ltm8027 Linear Technology Corporation, ltm8027 Datasheet - Page 14

• Manufacturer Part Number: ltm8027
• Description: 60v, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTM8027
Example: Select R
a 15V minimum input voltage)
The V
ample the V
can be disabled by connecting the RUN pin to the V
through a large value pull-up resistor. This pin contains
a low impedance clamp at 6V, so the RUN pin will sink
current from the pull-up resistor (R
Because this arrangement will clamp the RUN pin to 6V,
it will violate the 5V absolute maximum voltage rating of
the pin. This is permitted, however, as long as the absolute
maximum input current rating of 1mA is not exceeded.
Input RUN pin currents of <100μA are recommended: a
1M or greater pull-up resistor is typically used for this
confi guration.
Soft-Start
The desired soft-start time (t
C
The amount of time in which the power supply must be
under a V
before the SS pin voltage enters its active region is ap-
proximated by the following formula:
14
SS
R
I
C
t
RUN
FAULT
capacitor as follows:
SS
A
IN
= 49.9k •
=
=
turn off voltage is 15% below turn on. In the ex-
IN
V
2µA • t
=
1.231V
IN
IN(OFF)
, V
R
C
– 6V
PU
CC
SS
⎛
⎝ ⎜
50µA
SS
or V
14.5V
• 0.65V
B
1.4V
would be 12.3V. The shutdown function
= 49.9k, V
SHDN
– 1
⎞
⎠ ⎟
UVLO fault condition (t
= 464k
SS
IN(ON)
) is programmed via the
PU
= 14.5V (based upon
):
FAULT
IN
pin
)
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LTM8027. However, these capacitors
can cause problems if the LTM8027 is plugged into a live
supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
source forms an under damped tank circuit, and the volt-
age at the V
nominal input voltage, possibly exceeding the LTM8027’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LTM8027 into an
energized supply, the input network should be designed to
prevent this overshoot by introducing a damping element
into the path of current fl ow. This is often done by adding
an inexpensive electrolytic bulk capacitor across the input
terminals of the LTM8027. The criteria for selecting this
capacitor is that the ESR is high enough to damp the ringing,
and the capacitance value is several times larger than the
LTM8027 ceramic input capacitor. The bulk capacitor does
not need to be located physically close to the LTM8027;
it should be located close to the application board’s input
connector, instead.
Synchronization
The oscillator can be synchronized to an external clock.
Choose the R
at least 10% below the desired synchronization frequency.
It is recommended that the SYNC pin be driven with a
square wave that has amplitude greater than 2.3V, pulse
width greater than 1μs and rise time less than 500ns. The
rising edge of the sync wave form triggers the discharge
of the internal oscillator capacitor.
IN
T
resistor such that the resultant frequency is
pin of the LTM8027 can ring to twice the
8027f