LTC2972 datasheet(92/112 Pages) LINER

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LTC2972

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APPLICATIONS INFORMATION

MEASURING INPUT VOLTAGE

READ_VIN returns input voltage on the VIN_SNS pin.

TUE_VIN reflects READ_VIN measurement error. Large

rippleontheVIN_SNSpinmayaffectREAD_VIN,READ_PIN,

andMFR_EINaccuracy.Toreducerippleinducedmeasure-

ment errors, consider adding the optional anti-aliasing

filter components shown in Figure 27. R3 and C3 filter

VIN_SNS with 30dB attenuation at 62.5kHz. R1, R2, C1, C2

add additional filtering for IIN_SNS inputs and prevent the

internal diodes, D1 to D4, from turning on.

MEASURING INPUT POWER

READ_PINreturnsinputpowerinwattscalculatedfromthe

product of the most recent VVIN_SNSand IIN_SNS measure-

ments. Although the Electrical Characteristics table only

specifies READ_PIN Total Unadjusted Error (TUE_PIN)

under typical conditions, TUE_PIN is actually bounded

by the sum of TUE_IIN and TUE_VIN.

TUE_PIN ≤ TUE_IIN + TUE_VIN

For example, if 15mV < |VIIN_SNS| < 50mV with the current

sense amplifier set for medium range, TUE_IIN ≤ 1% and

TUE_PIN is less than 1% (TUE_IIN) + 0.5% (TUE_VIN)

= 1.5%.

Since current sense ranges include positive and negative

inputs, READ_PIN returns signed values indicating power

transfer magnitude and direction.

MEASURING INPUT ENERGY

The 12 byte data block, MFR_EIN, contains a 48-bit accu-

mulated energy measurement in mJ, Energy_value[47:0],

and a 48-bit elapsed time in milliseconds since energy

began accumulating, Energy_time[47:0]. Refer to INPUT

CURRENT AND ENERGY and MFR_COMMAND_PLUS

sections of the PMBUS COMMAND DESCRIPTION for

accumulated energy and elapsed time data access details.

Energy_value can accumulate up to (248-1) mJ of energy

before wrapping. Energy can accumulate for (248-1) ms,

or about 8925 years, before Energy_time wraps. Ac-

cumulation of negative power measurements decreases

Energy_value, and the energy meter saturates when it

reaches 0 millijoules.

Theenergymetertimebaseerror(TUE_ETB)specifieserror

in the internal energy time base accuracy: Energy_time is

accurate with maximum error of TUE_ETB. Accumulated

energyincludeserrorsfromcurrentsensemeasurements,

voltage sense measurements, and the internal time base.

Energy_value error (TUE_EIN) is bounded by the summa-

tion of TUE_IIN, TUE_VIN, and TUE_ETB:

TUE_EIN ≤ TUE_IIN + TUE_VIN + TUE_ETB

For example, if VIIN_SNS = 20mV with high current sense

amplifier range, TUE_IIN is less than 1% error, TUE_VIN

is less than 0.5% error, and TUE_ETB is less than 1% er-

ror. Therefore the energy measurement error (TUE_EIN)

is less than 2.5%.

SEQUENCE, SERVO, MARGIN AND RESTART

OPERATIONS

Command Units On or Off

Three control parameters determine how a particular

channel is turned on and off: The CONTROL pins, the

OPERATION command and the value of the input voltage

measured at the VIN_SNS pin (VIN). In all cases, VIN must

exceed VIN_ON in order to enable the device to respond

to the CONTROL pins or OPERATION commands. When

VIN drops below VIN_OFF an immediate OFF or sequence

off after TOFF_DELAY of both channels will result (See

Mfr_config_track_enn). Refer to the OPERATION sec-

tion in the data sheet for a detailed description of the

ON_OFF_CONFIG command.

Some examples of typical ON/OFF configurations are:

1. A DC/DC converter may be configured to turn on any

time VIN exceeds VIN_ON.

2. A DC/DC converter may be configured to turn on only

when it receives an OPERATION command.

3. A DC/DC converter may be configured to turn on only

via the CONTROL pin.

4. A DC/DC converter may be configured to turn on only

when it receives an OPERATION command and the

CONTROL pin is asserted.

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