LTC3889 datasheet(28/118 Pages) AD

28 / 118 page

LTC3888/LTC3888-1/

LTC3888-2

Rev. B

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OPERATION

OUTPUT VOLTAGE SENSING AND MONITORING

Both on-chip voltage control loops allow remote, differ-

ential sensing of the load voltage with VSENSE± pins. The

amplifier outputs provide a 1X buffered version of this

sensed differential referenced to package GND. These

outputs are suitable for driving a resistor divider to GND

supplying FB (LTC3888/3888-2), VFB (LTC3888-1) to set

the desired output voltage.

VOUT supervisor UV and OV fault limits are set from

EEPROM or PMBus command. Factory EEPROM sets

the supervisor limits and margin values as a percentage

of VOUT (LTC3888/3888-2), VFB (LTC3888-1) and hence

VOUT. If these are modified by PMBus command, they are

specified in absolute volts.

The telemetry ADC is also fully differential and makes

measurements of regulated output voltages at VSENSE±.

Conversion results are returned by READ_VOUT.

OUTPUT CURRENT SENSING AND MONITORING

Every PWM channel on the LTC3888 accepts an output

current monitor signal from the power stage it controls. A

proprietary input interface uses this information to control

output pulse width on that phase. When the ISENSE pins

for a channel are multiplexed to the differential inputs of

the LTC3888 monitor ADC, they use IREF as the nega-

tive input. These ADC channels have a differential input

range of approximately ±400mVdc. The internal ADC

anti-aliasing filter and conversion rate produce an aver-

age reading of the ISENSE input voltage relative to IREF.

The resulting value is returned by the READ_IOUT PMBus

command. To allow reduction in round-robin latency, bit 6

of MFR_PWM_CONFIG_LTC3888 determines if the ADC

converts ISENSE for all eight phases, or only the two mas-

ter channels. If only ISENSE0 and ISENSE1 are monitored,

the IOUT results for the remaining six channels are set

to 0A. READ_IOUT returns readings for the two master

channels, MFR_TOTAL_IOUT returns the sum of all out-

put currents for on-chip phases assigned to each master

channel, and MFR_READ_ALL_IOUT returns output cur-

rent for each individual phase using block read format.

TEMPERATURE SENSE

External temperature can be monitored by most modern

power stages. This is normally presented on a highest-

indicated wire-OR analog bus for each rail, where the

device with highest internal temperature sets the bus volt-

age. The internal ADC converts these temperature inputs

at one-third the rate of voltage or current channels, and

those results are returned by READ_TEMPERATURE_1.

READ_TEMPERATURE_2 returns the internal junction

temperature of the LTC3888 using an on-chip diode with

accurate ΔVBE measurement and calculation.

RESISTOR CONFIGURATION PINS

Six input pins can be used to configure key operating

parameters with selected 1% resistors between each pin

and GND. RSET establishes the proper bias levels for the

remaining five pins and requires an 18.7kΩ resistor to

GND. The remaining RCONFIG pins are ASEL0, ASEL1,

VOUT0_CFG, VOUT1_CFG (LTC3888/3888-2) or ILIM0_

CFG, ILIM1_CFG (LTC3888-1), and PWM_CFG. If any of

these five pins are left open, the value stored in the cor-

responding EEPROM command(s) is used. The resistor

configuration pins are only measured during power-up

and execution of RESTORE_USER_ALL or MFR_RESET.

If bit 6 of MFR_CONFIG_ALL is set in EEPROM, all resistor

configuration pins except ASEL0 and ASEL1 are ignored.

Per the PMBus specification, all pin-programmed param-

eters can be overridden at any time by commands from

the digital interface.

The ASEL0/1 pin settings are described in application

Table 12. These pins can be used to select the entire

LTC3888 device address. ASEL0 always programs the

bottom four bits of the device address for the LTC3888

unless left open. ASEL1 can be used to program the

three most-significant bits. Either portion of the address

can also be retrieved from the MFR_ADDRESS value in

EEPROM. If both pins are left open, the full 7-bit MFR_

ADDRESS value stored in EEPROM is used to determine

the device address. The LTC3888 always responds to

7-bit global addresses 0x5A and 0x5B. MFR_ADDRESS

should not be set to either of these values or 0x7C.

LTC3889 Datasheet(PDF) 28 Page - Analog Devices

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