Datasheet

LTM4649
16
4649f
For more information www.linear.com/LTM4649
APPLICATIONS INFORMATION
SW Pins
The SW pin is generally for testing purposes by monitor-
ing the pin. The SW pin can also be used to dampen out
switch node ringing caused by LC parasitic in the switched
current path. Usually a series R-C combination 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
= 2π • f • L
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
2
π
•f •C
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 Monitoring
Measuring the absolute temperature of a diode is pos-
sible due to the relationship between current, voltage
and temperature described by the classic diode 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
temperature relationship that makes diodes suitable
temperature sensors. The I
S
term in the equation above
is the extrapolated current through a diode junction when
the diode has zero volts across the terminals. The I
S
term
varies from process to process, varies with temperature,
and by definition must always be less than I
D
. Combining
all of the constants into one term:
K
D
=
η • k
q
where K
D
= 8.62
−5
, and knowing ln(I
D
/I
S
) is always posi-
tive because I
D
is always greater than I
S
, leaves us with
the equation that:
V
D
= T(KELVIN) • K
D
• ln
I
D
I
S
where V
D
appears to increase with temperature. It is com-
mon knowledge that a silicon diode biased with a current
source has an approximately –2mV/°C temperature rela-
tionship (Figure 7), which is at odds with the equation. In
fact, the I
S
term increases with temperature, reducing the
ln(I
D
/I
S
) absolute value yielding an approximately –2mV/°C
composite diode voltage slope.
An external diode connected PNP transistor can be pulled
up to V
IN
with a resistor to set the current to 100µA for
using this diode connected transistor as a general tem-
perature monitor by monitoring the diode voltage drop
with temperature. See Figure 21 for an example.