Detailed Description
The MAX15027/MAX15028 low-dropout linear regulators
operate from input voltages as low as 1.425V and deliver
up to 1A of continuous output current with a maximum
dropout voltage of only 225mV.
The MAX15028 operates with an input voltage as low
as 1.425V if the bias voltage (VBIAS) of 3V to 5.5V is
available. The power is applied at IN while the control is
provided through BIAS input. The current drawn by BIAS is
negligible when the LDO goes into shutdown. This feature
is especially useful in automotive applications where the
BIAS input is derived from an always-on LDO that expects
to provide minimal power during the key-off condition.
The pMOS output stage can be driven from input voltages
down to +1.425V without sacrificing stability or transient
performance. The output voltage of all the regulators is
adjustable from 0.5V to VIN and is ±2% accurate over
load and line variations, from -40°C to +125°C. Since
these regulators have a pMOS output device, supply
current is not a significant function of load or input head-
room.
Internal p-Channel Pass Transistor
The devices feature a 75mΩ (typ) p-channel MOSFET
pass transistor. Unlike similar designs using pnp pass
transistors, p-channel MOSFETs require no base drive,
reducing quiescent current.
pnp-based regulators also waste considerable current in
dropout when the pass transistor saturates and use high
base-drive currents under large loads. The devices do
not suffer from these problems and consume only 275μA
(typ) of quiescent current under heavy loads, as well as
in dropout.
Short-Circuit/Thermal Fault Protection
The devices are fully protected from a short circuit at
the output through current limiting and thermal-overload
circuitry. In the fault condition when the output is shorted
to ground, the output current is limited to a maximum of
2A. Under these conditions, the device quickly heats up.
When the junction temperature reaches +165°C (typ), the
thermal-overload circuitry turns off the output, allowing the
part to cool down. When the junction temperature cools to
+150°C (typ), the output turns back on and reestablishes
regulation. Current limiting and thermal protection contin-
ue until the fault is removed. For continuous operation, do
not exceed the absolute maximum junction-temperature
rating of TJ = +150°C.
Soft-Start
The devices feature a soft-start function that slowly ramps
up the output voltage of the regulator based on the value
of the capacitor (CSS) connected from SS to GND. Upon
power-up, CSS is charged with a 5μA (typ) current source
through SS. The voltage at SS is compared to the internal
0.5V reference (VREF). The feedback voltage for regula-
tion (VREG) is the lower of VSS or VREF. As VSS rises,
the regulation voltage (VREG) rises at the same rate.
Once VSS reaches and rises above VREF, the regulation
voltage then tracks the reference voltage since it is the
lower of VSS and VREF. The value of CSS determines the
length of the soft-start time, tSS. Use the following formula
to determine CSS.
CSS = 10-5 x tSS
where CSS is in farads and tSS is in seconds.
Shutdown Mode
The devices include an enable input. To shut down the
IC, drive EN low. In shutdown mode, the current drawn
by BIAS is less than 2μA. This feature is extremely use-
ful in an automotive application where the BIAS input
is derived from an always-on LDO expecting to provide
minimal dark current. For normal operation, drive EN high
or connect EN to IN (BIAS for MAX15028) for continuous
on operation. During shutdown, an internal 10kΩ resistor
is connected between OUT and GND.
MAX15027/MAX15028 1.425V to 3.6V Input, 1A Low-Dropout
Regulators with BIAS Input
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