LTC2972 datasheet(101/112 Pages) LINER

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LTC2972

101

2972fa

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APPLICATIONS INFORMATION

Let CCM2 = 3.3nF. Note that since CCM2 is much less than

CCM1 the loading effects of the second stage filter on the

matched first stage are not significant. Consequently, the

delay time constant through the filter for the current sense

waveform will be approximately 3μs.

Measuring Multiphase Currents

For current sense applications with more than one phase,

R-C averaging may be employed. Figure 38 shows an

example of this approach for a 3-phase system with DCR

current sensing. The current sense waveforms are aver-

aged together prior to being applied to the second stage of

the filter consisting of RCM2 and CCM2. Because the RCM1

resistors for the three phases are in parallel, the value of

RCM1 must be multiplied by the number of phases. Also

note that since the DCRs are effectively in parallel, the

value for IOUT_CAL_GAIN will be equal to the inductor’s

DCR divided by the number of phases. Care should be

taken in the layout of the multiphase inductors to keep the

PCB trace resistance from the DC side of each inductor to

the summing node balanced in order to provide the most

accurate results.

Measuring Output Current in Buffered IMON

Telemetry Mode

The LTC2972 can interface to power supplies that out-

put a signal proportional to their internally measured

output current by setting Mfr_config_imon = 1. This

mode increases the ISENSE input range from ±170mV

to –0.1V to 6V. Regulators whose IMON pin outputs

a current proportional to the average load current will

require a resistor to convert this current to a voltage

for digitizing by the LTC2972 ADC. The finite input

impedance of ISENSE input of the LTC2972 may affect

the absolute accuracy of the load current measurement.

Figure39 shows the LTC2972 connecting to the IMON

pin of the LT®3086 LDO regulator.

LT3086 IMON Design Example

The LT3086 will output a current equal to IIMON =

IOUT/1000. If the desired VIMON is 1.0V for a total output

current of IOUT = 2.0A then RIMON = VIMON/IIMON = 500Ω

and IOUT_CAL_GAIN should be programmed to 500mΩ.

Figure 38. Multiphase DCR Current Sensing Circuits

LTC2972

ISENSEP

ISENSEM

2972 F38

CCM2

RCM2

CCM2

CCM1

RCM2

RCM1

RCM1/3

DCR

SWX2

SWX3

TO LOAD

SWX1

2972 F39

SHDN

IMON

OUT

SET

GND

LT3086

VIN

VIMON

VOUT

RSET

RIMON

IMON =

IOUT

1000

VIMON =IMON •RIMON

IOUT_CAL_GAIN = RIMON

1000

ISENSEP

LTC2972

ISENSEM

MFR_CONFIG_IMON_SEL = 1

Figure 39. IMON Current Sensing

LTC2972 Datasheet(PDF) 101 Page - Linear Technology

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