LTM4620 View Datasheet(PDF) - Linear Technology
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LTM4620 Datasheet PDF : 40 Pages
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LTM4620
APPLICATIONS INFORMATION
controlled from a single control. See the Typical Applica-
tion circuits in Figure 23.
INTVCC and EXTVCC
The LTM4620 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.
EXTVCC allows an external 5V supply to power the LTM4620
and reduce power dissipation from the internal low dropout
5V regulator. The power loss savings can be calculated by:
(VIN – 5V) • 30mA = PLOSS
EXTVCC has a threshold of 4.7V for activation, and a
maximum rating of 6V. When using a 5V input, connect
this 5V input to EXTVCC also to maintain a 5V gate drive
level. EXTVCC must sequence on after VIN, and EXTVCC
must sequence off before VIN.
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 VOUTS1 or VOUTS2.
In parallel operation, the DIFFP and DIFFN are connected
properly at the output, and DIFFOUT is connected to
one of the VOUTS pins. Review the parallel schematics in
Figure 24 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) = 2Ï€fL,
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/(2Ï€fC). 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
ï£IS
,
Where VT 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 ID has an exponential temperature
dependence that can be understood from the typical em-
pirical equation for IS:
IS = I0 exp (–VG0/VT),
Where Io is some process and geometry dependent current
(Io is typically around 20 orders of magnitude larger than
IS at room temperature, so Io is much larger than typical
values of ID), and VG0 is the band gap voltage of 1.2V
extrapolated to absolute zero of –273°C Kelvin. Figure 8
shows a plot of the diode temperature characteristic of
the diode connected PNP transistor biased with a 100µa
4620fb
20
For more information www.linear.com/LTM4620
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