Rainbow Electronics MAX15028 - Manual (Page 8)

MAX15027/MAX15028
Internal p-Channel Pass Transistor
The MAX15027/MAX15028 feature a 75m
Ω (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
MAX15027/MAX15028 do not suffer from these prob-
lems and consume only 275µA (typ) of quiescent cur-
rent under heavy loads, as well as in dropout.
Short-Circuit/
Thermal Fault Protection
The MAX15027/MAX15028 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 temper-
ature reaches +165°C (typ), the thermal-overload cir-
cuitry 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 regu-
lation. Current limiting and thermal protection continue
until the fault is removed. For continuous operation, do
not exceed the absolute maximum junction-tempera-
ture rating of T
J = +150°C.
Soft-Start
The MAX15027/MAX15028 feature a soft-start function
that slowly ramps up the output voltage of the regulator
based on the value of the capacitor (C
SS
) connected
from SS to GND. Upon power-up, C SS is charged with
a 5µA (typ) current source through SS. The voltage at
SS is compared to the internal 0.5V reference (V
REF ).
The feedback voltage for regulation (V REG ) is the lower
of V SS or V REF . As V SS rises, the regulation voltage
(V
REG
) rises at the same rate. Once V
SS
reaches and
rises above V REF , the regulation voltage then tracks the
reference voltage since it is the lower of V SS and V REF .
The value of C SS determines the length of the soft-start
time, t SS . Use the following formula to determine C SS .
C SS = 10
-5
x t SS
where C SS is in farads and t SS is in seconds.
Shutdown Mode
The MAX15027/MAX15028 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 useful 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 inter-
nal 10k
Ω resistor is connected between OUT and GND.
Applications Information
Programming the Output Voltage
The MAX15027/MAX15028 feature an adjustable output
voltage from 0.5V to V
IN
using two external resistors
connected as a voltage-divider to FB as shown in
Figure 1. The output voltage is set by the following
equation:
where typically V FB = 0.5V. Choose R2 to be 10k
Ω. Or, to
optimize load-transient response for no load to full load
transients, use the resistor-divider as a minimum load
and choose R2 to be 500
Ω. To simplify resistor selection:
R R
V
V
OUT
FB
1 2 1
=
⎛
⎝⎜ ⎞
⎠⎟
−
V V
R
R
OUT FB
= +
⎛
⎝⎜ ⎞
⎠⎟
1
1
2
1.425V to 3.6V Input, 1A Low-Dropout
Regulators with BIAS Input
8 _______________________________________________________________________________________
0.1uF
ENABLE
SHUTDOWN
R2
R1
INPUT
1.2V TO 3.6V
ALWAYS-ON
INPUT 3V
TO 5.5V
OUTPUT
4.7 µF
1 µF
IN
BIAS
OUT
FB
I.C.
GND
EN
SS
MAX15028
Figure 1. MAX15028 Typical Application Circuit