Datasheet

LTM4628
18
4628fe
For more information www.linear.com/LTM4628
APPLICATIONS INFORMATION
INTV
CC
and EXTV
CC
The LTM4628 module has an internal 5V low dropout
regulator that is derived from the input voltage. This
regulator is used to power the control circuitry and the
power MOSFET drivers. This regulator can source up to
70mA, and typically uses ~30mA for powering the device
at the maximum frequency.
EXTV
CC
allows an external 5V supply to power the LTM4628
and reduce power dissipation from the internal low dropout
5V regulator. The power loss savings can be calculated by:
(V
IN
– 5V) • 30mA = P
LOSS
EXTV
CC
has a threshold of 4.7V for activation, and a maxi-
mum rating of 6V. When using a 5V input, connect this
5V
input to EXTV
CC
also to maintain a 5V gate drive level.
V
IN
has to be sequenced on before EXTV
CC
, and EXTV
CC
must be sequenced off before V
IN
.
Differential Remote Sense Amplifier
An accurate differential remote sense amplifier is provided
to sense low output voltages accurately at the remote
load points. This is especially true for high current loads.
The amplifier can be used on one of the two channels, or
on a single parallel output. It is very important that the
DIFFP and
DIFFN are connected properly at the output,
and
DIFFOUT is connected to either V
OUTS1
or V
OUTS2
.
In parallel operation, the DIFFP and DIFFN are connected
properly at the output, and DIFFOUT is connected to
one of the V
OUTS
pins. Review the parallel schematics in
Figure 31 and review Figure 2.
SW Pins
The
SW pins are generally for testing purposes by moni-
toring these
pins. These pins can also be used to dampen
out
switch node ringing caused by LC parasitic in the
switched current paths. Usually a series R-C combina-
tion is
used, called a snubber circuit. The resistor will
dampen
the resonance and the capacitor is chosen to
only affect the high frequency ringing across the resistor.
If the stray inductance or capacitance can be measured or
approximated then
a somewhat analytical technique can
be used to select the snubber values. The inductance is
usually easier to predict. It combines the power path board
inductance in combination with the MOSFET interconnect
bond wire inductance.
First the SW pin can be monitored with a wide bandwidth
scope with a high frequency scope probe. The ring fre
-
quency can be measured for its value. The impedance Z
can be calculated:
Z
L
= 2πfL,
where f is the resonant frequency of the ring, and L is the
total parasitic inductance in the switch path. If a resistor
is selected that is equal to Z, then the ringing should be
dampened. The snubber capacitor value is chosen so that
its impedance is equal to the resistor at the ring frequency.
Calculated by: Z
C
= 1/(2πfC). These values are a good place
to start with. Modification to these components should
be made to attenuate the ringing with the least amount
the power loss.
Temperature Diode Monitoring
The LTM4628 has an on board 1N4148 silicon diode at the
TEMP pin that can be used to monitor temperature. The
diode is mounted very close to internal power switches.
The forward voltage of a silicon diode is temperature
dependent based on the following equation:
I
D
= I
S
• e
V
D
η • V
T






or
V
D
= η • V
T
• ln
I
D
I
S
where I
D
is the diode current, V
D
is the diode voltage, η is
the ideality factor (typically close to 1.0) and I
S
(satura-
tion current) is a process dependent parameter. V
T
can
be broken out to:
V
T
=
k • T
q
where T is the diode junction temperature in Kelvin, q is
the electron charge and k is Boltzmann’s constant. V
T
is
approximately 26mV at room temperature (298K) and
scales linearly with Kelvin temperature. It is this linear