Low-Voltage, Low-Dropout Linear Regulators

with External Bias Supply

8

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Pin Description

PIN

MAX1982

MAX1983

NAME

FUNCTION

1

1

BIAS

Bias Voltage. Input voltage to the control circuitry. BIAS powers all control blocks and the gate

of the pass transistor. Bypass BIAS to GND with a 0.1µF ceramic capacitor.

2

2

GND

Ground

33

SHDN

Shutdown Control Input. Drive

SHDN low to enter the low-power shutdown state. Connect

SHDN to BIAS for normal operation.

4—

PGOOD

Power-Good Output. The open-drain PGOOD output goes high at least 1ms after the output

voltage has attained regulation. PGOOD asserts low 10µs after the output falls by 10%.

—

4

ADJ

Feedback for Adjustable Output. Connect ADJ to the midpoint of a resistor-divider between

OUT and GND for an adjustable output voltage between 0.8V and 2.0V.

5

5

OUT

Regulator Output. Bypass OUT to GND with a 10µF to 22µF low-ESR ceramic capacitor.

66

IN

Internal Pass Transistor Input. IN connects to the drain of the internal power switch. Bypass IN

to GND with a 10µF ceramic capacitor.

Detailed Description

The MAX1982/MAX1983 are low-voltage, low-dropout

linear regulators with an external bias supply input (see

Figures 1 and 2). BIAS is powered by a 4.5V to 5.5V

supply that is commonly available in laptop and desk-

top computers. The 5V bias supply drives the gate of

the internal pass transistor, while a lower voltage input

at the drain of the transistor (IN) is regulated to provide

efficiency. These devices are ideal for applications that

require low-voltage outputs from low-voltage inputs.

The MAX1982 delivers 1.2V (±3%) at 300mA from an

input voltage of 1.25V to 5.5V. The MAX1983 delivers

an adjustable output voltage from 0.8V to 2V.

The MAX1982 features an open-drain PGOOD output

that transitions high 1ms after the output reaches regu-

lation. PGOOD goes low within 10µs of the output

falling out of regulation by 120mV.

Both devices feature current- and thermal-limiting cir-

cuitry that protect the MAX1982/MAX1983 from dam-

age during fault conditions.

Bias Supply Input

The BIAS input powers the control circuitry and pro-

vides the pass transistor gate drive. Power BIAS from a

well-regulated 5V (

±10%) supply. Current drawn from

the BIAS supply remains relatively constant with varia-

Characteristics). Bypass BIAS with a 0.1µF or greater

capacitor as close to the device as possible. The bias

supply current remains low (250µA) when the

MAX1982/MAX1983 are in dropout.

Power-Supply Input

IN connects to the drain of the internal N-channel

power transistor. IN may be as low as 1.25V, minimizing

power dissipation. Bypass IN with a 10µF or greater

capacitor as close to the device as possible.

Shutdown

The MAX1982/MAX1983 feature a low-power shutdown

mode that reduces quiescent current drawn to 1µA

(typ), and BIAS current consumption to less than 1µA

(typ). Driving SHDN low disables the voltage reference,

error amplifier, gate-drive circuitry, and pass transistor

(see Figure 2), and the device output enters a high-

impedance state. Connect SHDN to BIAS for normal

operation.

PGOOD Output

The MAX1982 provides an open-drain PGOOD output

that goes high 1ms (min) after the output attains regula-

tion (within 10% of the nominal output voltage). PGOOD

transitions low 10µs after the output falls out of regula-

tion by 120mV, or when the device enters shutdown.

Connect a pullup resistor from PGOOD to BIAS for a

logic-level output. Use a 100k

Ω resistor to minimize

current consumption.