LTM4675 datasheet(21/128 Pages) LINER

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LTC3884/LTC3884-1

3884fe

For more information www.linear.com/LTC3884

OPERATION

Fault reporting and shutdown behavior are fully con-

figurable. Two individual FAULT0, FAULT1 outputs are

provided, both of which can be masked independently.

Three dedicated pins for ALERT, PGOOD0/1 functions are

provided. The shutdown operation also allows all faults

to be individually masked and can be operated in either

unlatched (hiccup) or latched modes.

Individual status commands enable fault reporting over

the serial bus to identify the specific fault event. Fault or

warning detection includes the following:

Output Undervoltage/Overvoltage

Input Undervoltage/Overvoltage

Input and Output Overcurrent

Internal Overtemperature

External Overtemperature

Communication, Memory or Logic (CML) Fault

MAIN CONTROL LOOP

The LTC3884 is a constant-frequency, current mode step-

down controller containing two channels operating with

user-defined relative phasing. During normal operation the

top MOSFET is turned on when the clock for that channel

sets the RS latch, and turned off when the main current

comparator, ICMP, resets the RS latch. The peak inductor

current at which ICMP resets the RS latch is controlled by

the voltage on the ITH pin which is the output of each er-

ror amplifier, EA. The EA negative terminal is equal to the

differential voltage between VSENSE+ and VSENSE– divided

by 5.5 (or 2.75 if MFR_PWM_MODE[1] = 1). The positive

terminal of the EA is connected to the output of a 12-bit

DAC with values ranging from 0V to 1.024V. The output

voltage, through feedback of the EA, will be regulated to

5.5 times the DAC output (or 2.75 times). The DAC value

is calculated by the part to synthesize the user's desired

output voltage. The output voltage is programmed by the

user either with the resistor configuration pins detailed

in Table 3 or by the PMBus VOUT command (either from

EEPROM or by PMBus command). Refer to the PMBus

commandsectionofthedatasheetorthePMBusspecifica-

tion for more details. The PMBus VOUT_COMMAND can

be executed at any time while the device is running. This

command will typically have a latency less than 10ms. The

user is encouraged to refer to the PMBus Power System

Management Protocol Specification to understand how to

program the LTC3884.

http://www.pmbus.org/specs.html

Continuing the basic operation description, the current-

mode controller will turn off the top gate when the peak

current is reached. If the load current increases, sense

voltagewillslightlydroopwithrespecttotheDACreference.

This causes the ITH voltage to increase until the average

inductor current matches the new load current. After the

top MOSFET has turned off, the bottom MOSFET is turned

on. In continuous conduction mode, the bottom MOSFET

stays on until the end of the switching cycle.

EEPROM

The LTC3884 contains internal EEPROM (nonvolatile

memory) with Error Correction Coding (ECC) to store user

configuration settings and fault log information. EEPROM

endurance retention and mass write operation time are

specified in the Electrical Characteristics and Absolute

Maximum Ratings sections. Write operations above TJ =

85°C are possible although the Electrical Characteristics

are not guaranteed and the EEPROM will be degraded.

Read operations performed at temperatures between

–40°C and 125°C will not degrade the EEPROM. Writing

to the EEPROM above 85°C will result in a degradation of

retentioncharacteristics.Thefaultloggingfunction,which

is useful in debugging system problems that may occur

at high temperatures, only writes to fault log EEPROM

locations. If occasional writes to these registers occur

above 85°C, the slight degradation in the data retention

characteristics of the fault log will not take away from the

usefulness of the function.

It is recommended that the EEPROM not be written

when the die temperature is greater than 85°C. If the die

temperature exceeds 130°C, the LTC3884 will disable all

EEPROM write operations. All EEPROM write operations

will be re-enabled when the die temperature drops below

125°C. (The controller will also disable all the switching

when the die temperature exceeds the internal overtem-

perature fault limit 160°C with a 10°C hysteresis)

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