3.3V/5V/Adjustable-Output,

Step-Up DC-DC Converters

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5

_______________Detailed Description

Operating Principle

The MAX756/MAX757 combine a switch-mode regulator

with an N-channel MOSFET, precision voltage reference,

and power-fail detector in a single monolithic device.

The MOSFET is a “sense-FET” type for best efficiency,

and has a very low gate threshold voltage to ensure

start-up under low-battery voltage conditions (1.1V typ).

Pulse-Frequency

Modulation Control Scheme

A unique minimum off time, current-limited, pulse-frequen-

cy modulation (PFM) control scheme is a key feature of

the MAX756/MAX757. This PFM scheme combines the

advantages of pulse-width modulation (PWM) (high output

power and efficiency) with those of a traditional PFM

pulse-skipper (ultra-low quiescent currents). There is no

oscillator; at heavy loads, switching is accomplished

through a constant peak-current limit in the switch, which

allows the inductor current to self-oscillate between this

peak limit and some lesser value. At light loads, switching

frequency is governed by a pair of one-shots, which set a

minimum off-time (1µs) and a maximum on-time (4µs).

The switching frequency depends on the load and the

input voltage, and can range as high as 500kHz.

The peak switch current of the internal MOSFET power

switch is fixed at 1A ±0.2A. The switch's on resistance

is typically 0.5

Ω, resulting in a switch voltage drop

Conventional PWM converters generate constant-fre-

quency switching noise, whereas this architecture pro-

duces variable-frequency switching noise. However,

the noise does not exceed the switch current limit times

the filter-capacitor equivalent series resistance (ESR),

unlike conventional pulse-skippers.

Voltage Reference

The precision voltage reference is suitable for driving

external loads such as an analog-to-digital converter.

It has guaranteed 250µA source-current and 20µA

sink-current capability. The reference is kept alive

even in shutdown mode. If the reference drives an

external load, bypass it with 0.22µF to GND. If the ref-

erence is unloaded, bypass it with at least 0.1µF.

Control-Logic Inputs

The control inputs (3/5, SHDN) are high-impedance

MOS gates protected against ESD damage by normally

reverse-biased clamp diodes. If these inputs are dri-

ven from signal sources that exceed the main supply

voltage, the diode current should be limited by a series

resistor (1M

Ω suggested). The logic input threshold

level is the same (approximately 1V) in both 3.3V and

5V modes. Do not leave the control inputs floating.

__________________Design Procedure

Output Voltage Selection

The MAX756 output voltage can be selected to 3.3V or

5V under logic control, or it can be left in one mode or

the other by tying 3/5 to GND or OUT. Efficiency varies

depending upon the battery and the load, and is typi-

cally better than 80% over a 2mA to 200mA load range.

The device is internally bootstrapped, with power

derived from the output voltage (via OUT). When the

output is set at 5V instead of 3.3V, the higher internal

supply voltage results in lower switch-transistor on

resistance and slightly greater output power.

Bootstrapping allows the battery voltage to sag to less

than 1V once the system is started. Therefore, the bat-

If the battery voltage exceeds the programmed output

voltage, the output will follow the battery voltage. In

many systems this is acceptable; however, the output

voltage must not be forced above 7V.

The output voltage of the MAX757 is set by two resis-

tors, R1 and R2 (Figure 1), which form a voltage divider

between the output and the FB pin. The output voltage

is set by the equation:

To simplify resistor selection:

Since the input bias current at FB has a maximum

value of 100nA, large values (10k

Ω to 200kΩ) can be

used for R1 and R2 with no significant loss of accuracy.

For 1% error, the current through R1 should be at least

100 times FB’s bias current.

Low-Battery Detection

The MAX756/MAX757 contain on-chip circuitry for low-

battery detection. If the voltage at LBI falls below the reg-

ulator’s internal reference voltage (1.25V), LBO (an open-

drain output) sinks current to GND. The low-battery mon-

itor's threshold is set by two resistors, R3 and R4 (Figure

1), which forms a voltage divider between the input volt-

age and the LBI pin. The threshold voltage is set by R3

and R4 using the following equation: