Datasheet

LTM8055/LTM8055-1
9
Rev C
For more information www.analog.com
OPERATION
The LTM8055/LTM8055-1 is a standalone nonisolated
buck-boost switching DC/DC power supply. The buck-boost
topology allows the LTM8055/LTM8055-1 to regulate its
output voltage for input voltages both above and below
the magnitude of the output. The maximum output current
depends upon the input voltage. Higher input voltages
yield higher maximum output current.
This converter provides a precisely regulated output volt
-
age programmable
via an external resistor divider from
1.2V to 36V. The input voltage range is 5V to 36V, but
the LTM8055/LTM8055-1 may be operated at lower input
voltages if SV
IN
is powered by a voltage source above 5V.
A simplified block diagram is given on the previous page.
The LTM8055/LTM8055-1 contains a current mode con
-
troller, power switching elements, power inductor and
a modest amount of input and output capacitance. The
LTM8055/LTM8055-1 is a fixed frequency PWM regulator.
The switching frequency is set by connecting the appropri
-
ate resistor value from the RT
pin to GND.
The output voltage of the LTM8055/LTM8055-1 is set by
connecting the FB pin to a resistor divider between V
OUT
and GND.
In addition to regulating its
output voltage
, the LTM8055/
LTM8055-1 is equipped with average current control loops
for both the input and output. Add a current sense resistor
between I
IN
and V
IN
to limit the input current below some
maximum value. The I
INMON
pin reflects the current flowing
though the sense resistor between I
IN
and V
IN
.
A current sense resistor between V
OUT
and I
OUT
allows
the LTM8055/LTM8055-1 to accurately regulate its output
current to a maximum value set by the value of the sense
resistor. When the resistor is present, the I
OUTMON
pin
reflects the output current flowing through V
OUT
.
In general, the LTM8055/LTM8055-1 should be used with an
output sense resistor to limit the maximum output current,
as buck-boost regulators are capable of delivering large
currents when
the output voltage is lower than the input,
if demanded.
Furthermore, while the LTM8055/LTM8055-1 does not
require an output sense resistor to operate, it uses informa-
tion from
the sense resistor to optimize its performance.
If an out-put sense resistor is not used, the efficiency
or output ripple may degrade, especially if the current
through the integrated inductor is discontinuous. In some
cases,
an output
sense resistor is required to adequately
protect the LTM8055/LTM8055-1 against output overload
or short-circuit.
A voltage less than 1.2V applied to the CTL pin reduces
the maximum output current. Drive CTL to about 50mV
to stop switching. The current flowing through the sense
resistor is reflected by the output voltage of the I
OUTMON
pin.
Driving the SYNC pin will synchronize the LTM8055/
LTM8055-1 to an external clock source. The CLKOUT
pin sources a signal that is the same frequency but ap
-
proximately 180° out of
phase with the internal oscillator.
If more output current is required than a single LTM8055/
LTM8055-1 can provide, multiple devices may be oper
-
ated in parallel. Refer to the Parallel Operation section of
Applications Information for more details.
An internal regulator provides power to the control circuitry
and the gate driver to the power MOSFETs. This internal
regulator draws power from the SV
IN
pin. The RUN pin
is used to place the LTM8055/LTM8055-1 in shutdown,
disconnecting the output and reducing the input current
to less than 1μA.
The LTM8055/LTM8055-1 is equipped with a thermal
shutdown that inhibits power switching at high junction
temperatures. The
activation threshold of this function is
above 125°C to avoid interfering with normal operation,
so prolonged or repetitive operation under a condition in
which the thermal shutdown activates may damage or
impair the reliability of the device.
The LTM8055 features an integrated compensation network
that works well under most conditions. Some applications,
however, benefit from a different compensation network.
In such cases, use the LTM8055-1, which has no internal
compensation network. Apply an appropriate external
compensation network for optimal and proper operation.
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