Datasheet

LTC3880/LTC3880-1
22
3880fc
For more information www.linear.com/LTC3880
Figure 4. Temperature Sense Circuit
TSNS
MMBT3906
LTC3880 10nF
SGND
SGND
3880 F04
OPERATION
near the inductor and used to modify the internal current
limit circuit to maintain an essentially constant current
limit with temperature. In this application, the I
SENSEn
+
pin is connected to the FET side of the capacitor while the
I
SENSEn
–
pin is placed on the load side of the capacitor.
The current sensed from the input is then given by the
expression V
DCR
/DCR. V
DCR
is digitized by the LTC3880’s
telemetry ADC with an input range of ±128mV, a noise
floor of 7µV
RMS
, and a peak-peak noise of approximately
46.5µV. The LTC3880 computes the inductor current using
the DCR value stored in the IOUT_CAL_GAIN command
and the temperature coefficient stored in command
MFR_IOUT_CAL_GAIN_TC. The resulting current value
is returned by the READ_IOUT command.
LOAD SHARING
Multiple LTC3880’s can be arrayed in order to provide a
balanced load-share solution by bussing the necessary
pins. Figure 3 illustrates the shared connections required
for load sharing.
The frequency must only be programmed on one of the
LTC3880s. The other(s) must be programmed to External
Clock.
EXTERNAL/INTERNAL TEMPERATURE SENSE
External temperature can be best measured using a remote
diode-connected PNP transistor such as the MMBT3906.
The emitter should be connected to the TSNSn pin while the
base and collector terminals of the PNP transistor should
be returned to the LTC3880’s SGND pin, preferably using a
star connection. It is possible to connect the collector of the
PNP to the source of the bottom MOSFET. This may optimize
board layout allowing the PNP closer proximity to the power
FETs. The base of the PNP must still be tied to signal ground.
For best noise immunity, the connections should be routed
differentially and a 10nF capacitor should be placed in
parallel with the diode connected PNP. Two different currents
are applied to the diode (nominally 2µA and 32µA) and the
temperature is calculated from the ∆V
BE
measurement. The
external transistor temperature is digitized by the telem-
etry ADC, and the value is returned by the PMBus READ_
TEMPERATURE_1 (Chn) command.
The READ_TEMPERA
TURE_2 command returns the junc-
tion temperature of the LTC3880 using an on-chip diode.
The slope of the external temperature sensor can be
modified with the temperature slope coefficient stored in
MFR_TEMP_1_GAIN. Typical PNPs require temperature
slope adjustments slightly less than
1. The MMBT3906 has
a recommended value in this command of approximately
MFR_TEMP_1_GAIN = 0.991 based on the ideality factor
of 1.01. Simply invert the ideality factor to calculate the
MFR_TEMP_1_GAIN. Different manufacturers and differ-
ent lots may have different ideality factors. Consult with
the manufacturer to set this value.
The offset of the external temperature sense can be adjusted
by MFR_TEMP_1_OFFSET. A value of 0 in this register sets
the temperature offset to –273.15°C.
If the PNP cannot be placed in direct contact with the
inductor, the slope or offset can be increased to account
for temperature mismatches. If the user is adjusting the
slope, the intercept point is at absolute zero, –273.15°C, so
small adjustments in slope can change the apparent mea
-
sured temperature significantly. Another way to artificially
increase the slope of the temperature term is to increase
the MFR_IOUT_CAL_GAIN_TC term. This will modify the
temperature slope with respect to room temperature.