Tiny 500mA, 4MHz/2MHz Synchronous

Step-Down DC-DC Converters

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5

PIN

NAME

FUNCTION

1LX

Inductor Connection to the Internal Drains of the p-channel and n-channel MOSFETs. High impedance

during shutdown.

2, 5

GND

Ground. Connect these pins together directly under the IC.

3

OUT

Output Sense Input. Bypass with a ceramic capacitor as close as possible to pin 3 (OUT) and pin 2 (GND).

OUT is internally connected to the internal feedback network.

4

SHDN

Active-Low Shutdown Input. Connect to IN or logic-high for normal operation. Connect to GND or logic-low for

shutdown mode.

6IN

Supply Voltage Input. Input voltage range is 2.7V to 4.9V. Bypass with a ceramic capacitor as close as

possible to pin 6 (IN) and pin 5 (GND).

Pin Description

Detailed Description

The MAX8640Y/MAX8640Z step-down converters deliv-

er over 500mA to outputs from 0.8V to 2.5V. They utilize

a proprietary hysteretic PWM control scheme that

switches at up to 4MHz (MAX8640Z) or 2MHz

(MAX8640Y), allowing some trade-off between efficien-

cy and size of external components. At loads below

100mA, the MAX8640Y/MAX8640Z automatically switch

to pulse-skipping mode to minimize the typical quies-

cent current (24µA). Output ripple remains low at all

loads, while the skip-mode switching frequency

remains ultrasonic down to 1mA (typ) loads. Figure 1 is

the simplified functional diagram.

Control Scheme

A proprietary hysteretic PWM control scheme ensures

high efficiency, fast switching, fast transient response,

low output ripple, and physically tiny external compo-

nents. This control scheme is simple: when the output

voltage is below the regulation threshold, the error

comparator begins a switching cycle by turning on the

high-side switch. This switch remains on until the mini-

mum on-time expires and the output voltage is above

the regulation threshold or the inductor current is above

the current-limit threshold. Once off, the high-side

switch remains off until the minimum off-time expires

and the output voltage falls again below the regulation

threshold. During the off period, the low-side synchro-

nous rectifier turns on and remains on until either the

high-side switch turns on again or the inductor current

approaches zero. The internal synchronous rectifier

eliminates the need for an external Schottky diode.

Voltage-Positioning Load Regulation

The MAX8640Y/MAX8640Z utilize a unique feedback

network. By taking DC feedback from the LX node, the

usual phase lag due to the output capacitor is

removed, making the loop exceedingly stable and

allowing the use of very small ceramic output capacitors.

This configuration yields load regulation equal to the

inductor’s series resistance multiplied by the load current.

This voltage-positioning load regulation greatly reduces

overshoot during load transients, effectively halving the

peak-to-peak output-voltage excursions compared to tra-

ditional step-down converters. See the Load-Transient

Response in the Typical Operating Characteristics.

Shutdown Mode

Connecting SHDN to GND or logic low places the

MAX8640Y/MAX8640Z in shutdown mode and reduces

supply current to 0.1µA (typ). In shutdown, the control

circuitry and internal MOSFET switches turn off and LX

becomes high impedance. Connect SHDN to IN or

logic high for normal operation.

PWM

LOGIC

MAX8640Y

MAX8640Z

GND

LX

IN

SHDN

0.6V

OUT

Figure 1. Simplified Functional Diagram