LTM4673 datasheet(60/132 Pages) AD

60 / 132 page

LTM4683

Rev. 0

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APPLICATIONS INFORMATION

at the natural rate determined by the load impedance. If

the controller is in discontinuous-conduction mode, the

controller will not pull a negative current, and the output

will be pulled low by the load, not the power stage. The

maximum fall time is limited to 1.3 seconds. The shorter

the TOFF_FALL time is set, the larger the discrete steps

in the TOFF_FALL ramp will appear. The number of steps

in the ramp is equal to TOFF_FALL/0.1ms.

UNDERVOLTAGE LOCKOUT

The LTM4683 is initialized by an internal threshold-

based UVLO where VIN must be approximately 4V and

INTVCC_nn, VDD33_nn, and VDD25_nn must be within

approximately 20% of their regulated values. In addition,

VDD33_nn must be within approximately 7% of the tar-

geted value before the RUNn pin is released. After the

part has initialized, an additional comparator monitors

VIN. The VIN_ON threshold must be exceeded before the

power sequencing can begin. When the VIN drops below

the VIN_OFF threshold, the SHARE_CLK_nn pin will be

pulled low, and the VIN must increase above the VIN_ON

threshold before the controller will restart. The normal

start-up sequence will be allowed after the VIN_ON

threshold is crossed. If FAULTn is held low when VIN is

applied, ALERTnn will be asserted low even if the part is

programmed not to assert ALERTnn when FAULTn is held

low. If I2C communication occurs before the LTM4683 is

out of reset and only a portion of the command is seen by

the part, this can be interpreted as a CML fault. If a CML

fault is detected, ALERTnn is asserted low.

It is possible to program the contents of the NVM in the

application if the VDD33_nn supply is externally driven

directly to VDD33_nn or through VBIAS. This will activate the

digital portion of the LTM4683 without engaging the high

voltage sections. PMBus communications are valid in this

supply configuration. If VIN has not been applied to the

LTM4683, bit 3 (NVM Not Initialized) in MFR_COMMON

will be asserted low. If this condition is detected, the part

will only respond to addresses 5A and 5B. To initialize the

part issue, the following set of commands: global address

0x5B command 0xBD data 0x2B followed by global

address 5B command 0xBD and data 0xC4. The part will

now respond to the correct address. Configure the part

as desired, then issue a STORE_USER_ALL. When VIN

is applied, an MFR_RESET command must be issued to

allow the PWM to be enabled and valid ADC conversions

to be read.

FAULT DETECTION AND HANDLING

The LTM4683 FAULTn pins are configurable to indicate a

variety of faults, including overvoltage (OV), undervolt-

age (UV), overcurrent (OC), overtemperature (OT), timing

faults, and peak overcurrent faults. In addition, the FAULTn

pins can be pulled low by external sources, indicating

a fault in some other portion of the system. The fault

response is configurable and allows the following options:

Ignore

Shut Down Immediately—Latch Off

Shut Down Immediately—Retry Indefinitely at the

Time Interval Specified in MFR_RETRY_DELAY

See the PMBus Command Summary and the PMBus

Command Details sections and refer to the PMBus speci-

fication for more details.

The OV response is automatic. If an OV condition is

detected, TGn goes low, and BGn is asserted.

Fault logging is available on the LTM4683. The fault log-

ging is configurable to automatically store data when a

fault occurs that causes the unit to fault off. The header

portion of the fault logging table contains peak values. It

is possible to read these values at any time. This data will

be useful while troubleshooting the fault.

If the LTM4683 internal temperature is in excess of 85°C,

writes into the NVM (other than fault logging) are not

recommended. The data will still be held in RAM, unless

the 3.3V supply UVLO threshold is reached. If the die

temperature exceeds 130°C, all NVM communication is

disabled until the die temperature drops below 120°C.

LTM4673 Datasheet(PDF) 60 Page - Analog Devices

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