LTC3882-1
60
Rev A
For more information
APPLICATIONS INFORMATION
External Temperature Sense
The LTC3882-1 facilitates external measurement of the
power stage temperature of each channel with several
silicon-junction-based means. The voltage produced by
the remote sense circuit is digitized by the internal ADC,
and the computed temperature value is returned by the
paged READ_TEMPERATURE_1 telemetry command.
The most accurate external temperature measurement
can be made using a diode-connected PNP transistor
such as the MMBT3906 as shown in Figure 43 with bit 5
of MFR_PWM_MODE_LTC3882-1 set to 0 (∆V
BE
method).
The BJT should be placed in contact with or immediately
adjacent to the power stage inductor. Its emitter should be
connected to the TSNS
n
pin while the base and collector
terminals of the PNP transistor must be returned to the
LTC3882-1 GND paddle using a Kelvin connection. For
best noise immunity, the connections should be routed
differentially and a 10nF capacitor should be placed in
parallel with the diode-connected PNP.
The LTC3882-1 also supports direct junction voltage mea-
surements when bit 5 of MFR_PWM_MODE_LTC3882-1 is
set to one. The factory defaults support a resistor-trimmed
dual diode network as shown in Figure 44. However, this
measurement method can be applied to simple single-diode
circuits of the type shown in Figure 43 with parameter
adjustments as described below. This second measure-
ment method is not generally as accurate as the first, but
it supports legacy power blocks or may prove necessary if
high noise environments prevent use of the ∆V
BE
approach
with its lower signal levels.
For either method, the slope of the external temperature
sensor can be modified with the coefficient stored in
MFR_TEMP_1_GAIN. With the ∆V
BE
approach, 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 different lots may have dif-
ferent ideality factors. Consult with the manufacturer to
set this value. Bench characterization over temperature is
recommended when adjusting MFR_TEMP_1_GAIN for
the direct p-n junction measurement.
The offset of the external temperature sense can be ad-
justed by MFR_TEMP_1_OFFSET. For the ∆V
BE
method a
value of 0 in this register sets the temperature offset to
–273.15°C. For a direct p-n junction measurement, this
parameter adjusts the nominal circuit voltage at 25°C away
from that shown in Figure 44.
To ensure proper use of these temperature adjustment
parameters, refer to the specific formulas given for the two
methods by the MFR_PWM_MODE_LTC3882-1 command
in the later section covering PMBus command details, as
well as Application Note 137.
Resistor Configuration Pins
As a factory default, the LTC3882-1 is programmed to use
external resistor configuration, allowing output voltage,
PWM frequency and phasing, and the PMBus address to
be set without programming the part through its serial
interface or purchasing devices with custom EEPROM
contents. The RCONFIG pins all require a resistor divider
between V
DD25
and GND. The RCONFIG pins are only
interrogated at initial power up and during a reset, so
modifying their values on the fly is not recommended.
RCONFIG pins on the same IC can be shared with a single
resistor divider if they require identical programming.
Resistors with a tolerance of 1% or better must be used
to assure proper operation. In the following tables, R
TOP
is connected between V
DD25
and the RCONFIG pin, while
R
BOT
is connected between the pin and GND. Noisy clock
signals should not be routed near these pins.
Figure 43. External ΔV
BE
Temperature Sense
Figure 44. 2D+R Temperature Sense
TSNS
MMBT3906
LTC3882-1
10nF
GND
GND
38821 F43
TSNS
LTC3882-1
1nF
495µA
1.35V AT 25°C
GND
GND
38821 F44
