MAX8863 查看數據表（PDF） - Maxim Integrated
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MAX8863 Datasheet PDF : 10 Pages
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MAX8863T/S/R,
MAX8864T/S/R
Low-Dropout, 120mA Linear Regulators
CIN
BATTERY 1µF
IN
OUT
MAX8863
MAX8864
SHDN
SET
GND
OUTPUT
VOLTAGE
R1 20pF
COUT RL
R2
1µF
Figure 2. Adjustable Output Using External Feedback
Resistors
Internal p-Channel Pass Transistor
The MAX8863/MAX8864 feature a 1.1Ω typical pMOSFET
pass transistor. This provides several advantages over
similar designs using PNP pass transistors, including
longer battery life.
The pMOSFET requires no base drive current, which
reduces quiescent current considerably. PNP-based
regulators waste considerable amounts of current 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 problems,
and consume only 80μA of quiescent current, whether in
dropout, light load, or heavy load applications (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:
VOUT = VSET (1 + R1 / R2)
where VSET = 1.25V. To simplify resistor selection:
= R1
R2
VOUT
VSET
− 1
Choose R2 = 100kΩ 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 the
VSET tolerance is typically less than ±25mV, the output can
be set using fixed resistors instead of trim pots. Connect
a 10pF to 25pF capacitor across R1 to compensate for
layout-induced parasitic capacitances.
In preset voltage mode, impedances between SET and
ground should be less than 100kΩ. 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 transistor, 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 voltage,
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 dissipation
in the MAX8863/MAX8864. When the junction temperature
exceeds TJ = +170°C, the thermal sensor 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
temperatures above +125°C) may cause a momentary
overshoot (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
maximum junction temperature rating of TJ = +150°C.
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