LTM4664 datasheet(34/132 Pages) AD

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LTM4681

Rev. 0

For more information www.analog.com

OPERATION

power from SVIN_nn. If VBIAS is on at 5.5V output and VIN

is higher than 7.0V, the 5.5V regulator is turned off and

an internal switch is turned on, connecting VBIAS. Using

the VBIAS allows the INTVCC power to be derived from a

high efficiency internal source. VBIAS can provide power

to the internal 3.3V linear regulators when VIN is present,

which allows the LTM4681 controllers to be initialized and

programmed even with channels off.

The INTVCC_nn regulator is powered from the SVIN_nn pin,

the power through the IC is equal to SVIN_nn • IINTVCCnn.

The gate charge current is dependent on operating fre-

quency. The INTVCC_nn regulator can supply up to 100mA,

and the typical INTVCC_nn current for the LTM4681 is

~50mA. A 12V input voltage would equate to a difference

of 7V per controller drop across the internal controller,

when multiplied by 50mA equals a 350mW power loss.

This loss can be eliminated by ultilizing the VBIAS regulator.

Do not tie INTVCC_nn on the LTM4681 to an external sup-

ply because INTVCC_nn will attempt to pull the external

supply high and hit current limit, significantly increasing

the die temperature.

For applications where VIN is 5V, tie the SVIN_nn and

INTVCC_nn pins together to the 5V input through a 1Ω

resistor as shown in Test Circuit 2.

OUTPUT CURRENT SENSING AND SUB MILLIOHM

DCR CURRENT SENSING

The LTM4681 use a unique sub-milliohm inductor cur-

rent sensing technique that provides a high level signal

to noise ratio while sensing very low signals in current

mode operation. This enables higher conversion efficien-

cies with the use of the internal sub-milliohm inductors in

heavy load applications. The current limit threshold can

be accurately set with the MFR_PWM_MODE[7] for High

and Low range (see page 97).

The internal DCR sensing network, thus current limit are

calculated based on the DCR of the inductor at room tem-

perature. The DCR of the inductor has a large temperature

coefficient, approximately 3900ppm/°C. The temperature

coefficient of the inductor is written to the MFR_IOUT_

CAL_GAIN_TC register. The external temperature is sensed

near the inductor and used to modify the internal current

limit circuit to maintain an essentially constant current

limit with temperature. The current sensed is then digitized

by the LTM4681’s telemetry ADC with an input range of

±128mV, a noise floor of 7µVRMS, and a peak-peak noise of

approximately 46.5µV. The LTM4681 computes the induc-

tor current using the DCR value stored in the IOUT_CAL_

GAIN command and the temperature coefficient stored in

command MFR_IOUT_CAL_GAIN_TC. The resulting cur-

rent value is returned by the READ_IOUT command.

INPUT CURRENT SENSING

To sense the total input current consumed by the

LTM4681’s power stages , a sense resistor is placed

between the supply voltage and the drain of the top

N-channel MOSFET. The IIN_nn+ and IIN_nn– pins are con-

nected to the sense resistor. The filtered voltage is ampli-

fied by the internal high side current sense amplifier and

digitized by the LTM4681’s telemetry ADC. The input cur-

rent sense amplifier has three gain settings of 2x, 4x, and

8x set by the bit[3:2] of the MFR_PWM_CONFIG com-

mand. The maximum input sense voltage for the three

gain settings is 50mV, 25mV, and 10mV respectively. The

LTM4681 computes the input current using the internal

RSENSE value stored in the IIN_CAL_GAIN command. The

resulting measured power stage current is returned by

the READ_IIN command. IIN_01+, IIN_01– for controller

1 (channel 0 and 1), and IIN_23+, IIN_23– for controller 2

(channel 2 and 3).

The LTM4681 uses a 1Ω resistor to measure the SVIN_nn

pin supply current being consumed by each LTM4681

internal controller. This value is returned by the MFR_

READ_ICHIP command. The chip current is calculated by

using the 1Ω value stored in the MFR_ICHIP_CAL_GAIN

command. Refer to the subsection titled Input Current

Sense Amplifier in the Applications Information section

for further details.

PolyPhase LOAD SHARING

Multiple LTM4681s can be arrayed in order to provide a

balanced load-share solution by bussing the necessary

pins. Figure 50 illustrates a 8-Phase design sharing con-

nections required for load sharing.

LTM4664 Datasheet(PDF) 34 Page - Analog Devices

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