LT3757 datasheet(20/48 Pages) AD

20 / 48 page

LTC7841

Rev. 0

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OPERATION

Internal Reference DAC

The LTC7841 has an internal 9-bit DAC that allows the

internal reference voltage to be set to any value in the

range of 0 to 2.4V with 0.2%/bit resolution. The digital

value is changed with the MFR_VOUT_COMMAND com-

mand through the PMBus interface. When a change in

the reference is detected, the reference is ramped (0.2%/

step) from its current value to the new value at a rate set

by the capacitor value connected to the CSLEW pin. The

slew rate can also be changed by the PMBus interface

using the MFR_CONFIG command. Using this command,

the slew rate can be set to one of three relative values:

nominal (1x), fast (2x) and slow (0.125x). The LTC7841

can be pre-loaded with two additional reference values

with the MFR_VOUT_MARGIN_HIGH and MFR_VOUT_

MARGIN_LOW commands. Once loaded, the reference

can then be switched between any of these three reg-

ister values by setting the appropriate margin bit in the

OPERATION register.

Whenever a power cycle, RUN pin turn-on, or MFR_RESET

event occurs, the MFR_VOUT_COMMAND register will be

re-initialized to zero. Margin high/low registers are also

initialized to zero at this time.

During VOUT changes, PLLIN/MODE pin setting is ignored

and forced continuous mode is used to guarantee that

VOUT is changed at the programmed slew rate regardless

of the load.

Telemetry Readback

The LTC7841 has an integrated 13-bit ADC that monitors

and performs conversions on the input and output volt-

age, input and output current, and either die temperature

or remote sense temperature. The values are refreshed

at a 25Hz rate and are readable through the PMBus inter-

face. A peak monitor is also available for each of these

telemetry measurements to provide that highest value

measured since the start of the monitor. The monitor is

reset by the MFR_CLEAR_PEAKS command, writing to

the individual peak register, or cycling the RUN pin.

Temperature Monitor and Thermal Warning/Shutdown

The LTC7841 has two internal die temperature monitors

— one that is digitized by the ADC and reported with

the READ_TEMPERATURE_1 command and a second

that is used to shut down the LTC7841 IC when the die

temperature exceeds 170°C. A third temperature moni-

tor is also available for external temperature sensing by

connecting an external temperature sense element to the

TSNS pin. This is useful for monitoring temperature close

to the power inductor or power MOSFETs where tem-

perature is typically highest. The LTC7841 reports either

internal die temperature or external temperature sense

voltage with the READ_TEMPERATURE_1 command. The

MFR_CONFIG register is used to set which one is active.

When internal die temperature is selected and the inter-

nal die temperature exceeds 150°C, the overtemperature

warning bit in the STATUS_WORD is set and the ALERT

pin pulls low to alert the PMBus master. This overtem-

perature warning bit is latched and remains set until the

host clears it. Both internal die temperature monitors have

about 20°C of hysteresis before the overtemperature con-

dition is cleared.

SERIAL INTERFACE

The LTC7841 serial interface is a PMBus compliant slave

device and can operate at any frequency between 10kHz

and 400kHz. The address is configurable using an external

resistor. In addition the LTC7841 always responds to the

global broadcast address of 0x5A or 0x5B (7 bit). The

serial interface supports the following protocols defined

in the PMBus specifications: 1) send command, 2) write

byte, 3) write word, 4) group, 5) read byte and 6) read

word. The PMBus write operations are not acted upon

until a complete valid message is received by the LTC7841

including the STOP bit.

Communication Failure

Attempts to access unsupported commands or writing

invalid data to supported commands will result in a CML

fault. The CML bit is set in the STATUS_WORD command

and the ALERT pin is pulled low.

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