Low-Dropout, 120mA Linear Regulators

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7

Internal P-Channel Pass Transistor

The MAX8863/MAX8864 feature a 1.1

Ω typical P-chan-

nel MOSFET pass transistor. This provides several

advantages over similar designs using PNP pass tran-

sistors, including longer battery life.

The P-channel MOSFET requires no base drive current,

which reduces quiescent current considerably. PNP-

based regulators waste considerable amounts of cur-

rent in dropout when the pass transistor saturates. They

also use high base-drive currents under large loads.

The MAX8863/MAX8864 do not suffer from these prob-

lems, and consume only 80µA of quiescent current,

whether in dropout, light load, or heavy load applica-

tions (see

Typical Operating Characteristics).

Output Voltage Selection

The MAX8863/MAX8864 feature Dual Mode operation:

they operate in either a preset voltage mode or an

adjustable mode.

In preset voltage mode, internal, trimmed feedback

resistors set the MAX886_R output to 2.80V, the

MAX886_S output to 2.84V, and the MAX886_T output to

3.15V. Select this mode by connecting SET to ground.

In adjustable mode, select an output between 1.25V

and 6.5V using two external resistors connected as a

voltage divider to SET (Figure 2). The output voltage is

set by the following equation:

Choose R2 = 100k

Ω to optimize power consumption,

accuracy, and high-frequency power-supply rejection.

The total current through the external resistive feedback

and load resistors should not be less than 10µA. Since

put can be set using fixed resistors instead of trim pots.

Connect a 10pF to 25pF capacitor across R1 to com-

pensate for layout-induced parasitic capacitances.

In preset voltage mode, impedances between SET and

ground should be less than 100k

Ω. Otherwise, spurious

conditions could cause the voltage at SET to exceed

the 60mV Dual Mode threshold.

Shutdown

A low input on the SHDN pin shuts down the

MAX8863/MAX8864. In shutdown mode, the pass tran-

sistor, control circuit, reference, and all biases are

turned off, reducing the supply current to typically

0.1nA. Connect SHDN to IN for normal operation. The

MAX8864 output voltage is actively discharged to

ground when the part is placed in shutdown (see

Typical Operating Characteristics).

Current Limit

The MAX8863/MAX8864 include a current limiter that

monitors and controls the pass transistor’s gate volt-

age, estimating the output current and limiting it to

about 280mA. For design purposes, the current limit

should be considered 120mA (min) to 420mA (max).

The output can be shorted to ground for an indefinite

time period without damaging the part.

Thermal Overload Protection

Thermal overload protection limits total power dissipa-

tion in the MAX8863/MAX8864. When the junction tem-

sends a signal to the shutdown logic, turning off the

pass transistor and allowing the IC to cool. The thermal

sensor will turn the pass transistor on again after the

IC’s junction temperature typically cools by 20°C,

resulting in a pulsed output during continuous thermal

overload conditions.

Thermal overload protection is designed to protect the

MAX8863/MAX8864 in the event of fault conditions.

Stressing the device with high load currents and high

input-output differential voltages (which result in die tem-

peratures above +125°C) may cause a momentary over-

shoot (2% to 8% for 200ms) when the load is completely

removed. This can be remedied by raising the minimum

load current from 0µA (+125°C) to 100µA (+150°C). For

continuous operation, do not exceed the absolute maxi-

MAX8863

MAX8864

OUT

SET

GND

IN

SHDN

1

µF

1

µF

BATTERY

OUTPUT

VOLTAGE

R1 20pF

R2

Figure 2. Adjustable Output Using External Feedback

Resistors

R1 R2

V

V

1

OUT

SET

=−

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