Datasheet

LTM4620A
20
4620afb
For more information www.linear.com/LTM4620A
APPLICATIONS INFORMATION
controlled from a single control. See the Typical Applica-
tion circuits in Figure 26.
INTV
CC
and EXTV
CC
The LTM4620A module has an internal 5V low dropout
regulator that is derived from the input voltage. This regu-
lator 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. This internal 5V supply is enabled
by either RUN1 or RUN2.
EXTV
CC
allows an external 5V supply to power the
LTM4620A and
reduce power dissipation from the internal
low dropout 5V regulator. The power loss savings can be
calculated by:
(V
IN
– 5V) • 30mA = PLOSS
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.
EXTV
CC
must sequence on after V
IN
, and EXTV
CC
must
sequence off before V
IN
. When designing a 5V output,
connect this 5V output to EXTV
CC
. Use an external 5V bias
on EXTV
CC
to improve efficiency.
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 29 and review
Figure 2. The diffamp can only be
used for output voltage ≤3.3V.
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 of power loss.
Temperature Monitoring (TEMP)
A diode connected PNP transistor is used for the TEMP
monitor function by monitoring its voltage over tempera
-
ture. The temperature dependence of this diode can be
understood in the equation:
D = nVTIn
ID
IS




,
Where V
T
is the thermal voltage (kT/q), and n, the ideality
factor is 1 for the two diode connected PNPs being used
in the LTM4620. Since I
D
has an exponential temperature
dependence that can be understood from the typical em-
pirical equation for I
S
:
I
S
= I
0 exp (–VG0/VT)
,
Where I
o
is some process and geometry dependent current
(I
o
is typically around 20 orders of magnitude larger than