MAX688 datasheet(6/12 Pages) MAXIM

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High-Accuracy, Low-Dropout

Linear Regulators

_______________________________________________________________________________________

______________________________________________________________Pin Description

NAME

FUNCTION

Positive input voltage, 2.7V to 11.0V

(MAX687)

ON activates the regulator when pulsed high. In order for the regulator to remain on, ON must remain high

until VOUT exceeds the internal shutdown threshold voltage. The MAX687 is shut down when VOUT < 2.96V,

and remains latched off until ON is pulsed high. When powered up, the MAX687 does not start up until ON

is pulsed high. Connect to VIN if not used.

SHDN

(MAX688/

MAX689)

SHDN is a three-level input that controls the mode of operation. The device is on when V SHDN > 1.25V.

The output is disabled and the supply current is reduced (IIN < 25µA, standby mode) when V SHDN < 1.2V,

and is fully off (IIN < 1µA, shutdown mode) when V SHDN < 0.2V. Connect SHDN to IN if the shutdown func-

tion is not used. PFO is low when SHDN is low (below 0.2V).

PFO

Power-Fail Output. PFO trips when VOUT is 170mV below nominal VOUT at IBASE = 1mA. PFO sources and

sinks current in the MAX687, but is an open drain in the MAX688/MAX689. When shut down, PFO is always

low and sinks current. Leave PFO open if not used.

BASE

Base drive for the external PNP transistor. Current limiting is controlled using a resistor (RB) connected

between BASE and BLIM. For maximum output current, connect BASE to BLIM.

BLIM

Base current limit. A resistor (RB) connected between BASE and BLIM sets the maximum base drive to the

PNP transistor: IBASE ≤ 0.1V / (RB + 5Ω). This limits quiescent current rise during dropout and also pro-

vides current limiting without using a current-sense resistor in the collector of the external PNP, thus not

impacting dropout. Output current limiting accuracy depends on how well the external PNP beta (hFE) is

controlled. Connect to BASE if not used.

OUT

Regulator Output. MAX687/MAX688, 3.3V; MAX689, 3.0V.

GND

Ground

Compensation Capacitor. Connect a non-polarized capacitor (10nF to 100nF) from CC to GND.

_______________Detailed Description

The MAX687/MAX688/MAX689 are precision low-

dropout linear regulators employing external PNP tran-

sistors to achieve a wide range of output currents at

voltages of 3.0V or 3.3V. The maximum base current

for the PNP can be limited using a resistor. Limiting the

base drive keeps high currents from being wasted

when the device is in dropout (e.g., at low input volt-

ages), and limits the regulator’s output current. The

dropout voltage is limited only by the PNP transistor’s

VCE(SAT). The Power-Fail Output (PFO) goes low when

the output voltage drops 170mV below the nominal

level.

The three devices differ in their output voltages, in their

shutdown-control functions, and in PFO’s output drive

(see Table 1).

The MAX687’s output voltage is internally monitored; a

falling VOUT is signaled by PFO going low. As the out-

put falls further, the MAX687 automatically enters a low-

power shutdown mode, where the base drive to the

external PNP is cut off. PFO trips at a minimum of

110mV below the nominal VOUT, and shutdown occurs

by 2.96V. PFO is guaranteed to trip before the device

Table 1. Device Functions

FUNCTION

MAX687

MAX688

MAX689

VOUT

3.3V

3.0V

Shutdown

Control

Automatic at low

VOUT. Latched.

Use ON to start.

External

SHDN

External

SHDN

PFO

Sources & Sinks

Open Drain

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