LTC4449 datasheet(60/108 Pages) LINER

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LTC3882-1

Rev A

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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 (∆VBEmethod).

The BJT should be placed in contact with or immediately

adjacent to the power stage inductor. Its emitter should be

connected to the TSNSn 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-

surementswhenbit5ofMFR_PWM_MODE_LTC3882-1is

set to one. The factory defaults support a resistor-trimmed

dual diode network as shown in Figure 44. However, this

measurementmethodcanbeappliedtosimplesingle-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

highnoiseenvironmentspreventuseofthe∆VBEapproach

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 ∆VBE 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 ∆VBE method a

value of 0 in this register sets the temperature offset to

–273.15°C. For a direct p-n junction measurement, this

parameteradjuststhenominalcircuitvoltageat25°Caway

from that shown in Figure 44.

To ensure proper use of these temperature adjustment

parameters, refer to the specific formulas given for the two

methodsbytheMFR_PWM_MODE_LTC3882-1command

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 VDD25 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, RTOP

is connected between VDD25 and the RCONFIG pin, while

RBOT is connected between the pin and GND. Noisy clock

signals should not be routed near these pins.

Figure 43. External ΔVBE Temperature Sense

Figure 44. 2D+R Temperature Sense

TSNS

MMBT3906

LTC3882-1

10nF

GND

38821 F43

TSNS

LTC3882-1

1nF

495µA

1.35V AT 25°C

GND

38821 F44

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