MAX8863T/S/R, MAX8864T/S/R
Low-Dropout, 120mA Linear Regulators
7
Internal P-Channel Pass Transistor
The MAX8863/MAX8864 feature a 1.1Ω 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:
V
OUT
= V
SET
(1 + R1 / R2)
where V
SET
= 1.25V. To simplify resistor selection:
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 V
SET
tolerance is typically less than ±25mV, the out-
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 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 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-
perature exceeds T
J
= +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 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-
mum junction temperature rating of T
J
= +150°C.
