Datasheet

MAX1982/MAX1983
Current Limit
The MAX1982/MAX1983 limit the output current to
600mA (typ) in the event of an overload or output short
circuit. The current limit prevents damage to the inter-
nal power transistor, but the device can enter thermal
shutdown if the power dissipation is great enough to
increase the die temperature above +160°C (see the
Thermal-Overload Protection section).
Thermal-Overload Protection
Thermal-overload protection limits the power dissipa-
tion in the MAX1982/MAX1983. When the die tempera-
ture exceeds +160°C, the pass transistor turns off,
allowing the device to cool. Normal operation resumes
when the die temperature cools by 20°C. A continuous
thermal-overload condition results in a pulsed output.
For continuous operation, do not exceed a junction
temperature of +150°C.
Applications Information
Output Voltage Selection
The MAX1982 output is fixed at 1.2V. The MAX1983
provides an adjustable output (0.8V to 2.0V). Connect
ADJ to a resistive voltage-divider between OUT and
GND as shown in Figure 4. Set the output voltage using
the following equation:
Set R2 at 40kΩ and choose R1 to achieve the desired
output voltage. Set V
IN
to higher than (V
OUT
+ 400mV)
to meet the dropout voltage requirement (see the
Input/Output (Dropout) Voltage section).
To ensure stability over the specified input voltage
range, the minimum output capacitance must be 10µF
with a maximum ESR of 35mΩ.
Operating Region and Power Dissipation
The maximum power dissipation of the MAX1982/
MAX1983 depends on the thermal resistance of the 6-pin
SOT23 package and the circuit board, the temperature
difference between the die and ambient air, and the rate
of airflow. The power dissipated in the device is:
The resulting maximum power dissipation is:
where T
J(MAX)
is the maximum junction temperature
(+150°C) and T
A
is the ambient temperature, θ
JC
is the
thermal resistance from the die junction to the package
case, and θ
CA
is the thermal resistance from the case
through the PC board, copper traces, and other materi-
als to the surrounding air. For optimum power dissipa-
tion, use a large ground plane with good thermal
contact to GND, and use wide input and output traces.
When 1 square inch of copper is connected to the
device, the maximum allowable power dissipation of a
6-pin SOT23 package is 696mW. The maximum power
dissipation is derated by 8.7mW/°C above T
A
= +70°C.
Extra copper on the PC board increases thermal mass,
and reduces thermal resistance of the board. Refer to
the MAX1982/MAX1983 EV kit for a layout example.
The MAX1982/MAX1983 deliver up to 300mA and oper-
ate with input voltages up to 5.5V, but not simultane-
ously. High output currents can only be achieved when
the input-output differential voltages are low (Figure 5).
Undervoltage Lockout (UVLO)
The undervoltage lockout (UVLO) circuit ensures that
the regulator starts up with adequate voltage for the
gate-drive circuitry to bias the internal pass transistor.
The UVLO circuitry monitors V
BIAS
P
TT
DISS MAX
J MAX A
JC CA
()
()
=
+
−
θθ
PI VV
D OUT IN OUT
=×
()
−
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
08 1
1
2
.
Low-Voltage, Low-Dropout Linear Regulators
with External Bias Supply
10 ______________________________________________________________________________________
0.1μF
BIAS
GND
SHDN
ON
OFF
IN
OUT
PGOOD
100kΩ
10μF
10μF
4.5V TO 5.5V
1.25V TO 5.5V
1.2V
MAX1982
Figure 3. MAX1982 Typical Application Circuit
0.1μF
BIAS
GND
SHDN
ON
OFF
IN
OUT
ADJ
R2
40kΩ
10μF
4.5V TO 5.5V
1.5V TO 5.5V
1.2V
MAX1983
10μF
R1
20kΩ
Figure 4. MAX1983 Typical Application Circuit