LTM4677 datasheet(24/116 Pages) AD

24 / 116 page

LTC3883/LTC3883-1

Rev. F

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OPERATION

in parallel with the diode connected PNP. Two different cur-

rentsareappliedtothediode(nominally2µAand32µA)and

thetemperatureiscalculatedfromthe∆VBEmeasurement.

Theexternaltransistortemperatureisdigitizedbythetelem-

etry ADC, and the value is returned by the PMBus READ_

TEMPERATURE_1 command.

The READ_TEMPERATURE_2 command returns the

junction temperature of the LTC3883 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

slopeadjustmentsslightlylessthan 1.TheMMBT3906has

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.

Theoffsetoftheexternaltemperaturesensecanbeadjusted

by MFR_TEMP_1_OFFSET. A value of 0 in this command

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,theinterceptpointisatabsolutezero,–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.

If an external temperature sense element is not used, the

TSNS pin must be shorted to GND. The UT_FAULT_LIMIT

must be set to –275°C, and the UT_FAULT_RESPONSE

must be set to ignore. The user also needs to set the

IOUT_CAL_GAIN_TC to a value of 0.

RCONFIG (RESISTOR CONFIGURATION) PINS

The pins FREQ_CFG, VOUT_CFG and VTRIM_CFG can be

used to select important operating parameters without

programming the configuration EEPROM. Connecting

thesepinstoexternalresistordividersselectstheswitching

frequency, output voltage and basic power management

supervisor parameters. The ASEL pin is used to select the

unique device bus address. Connect this pin to an external

resistor divider to select the device address. Always use

a resistor divider to select the device address. Setting

the device address in EEPROM is allowed, but can create

problems if the device address is somehow lost by the

host. It is safe and prudent to use the ASEL pin to set the

device address. If RCONFIG pins are floated, the value

stored in the corresponding NVM command is used. If

bit 6 of the MFR_CONFIG_ALL_LTC3883 configuration

command is asserted in NVM, the resistor inputs are

ignored upon power-up except for ASEL which is always

respected. The resistor configuration pins are only mea-

sured during a power-up reset or after an MFR_RESET

command is executed.

TheVOUT_CFGandVTRIMpinsettingsaredescribedinTables

12 and 13. These pins select the output voltage for the

LTC3883’s analog PWM controller. If both pins are open,

the VOUT_COMMAND command is loaded from NVM to

determine the output voltage.

The following parameters are set as a percentage of the

output voltage if the RCONFIG pins are used to determined

output voltage:

VOUT_OV_FAULT_LIMIT.................................... +10%

VOUT_OV_WARN_LIMIT .................................. +7.5%

VOUT_MAX....................................................... +7.5%

VOUT_MARGIN_HIGH .........................................+5%

POWER_GOOD_ON .............................................–7%

POWER_GOOD_OFF............................................–8%

VOUT_MARGIN_LOW..........................................–5%

VOUT_UV_WARN_LIMIT ..................................–6.5%

VOUT_UV_FAULT_LIMIT......................................–7%

The FREQ_CFG pin settings are described in Table 14. This

pinselectstheswitchingfrequencyandphaserelationship

between the PWM channel and SYNC pin. To synchronize

toanexternalclock,thepartmustbeputintoexternalclock

Figure 4. Temperature Sense Circuit

TSNS

MMBT3906

LTC3883

10nF

GND

3883 F04

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