Datasheet

LTM8055/LTM8055-1
14
Rev C
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Input Current Monitor (I
INMON
)
The I
INMON
pin produces a voltage equal to approximately
20 times the voltage of I
IN
– V
IN
. Since the LTM8055/
LTM8055-1 input current limit engages when I
IN
– V
IN
=
50mV, I
INMON
will be 1V at maximum input current.
Output Current Monitor (I
OUTMON
)
The I
OUTMON
pin produces a voltage proportional to the
voltage of V
OUT
– I
OUT
. Since the LTM8055/LTM8055-1
output current limit engages when V
OUT
– I
OUT
= 58mV,
I
OUTMON
will be 1.2V at maximum output current.
Synchronization
The LTM8055/LTM8055-1 switching frequency can be
synchronized to an external clock using the SYNC pin.
Driving SYNC with a 50% duty cycle waveform is a good
choice, otherwise maintain the duty cycle between about
10% and 90%. When synchronizing, a valid resistor value
(that is, a value that results in a free-running frequency of
100kHz to 800kHz) must be connected from RT to GND.
While an RT resistor is required for proper operation, the
value of this resistor is independent of the frequency of
the externally applied SYNC signal. Be aware, however,
that the LTM8055/LTM8055-1 will switch at the frequency
prescribed by the RT value if the SYNC signal
terminates,
so
choose an appropriate resistor value.
CLKOUT
The CLKOUT signal reflects the internal switching clock
of the LTM8055/LTM8055-1. It is phase shifted by ap
-
proximately 180° with respect
to the leading edge of the
internal clock. If CLKOUT is connected to the SYNC input
of another LTM8055/LTM8055-1, the two devices will
switch about 180° out of phase.
Input Precaution
In applications where the output voltage is deliberately
pulled up above the set regulation voltage or the FB pin is
abruptly driven to a new voltage, the LTM8055/LTM8055-1
may attempt to regulate the voltage by removing energy
from the load for a short period of time after the output
is pulled up. Since the LTM8055/LTM8055-1 is a syn
-
chronous switching converter, it delivers this energy to
the input. If there is nothing on the LTM8055/LTM8055-1
input to consume this energy, the input voltage may rise.
If the input voltage rises without intervention, it may rise
above the absolute maximum rating, damaging the part.
Carefully examine the input voltage behavior to see if the
application causes it to rise.
In many cases, the system load on the LTM8055/LTM8055-
1 input bus will be
sufficient
to absorb the energy delivered
by the μModule regulator. The power required by other
devices will consume more than enough to make up for
what the LTM8055/LTM8055-1 delivers. In cases where
the LTM8055/LTM8055-1 is the largest or only power
converter, this may not be true and some means may need
to be devised to prevent the LTM8055/LTM8055-1’s input
from rising too high. Figure 5a shows a passive crowbar
circuit that will dissipate energy during momentary input
overvoltage conditions. The break-down voltage of the
Zener diode is chosen in conjunction with the resistor R to
set the circuit’s trip point. The trip point is typically set well
above the maximum V
IN
voltage under normal operating
conditions. This circuit does not have a precision thresh-
old, and
is subject to both part-to-part and temperature
variations, so it is most suitable for applications where
the maximum input voltage is much less than the 40V
IN
absolute maximum. As stated earlier, this type of circuit
is best suited for momentary overvoltages.
Figure 5a is a crowbar circuit, which attempts to prevent
the input voltage from rising above some level by dumping
energy
to
GND through a power device. In some cases,
it is possible to simply turn off the LTM8055/LTM8055-1
when the input voltage exceeds some threshold. An ex
-
ample of this circuit is shown in Figure 5b. When the power
source on the output drives V
IN
above a predetermined
threshold, the comparator pulls down on the RUN pin
and stops switching in the LTM8055/LTM8055-1. When
this happens, the input capacitance needs to absorb the
energy stored within the LTM8055/LTM8055-1’s internal
inductor, resulting in an additional voltage rise. This volt
-
age rise depends upon the input capacitor size and how
much current is flowing from the LTM8055/LTM8055-1
output to input.
APPLICATIONS INFORMATION
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