Detailed Description

The MAX1595 charge pump provides either a 3.3V or 5V

regulated output. It delivers a maximum 125mA load cur-

rent. In addition, to boost regulating from a lower supply,

it is also capable of buck regulating from supplies that

exceed the regulated output by a diode drop or more.

Designed specifically for compact applications, a com-

plete regulator circuit requires only three small external

capacitors. An innovative control scheme provides con-

stant frequency operation from medium to heavy loads,

while smoothly transitioning to low-power mode at light

loads to maintain optimum efficiency. In buck mode,

switch S1 (in Figure 1) is switched continuously to IN,

while switch S2 alternates between IN and OUT. An

amount of charge proportional to the difference between

the output voltage and the supply voltage is stored on

tion point is reached. Maximum output ripple is propor-

tional to the difference between the supply voltage and

the output voltage, as well as to the ratio of the transfer

The MAX1595 consists of an error amplifier, a 1.23V

bandgap reference, internal resistive feedback network,

oscillator, high-current MOSFET switches, and shutdown

and control logic. Figure 1 shows an idealized unregulat-

ed charge-pump voltage doubler. The oscillator runs at a

50% duty cycle. During one half of the period, the trans-

than doubling the input voltage, the MAX1595 provides a

regulated output voltage of either 3.3V or 5.0V.

Shutdown

Driving SHDN low places the device in shutdown mode.

The device draws 0.1µA of supply current in this mode.

When driven high, the MAX1595 enters a soft-start

mode. Soft-start mode terminates when the output volt-

age regulates, or after 2ms, whichever comes first. In

shutdown, the output disconnects from the input.

Undervoltage Lockout

The MAX1595 has an undervoltage-lockout that deacti-

vates the devices when the input voltage falls below 1.6V.

Below UVLO, hysteresis holds the device in shutdown

until the input voltage rises 40mV above the lockout

threshold.

Applications Information

Using white LEDs to backlight LCDs is an increasingly

popular approach for portable information devices

(Figure 2). Because the forward voltage of white LEDs

IN

S1

S2

OUT

OSC

Figure 1. Unregulated Voltage Doubler

Regulated 3.3V/5.0V Step-Up/Step-Down

Charge Pump

4

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Pin Description

PIN

MAX1595

µMAX

MAX1595

THIN QFN

NAME

FUNCTION

1

12

AOUT

Analog Power and Sense Input for Error Amplifier/Comparator. Connect to OUT at

output filter capacitor.

21

SHDN

Shutdown Input. When SHDN = low, the device turns off; when SHDN = high, the device

activates. In shutdown, OUT is disconnected from IN.

3

2, 3

IN

Input Supply. Can range from 1.8V to 5.5V. Bypass to GND with a 1µF capacitor.

4

4

GND

Ground

5

5, 6

PGND

Power Ground

6

7, 8

CXN

Negative Terminal of the Charge-Pump Transfer Capacitor

7

9

CXP

Positive Terminal of the Charge-Pump Transfer Capacitor

8

10, 11

OUT

Output. Bypass to GND with output capacitor filter.