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MAX1763

1.5A, Low-Noise, 1MHz, Step-Up DC-DC Converter

Maxim Integrated 

1.5A, Low-Noise, 1MHz, Step-Up

DC-DC Converter

Table 2. Selecting the Operating Mode

CLK/SEL

0

MODE

Normal

operation

1

Forced PWM

External clock

Synchronized

500kHz to 1.2MHz PWM

FEATURES

High efficiency at all

loads. Fixed

frequency at all but

light loads.

Low noise, fixed

frequency at all loads.

Low noise, fixed

frequency at all loads.

Step-Up Converter

During DC-DC converter operation, the internal N-chan-

nel MOSFET switch turns on for the first part of each

cycle, allowing current to ramp up in the inductor and

store energy in a magnetic field. During the second

part of each cycle, the MOSFET turns off and inductor

current flows through the synchronous rectifier to the

output filter capacitor and the load. As the energy

stored in the inductor is depleted, the current ramps

down and the synchronous rectifier turns off, the N-

channel FET turns on, and the cycle repeats. At light

loads, depending on the CLK/SEL pin setting, output

voltage is regulated using either PWM or by switching

only as needed to service the load (Table 2).

Normal Operation

Pulling CLK/SEL low selects the MAX1763’s normal

operating mode. In this mode, the device operates in

PWM when driving medium to heavy loads, and at light

loads only, switches as needed. This optimizes efficien-

cy over the widest range of load conditions. In normal

operation mode, the output voltage regulates 1% higher

than in forced-PWM mode. See Efficiency vs. Load

Current in the Typical Operating Characteristics section.

Forced-PWM Operation

When CLK/SEL is high, the MAX1763 operates in a low-

noise forced-PWM mode. During forced-PWM opera-

tion, the MAX1763 switches at a constant frequency

(1MHz) and modulates the MOSFET switch pulse width

to control the power transferred per cycle and regulate

the output voltage. Switching harmonics generated by

fixed-frequency operation are consistent and easily fil-

tered. See the Noise Spectrum plot in the Typical

Operating Characteristics.

Synchronized-PWM Operation

In a variation of forced-PWM mode, the MAX1763 can

be synchronized to an external frequency by applying

a clock signal to CLK/SEL. This allows the user to

choose an operating frequency (from 500kHz to

1.2MHz) to avoid interference in sensitive applications.

For the most noise-sensitive applications, limit the

external synchronization signal duty cycle to less than

10% or greater than 90%. This eliminates the possibility

that noise from the power switching will coincide with

the synchronization signal. If the synchronization signal

edge falls on the power switching edge, a slight fre-

quency jitter may occur.

Synchronous Rectifier

The MAX1763 features an internal 130mΩ P-channel syn-

chronous rectifier to enhance efficiency. Synchronous

rectification provides a 5% efficiency improvement over

similar boost regulators that rely on diode rectifiers. In

PWM mode, the synchronous rectifier is turned on during

the second half of each switching cycle. In low-power

mode, an internal comparator turns on the synchronous

rectifier when the voltage at LX exceeds the boost regula-

tor output and turns it off when the inductor current drops

below 120mA. When setting output voltages greater than

4V, an external 0.5A Schottky diode must be connected

in parallel with the on-chip synchronous rectifier.

Low-Voltage Startup Oscillator

The MAX1763 uses a CMOS low-voltage startup oscil-

lator for a 1.1V guaranteed minimum startup input volt-

age. At startup, the low-voltage oscillator switches the

N-channel MOSFET until the output voltage reaches

2.15V. Above this level, the normal feedback and con-

trol circuitry take over. Once the device is in regulation,

it can operate down to 0.7V input because internal

power for the IC is derived from the output through the

OUT pin. Do not apply full system load until the output

exceeds 2.3V.

Shutdown, ONA, ONB

ONA and ONB turn the MAX1763 on or off. When ONA =

1 or ONB = 0, the device is on. When ONA = 0 and

ONB = 1, the device is off (Table 3). Logic high ON

control can be implemented by connecting ONB high

and using ONA for the control input. Momentary one-

pushbutton ON/OFF control is described in the

Applications Information section. Both ONA and ONB

have approximately (0.15 ✕ VOUT)V of hysteresis.

Reference

The MAX1763 has an internal 1.250V reference.

Connect a 0.22µF ceramic bypass capacitor to GND

within 0.2in (5mm) of the REF pin. REF can source up

to 50µA of external load current.

Gain Block

The MAX1763 gain block can function as a power-OK

comparator or can be used to build a linear regulator

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