Datasheet

LTM8055/LTM8055-1
13
Rev C
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the LTM8055/LTM8055-1 can still operate properly. Some
examples of this are provided in the Typical Applications
section.
Soft-Start
Soft-start reduces the input power sources’ surge cur
-
rents by
gradually increasing the controller’s current. As
indicated in the Block Diagram, the LTM8055/LTM8055-1
has an internal soft-start RC network. Depending upon the
load and operating conditions, the internal network may
be sufficient for the application. To increase the soft-start
time, simply add a capacitor from SS to GND.
Output Current Limit (I
OUT
)
The LTM8055/LTM8055-1 features an accurate average
output current limit set by an external sense resistor placed
between V
OUT
and I
OUT
as shown in Figure 3. V
OUT
and
I
OUT
internally connect to a differential amplifier that limits
the current when the voltage V
OUT
-I
OUT
reaches 58mV.
The current limit is:
I
OUT(LIM)
=
58mV
R
SENSE
where R
SENSE
is the value of the sense resistor in ohms.
Most applications should use an output sense resistor as
shown in Figure 3, if practical. The internal buck-boost
power stage is current limited, but is nonetheless capable
of delivering large amounts of current in an overload
condition, especially when the output voltage is much
lower than the input and the power stage is operating as
a buck converter.
tying the LL and MODE pins together can improve perfor
-
mance—see Switching Mode in this section.
In
high step-down voltage regulator applications, the
internal current limit can be quite high to allow proper
operation. This can potentially damage the LTM8055/
LTM8055-1 in overload or short-circuit conditions. Ap
-
ply an
output current sense resistor to set an appropriate
current limit to protect the LTM8055/LTM8055-1 against
these fault conditions.
Output Current Limit Control (CTL)
Use the CTL input to reduce the output current limit from
the value set by the external sense resistor applied between
V
OUT
and I
OUT
. The typical control range is between 0V
and 1.2V. The CTL pin does not directly affect the input
current limit. If this function is not used, leave
CTL
open.
Drive CTL to less than about 50mV to stop switching. The
CTL pin has an internal pull-up resistor to 2V.
Input Current Limit (I
IN
)
Some applications require that LTM8055/LTM8055-1 draw
no more than some predetermined current from the power
source. Current limited power sources and power sharing
are two examples. The LTM8055/LTM8055-1 features
an accurate input current limit set by an external sense
resistor placed between I
IN
and V
IN
as shown in Figure
4. V
IN
and I
IN
internally connect to a differential amplifier
that limits the current when the voltage I
IN
-V
IN
reaches
50mV. The current limit is:
I
IN(LIM)
=
50mV
R
SENSE
where R
SENSE
is the value of the sense resistor in ohms.
If input current limiting is not required, simply tie I
IN
to V
IN
.
APPLICATIONS INFORMATION
Figure 3. Set The LTM8055/LTM8055-1 Output Current Limit with
an External Sense Resistor
Figure 4. Set the LTM8055/LTM8055-1 Input Current
Limit with an External Sense Resistor
When the voltage across the output sense resistor falls
to about 1/10th of full scale, the LL pin pulls low. If there
is no output sense resistor, and I
OUT
is tied to V
OUT
, LL
will be active low. Applying an output sense resistor and
I
OUT
LTM8055/
LTM8055-1
V
OUT
R
SENSE
8055 F03
LOAD
I
IN
LTM8055/
LTM8055-1
R
SENSE
V
IN
POWER
SOURCE
8055 F04
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