Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the MAX8891/MAX8892. When the junction tempera-
ture exceeds T
J
= +160°C, the thermal sensor signals
the shutdown logic, turning off the pass transistor and
allowing the IC to cool down. The thermal sensor turns
the pass transistor on again after the IC’s junction tem-
perature drops by 10°C, resulting in a pulsed output dur-
ing continuous thermal-overload conditions.
Thermal-overload protection is designed to protect the
MAX8891/MAX8892 in the event of a fault condition. For
continual operation, do not exceed the absolute maxi-
mum junction temperature rating of T
J
= +150°C.
Operating Region and Power Dissipation
The MAX8891/MAX8892 maximum power dissipation
depends on the thermal resistance of the case and cir-
cuit board, the temperature difference between the die
junction and ambient, and the rate of airflow. The power
dissipation across the device is:
P = I
OUT
(V
IN
- V
OUT
)
The maximum power dissipation is:
P
MAX
= (T
J
- T
A
)/(θ
JC
+ θ
CA
)
where T
J
- T
A
is the temperature difference between the
MAX8891/MAX8892 die junction and the surrounding air,
θ
JC
is the thermal resistance of the package, and θ
CA
is
the thermal resistance through the PCB, copper traces,
and other materials to the surrounding air.
The GND pin of the MAX8891/MAX8892 performs the
dual function of providing an electrical connection to
ground and channeling heat away. Connect the GND
pin to ground using a large pad or ground plane.
Applications Information
Capacitor Selection
and Regulator Stability
Use a 1µF capacitor on the MAX8891/MAX8892 input
and a 1µF capacitor on the output. Larger input capacitor
values and lower ESRs provide better noise rejection and
line-transient response. Reduce output noise and
improve load-transient response, stability, and power-
supply rejection by using large output capacitors. Note
that some ceramic dielectrics exhibit large capacitance
and ESR variation with temperature. With dielectrics such
as Z5U and Y5V, it may be necessary to use a 2.2µF or
larger output capacitor to ensure stability at temperatures
below -10°C. With X7R or X5R dielectrics, 1µF is sufficient
at all operating temperatures. A graph of the region of
stable C
OUT
ESR vs. load current is shown in the
Typical
Operating Characteristics
.
PSRR and Transient Response
The MAX8891/MAX8892 are designed to deliver high
PSRR as well as low dropout and low quiescent currents
in battery-powered systems. The MAX8891/MAX8892
power-supply rejection is 65dB at 10kHz.
When operating from sources other than batteries,
improved supply-noise rejection and transient response
can be achieved by increasing the values of the input
and output bypass capacitors, and through passive fil-
tering techniques. The
Typical Operating Characteristics
show the MAX8891/MAX8892 line- and load-transient
responses.
Dropout Voltage
A regulator’s minimum dropout voltage determines the
lowest usable supply voltage. In battery-powered
systems, this determines the useful end-of-life battery
voltage. Because the MAX8891/MAX8892 use a
p-channel MOSFET pass transistor, their dropout volt-
age is a function of drain-to-source on-resistance
(R
DS(ON)
) multiplied by the load current (see the
Typical
Operating Characteristics
).
High PSRR, Low-Dropout, 150mA
Linear Regulators
6 Maxim Integrated
MAX8891/MAX8892
