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OPERATION
(see Figure 4b for inductor DCR sensing connections). The
filter time constant, R1 • C1, of the SNSD
L/DCR of the output inductor, while the filter at SNSA
have a bandwidth of five times larger than that of SNSD
i.e, R2 • C2 equals one-fifth of R1 • C1.
Thermal Balancing For Multiphase Operation
When LTC3875 is used as a single output multiphase
converter, the temperature of the whole system can be
balanced by enabling the thermal balancing function. This
prevents hot spots due to imperfection of current match-
ing and component mismatch. Therefore, it improves the
overall reliability of the power supply system.
Refer to Figure 2 for the following discussion of thermal
balancing for the LTC3875.
CHANNEL 1
CHANNEL 2
+
should match the
+
should
30µA
TCOMP1
+
THERMAL
–
SENSOR
OR NTC
ADJUST
g
CHANNEL
AMP
CURRENT
30µA
TCOMP2
+
THERMAL
–
SENSOR
OR NTC
ADJUST
g
CHANNEL
AMP
CURRENT
Figure 2. Thermal Balancing Technique for Multichip Operations
For more information
The thermal balancing can be enabled by setting the
ENTMPB pin to ground. Each channel has a TCOMP/ITEMP
pin which sources a 30µA precision current. By connecting
+
a linearized NTC network or a temperature sensing IC
,
placed near the hot spot of the converter from this pin to
SGND, the temperature of each channel can be sensed.
The sensed voltage from each channel is converted to a
current, which is programmable with resistor, R
at the TRSET pin. The current from each channel is then
summed together at the TAVG pin. The resistor value at
the TAVG is R
TCOMP
The voltage at TAVG is then a representation of the average
temperature of the whole system. By comparing the
phase temperature and average temperature, an internal
transconductance amplifier then adjusts the phase current
accordingly to match the phase temperature to the average
temperature of the system.
MIRROR
1:1
TRSET1
R
TCOMP
+
m
–
MIRROR
1:1
TRSET2
R
TCOMP
+
m
–
www.linear.com/LTC3875
LTC3875
/n, where n is the number of phases.
TAVG
R
AVG
REPEAT FOR
MULTICHIP
OPERATIONS
3875 F02
,
TCOMP
3875fb
15
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