M
Dual, Low-Dropout, 100mA Linear Regulators
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9
In preset voltage mode, internal, trimmed feedback
resistors set the MAX886_R outputs to 2.80V, the
MAX886_S outputs to 2.84V, and the MAX886_T out-
puts to 3.15V. Select this mode by connecting SET_ to
ground. If SET_ can’t be grounded in preset voltage
mode, limit impedances between SET_ and ground to
less than 100k
. Otherwise, spurious conditions could
cause the voltage at SET_ to exceed the 60mV Dual
Mode threshold.
In adjustable mode, select an output between 1.25V
and 5.5V 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 = 100k
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
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.
S hutdown
A low input on a
SHDN_
pin individually shuts down one
of the two outputs. In shutdown mode, the selected
pass transistor, control circuit, and all biases are turned
off. When both sections are turned off, the reference
and thermal shutdown are also turned off and the sup-
ply current is typically reduced to 0.16nA. Connect
SHDN_
to IN for normal operation. The MAX8866 output
voltages are actively discharged to ground when indi-
vidual regulators are shut down (see Typical Operating
Characteristics).
Current Limit
The MAX8865/MAX8866 include a current limiter for
each output section that monitors and controls the pass
transistor’s gate voltage, estimating the output current
and limiting it to about 220mA. For design purposes,
the current limit should be considered 120mA (min) to
320mA (max). The outputs can be shorted to ground for
an indefinite time period without damaging the part.
T hermal Overload Protec tion
Thermal overload protection limits total power dissipa-
tion in the MAX8865/MAX8866. When the junction tem-
perature exceeds T
J
= +170°C, the thermal sensor
sends a signal to the shutdown logic, turning off the
pass transistors and allowing the IC to cool. The ther-
mal sensor will turn the pass transistors 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
MAX8865/MAX8866 in the event of fault conditions.
Stressing the device with high load currents and high
input-output differential voltages (which result in elevat-
ed 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 T
J
= +150°C.
Operating Region and Power Dissipation
Maximum power dissipation of the MAX8865/MAX8866
depends on the thermal resistance of the case and cir-
cuit board, the temperature difference between the die
junction and ambient air, and the rate of air flow. The
power dissipation across the device is P = I
OUT
(V
IN
-
V
OUT
). The resulting maximum power dissipation is:
P
MAX
= (T
J
- T
A
) /
θ
J A
where (T
J
- T
A
) is the temperature difference between
the MAX8865/MAX8866 die junction and the surround-
ing air, and
θ
J A
is the thermal resistance of the pack-
age to the surrounding air (244°C/W).
R1 = R2
V
V
OUT
SET
_
_
1
MAX8865
MAX8866
SHDN_
OUT_
SET_
GND
IN
C
OUT
1
μ
F
C
IN
2μF
BATTERY
OUTPUT
VOLTAGE
R1
20pF
R2
R
L
Figure 2. Adjustable Output Using External Feedback
Resistors