LTM8001
11
8001fd
For more information www.linear.com/LTM8001
operaTion
The LTM8001 consists of two major parts: the first is a
standalone nonisolated step-down switching DC/DC power
converter that can deliver up to 5A of output current. The
second part is an array of five parallelable 1.1A LDOs. The
DC/DC converter provides a precisely regulated output
voltage programmable via one external resistor from 1.2V
to 24V. The input voltage range is 6V to 36V. Given that it
is a step-down converter, make sure that the input volt
-
age is high enough to support the desired output voltage
and load current
.
The linear regulator array consists of
five low drop-out regulators, of which three inputs are
dedicated to the buck converter’s output (V
OUT0
) and two
tie to an undedicated input (V
IN45
). Each individual linear
regulator may be set to a unique voltage through its SET
pin, or may be paralleled with other LDOs by tying their
respective SET and V
OUT
pins together.
The LTM8001 step-down switching converter utilizes fixed
frequency, average current mode control to accurately
regulate the output current. This results in a constant-
voltage, constant-current output characteristic, making
the LTM8001’s step-down regulator well suited for many
supercapacitor and battery charging applications. As shown
in the Typical Performance Characteristics, the current limit
works in both directions. The control loop will regulate the
current in the internal inductor. Once the V
OUT0
output has
reached the regulation voltage determined by the resistor
from the FBO pin to ground, the voltage regulation loop will
reduce the output current and maintain the output voltage.
The ILIM input may be used to set the maximum allowable
current output of the LTM8001. The analog control range
of the ILIM pin is from 0V to 1.5V. If the ILIM pin is raised
above 1.5V, there is little or no effect.
The RUN pin functions as a precision enable for the step-
down switching converter connected to V
OUT0
. If all V
OUT1-3
LDO inputs including BIAS are tied to V
OUT0
, the RUN pin
will also implicitly enable or disable these LDOs as well. If
an external power source is applied to BIAS123 alone or
in combination with V
OUT0
, RUN will not disable V
OUT1-3
.
Refer to the Applications Information section Shorted
Input Protection if V
OUT0
is forced above V
IN0
. When the
voltage at the RUN pin is lower than 1.55V, switching is
terminated. Below the turn-on threshold, the RUN pin sinks
5.5μA. This current can be used with a resistor between
RUN and V
IN0
to set hysteresis. Please refer to the UVLO
and Shutdown section in the Applications Information
for further details. During start-up, the SS pin is held low
until the part is enabled, after which the capacitor at the
soft-start pin is charged with an 11μA current source.
The LTM8001 is equipped with thermal shutdown circuitry
to protect the device during momentary overload condi
-
tions. It is set above the 125°C absolute maximum internal
temperature rating to avoid interfering with normal speci-
fied
operation, so internal device temperatures will exceed
the absolute maximum rating when the overtemperature
protection is active
.
Thus, continuous or repeated activa
-
tion of the thermal shutdown may impair device reliability.
During thermal shutdown, all switching is terminated and
the SS pin is driven low.
The switching frequency is determined by a resistor at
the RT pin. The LTM8001 may also be synchronized to
an external clock through the use of the SYNC pin. Please
see the Switching Frequency Synchronization section in
the Applications Information for further details.
The V
OUT1-5
linear regulators are easy to use and have
all the protection features expected in high performance
regulators. Included are short-circuit protection and safe
operating area protection, as well as thermal shutdown.
These linear regulators are especially well suited to ap
-
plications needing multiple rails. Their architecture allows
their
outputs
to be adjusted down to zero volts. The output
voltage is set by a single resistor, handling modern low
voltage digital ICs as well as allowing easy parallel opera
-
tion and simplified thermal management.
The linear regulators can be operated in two modes.
One
mode has the BIAS123 and BIAS45 pins connected to the
linear regulator power input pins (V
OUT0
and V
IN45
) which
gives a limitation of about 1.6V dropout. In the other mode,
the BIAS123 and BIAS45 pins can be tied to a voltage at
least 1.6V above their highest respective outputs. The linear
regulator power input (V
OUT0
and V
IN45
) can then be set
to a lower voltage that meets the dropout requirement,
minimizing the power dissipation.
