LTM4677 datasheet(22/116 Pages) AD

22 / 116 page

LTC3883/LTC3883-1

Rev. F

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OPERATION

theprogrammedFREQUENCY_SWITCHcommandvalue,

the SYNC pin will pull low at the desired clock rate with

500ns low pulse. Care must be taken in the application to

assure the capacitance on SYNC is minimized to assure

the pull-up resistor versus the capacitor load has a low

enough time constant for the application. In addition,

a phase-locked loop (PLL) is available to synchronize

the internal oscillator to an external clock source that is

connected to the SYNC pin. All phase relationships are

between the falling edge of SYNC and the rising edge

of the LTC3883 TG output. Multiple LTC3883s can be

synchronized in order to realize PolyPhase arrays.

OUTPUT VOLTAGE SENSING

The differential amplifier allows remote, differential sens-

ing of the load voltage with VSENSEn pins. The telemetry

ADC is fully differential and makes measurements of the

output voltage at the VSENSEn pins.

OUTPUT CURRENT SENSING

For DCR current sense applications, a resistor in series

with a capacitor is placed across the inductor. In this

configuration, the resistor is tied to the FET side of the

inductor while the capacitor is tied to the load side of the

inductor as shown in Figure 3. If the RC values are cho-

sen such that the RC time constant matches the inductor

time constant (L/DCR, where DCR is the inductor series

resistance), the resultant voltage (VDCR) appearing across

the capacitor will equal the voltage across the inductor

series resistance and thus represent the current flowing

through the inductor. The RC calculations are based on

the room temperature DCR of the inductor.

The RC time constant should remain constant, as a func-

tionoftemperature.Thisassuresthetransientresponseof

the circuit is the same regardless of the temperature. The

DCR of the inductor has a large temperature coefficient,

approximately 3900ppm/°C. The temperature coefficient

of the inductor must be written to the MFR_IOUT_CAL_

GAIN_TC command. The external temperature is sensed

near the inductor and is used to modify the internal current

limit circuit to maintain an essentially constant current

limit with temperature. In this application, the ISENSE+

pin is connected to the FET side of the capacitor while

the ISENSE– pin is placed on the load side of the capacitor.

The current sensed from the input is then given by the

expression VDCR/DCR. VDCR is digitized by the LTC3883’s

telemetry ADC with an input range of ±128mV, a noise

floor of 7µVRMS, and a peak-peak noise of approximately

46.5µV. The LTC3883 computes the inductor current

using the DCR value stored in the IOUT_CAL_GAIN com-

mand and the temperature coefficient stored in command

MFR_IOUT_CAL_GAIN_TC. The resulting current value is

returned by the READ_IOUT command.

AUTO CALIBRATION

Using a patent pending auto-calibration routine, the

LTC3883 can measure the actual DC resistance for DCR

current sense applications. The measured value is used

in READ_IOUT measurements and eliminates the need

for the user to know the actual resistance of the inductor.

Reference the subsection titled Inductor DCR Calibration

in the Applications Information section for further detail.

ACCURATE DCR TEMPERATURE COMPENSATION

The LTC3883 uses a patent pending algorithm to dy-

namically model the temperature rise from the external

temperature sensor to the inductor core. Refer to the

Accurate DCR Temperature Compensation subsection in

the Applications Information section for complete details.

INPUT CURRENT SENSING

To sense the total input current consumed by the LTC3883

and the power stage, a resistor is placed between the sup-

ply voltage and the drain of the top N-channel MOSFET.

The VIN_SNS and IIN_SNS pins are connected to the sense

resistorthrough100Ωfilterresistors.Bothpinsneedtobe

decoupledtoGND.Afiltercapacitorneedstobeconnected

across the VIN_SNS and IIN_SNS pins. Refer to Figure 25,

Low Noise Input Current Sense Circuit for further details.

The filtered voltage is amplified by the internal high side

current sense amplifier and digitized by the LTC3883’s

telemetry ADC. The input current sense amplifier has

three gain settings of 2x, 4x, and 8x set by the bits 5:4 of

the MFR_PWM_MODE command. The maximum input

sense voltage for the three gain settings is 50mV, 20mV,

and 8mV respectively. The LTC3883 computes the input

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