MAX8640YELT08 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX8640YELT08

Tiny 500mA, 4MHz/2MHz Synchronous Step-Down DC-DC Converters

Maxim Integrated 

MAX8640Y/MAX8640Z

Tiny 500mA, 4MHz/2MHz Synchronous

Step-Down DC-DC Converters

Pin Description

PIN NAME

FUNCTION

1

LX 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.

Active-Low Shutdown Input. A logic-low on SHDN disables the step-down DC-DC and resets the logic. A logic-

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SHDN high on SHDN enables the step-down DC-DC. Ensure that SHDN is low for ≥ 10µs (tSHDN) after VIN rises above its

undervoltage lockout threshold (UVLO) to reset the logic. See the Shutdown Mode section for more information.

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IN

Supply Voltage Input. Input voltage range is 2.7V to 5.5V. Bypass with a ceramic capacitor as close as

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

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 (28µA). 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

A logic-low on SHDN places the MAX8640Y/MAX8640Z

in shutdown mode by disabling the step-down DC-DC

and resetting its logic. In shutdown mode, the supply

current (ICC) is reduced to 0.01µA typical. Additionally,

the power MOSFETs between IN, LX, and GND

(Figure 1) are open such that LX is high impedance.

Ensure that SHDN is low for ≥ 10µs (tSHDN) after VIN

rises above its undervoltage lockout threshold (UVLO)

to reset the logic. In the majority of systems, this tSHDN

requirement is fulfilled naturally because the upstream

logic controlling SHDN is powered off of the same VIN

as the MAX8640Y/MAX8640Z. However, systems that

want an always on regulator without the burden of

enable/disable logic can use an R and C circuit on

SHDN as shown in Figure 2.

Maxim Integrated

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