset

output voltage (see the

Typical Operating Circuit).

The MAX5035D offers an adjustable output voltage. Set

the output voltage with a resistive voltage-divider con-

nected from the circuit’s output to ground (Figure 1).

Connect the center node of the divider to FB. Choose

R4 less than 15k

Ω, then calculate R3 as follows:

R

V

R

OUT

3

122

122

4

=

−

×

(.

)

.

V

R

R

V

UVLO TH

()

.

=+

⎛

⎝⎜

⎞

⎠⎟

×

1

1

2

185

MAX5035D

GND

BST

LX

SGND

FB

D1

50SQ100

VD

100

μH

5V

7.5V TO 76V

68

μF

0.1

μF

0.1

μF

68

μF

R1

R2

R3

41.2k

Ω

R4

13.3k

Ω

ON/OFF

Figure 1. Adjustable Output Voltage