Datasheet

LTM8052/LTM8052A
12
8052fc
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operaTion
The LTM8052/LTM8052A is a standalone nonisolated
constant-voltage, constant-current step-down switching
DC/DC power supply that can deliver up to 5A of positive
or 6A of negative output current. This µModule regulator
provides a precisely regulated output voltage program
-
mable via
one external resistor from 1.2V to 24V. The
input
voltage range is 6V to 36V. Given that the LTM8052/
LTM8052A is a step-down converter, make sure that the
input voltage is high enough to support the desired output
voltage and load current.
The LTM8052/LTM8052A is a 2-quadrant device, meaning
that it can both source and sink current in order to regu
-
late its output voltage. Most traditional voltage regulators
are
one quadrant; that is, they only source current. If the
load, for whatever reason, forces current into a traditional
regulator, the output voltage will typically rise. In a similar
situation, the LTM8052/LTM8052A will sink current to
keep the output voltage in regulation.
It should be clear that the above situation is only possible
if the load is providing energy to the LTM8052/LTM8052A
output. The LTM8052/LTM8052A will be able to maintain
the
output voltage at the target regulation point as long
as the current from the load does not exceed its nega-
tive current
limit.
If the current does exceed the negative
current limit, the LTM8052/LTM8052A output will start
to rise. If the output continues to rise, the LTM8052/
LTM8052A’s output overvoltage protection circuitry will
turn off the internal power switches, and the output will
be free to rise. The LTM8052A does not respond to an
output overvoltage. If the output voltage of either the
LTM8052 or the LTM8052A rises above the input, current
will flow through an internal power diode. The output will
be clamped to a diode drop above the input, and current
regulation will be lost.
When the LTM8052/LTM8052A is sinking current, it
maintains its output voltage regulation by power conver
-
sion, not power dissipation. This means that the energy
provided
to the LTM8052/LTM8052A is in turn delivered
to its input power bus. There must be something on this
power bus to accept or use the energy, or the LTM8052/
LTM8052A’s input voltage will rise. Left unchecked, the
energy can raise the input voltage
above the absolute
maximum
voltage and damage the LTM8052/LTM8052A.
Please see the Input Precautions section for further details.
For applications where only sourcing current (one quadrant
operation) is desired, use LTM8026.
The LTM8052/LTM8052A operates in forced continuous
mode. This means that the part will not skip cycles when
the load approaches zero amps. This may be particularly
useful in applications where the synchronization function
is used, or any time discontinuous switching is undesir
-
able. The LTM8052/LTM8052A will not operate in forced
continuous
mode when an input UVLO, output OVLO or
minimum duty cycle violation occurs.
As shown in the Block Diagram, the LTM8052/LTM8052A
contains a current mode controller, power switches,
power inductor, and a modest amount of input and output
capacitance.
The LTM8052/LTM8052A utilizes fixed frequency, average
current mode control to accurately regulate the inductor
current, independent from the output voltage. This is an
ideal solution for applications requiring a regulated cur
-
rent sour
ce. The control loop will regulate the current in
the
internal inductor. Once the output has reached the
regulation voltage determined by the resistor from the
ADJ pin to ground, the inductor current will
be reduced
by the voltage regulation loop.
The output current loop has two control inputs, deter-
mined by the voltage at the analog control pins, CTL_I
and CTL_T . CTL_I is typically used to set the maximum
allowable current output of the LTM8052/LTM8052A, while
CTL_T is typically used with a NTC thermistor to reduce
the output current in response to temperature. The lower
of the two analog voltages on CTL_I and CTL_T determines
the regulated output current. The analog control range
of both the CTL_I and CTL_T pin is from 0V to 1.5V. As
shown in the Typical Performance Characteristics section,
the positive and negative currents are not symmetric. The
negative current limit is offset by approximately 2A.
The RUN pin functions as a precision shutdown pin. When
the voltage at the RUN pin is lower than 1.55V, switch
-
ing 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
IN
to set a hysteresis. During start-
up, the SS pin is held low until the part is enabled, after