LTM4664 datasheet(63/132 Pages) AD

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LTM4681

Rev. 0

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APPLICATIONS INFORMATION

to avoid this problem. Multiple LTM4681s are required

to share one SYNC_nn pin in PolyPhase configurations.

For other configurations, connecting the SYNC_nn pins

to form a single SYNC signal is optional. If the SYNC_nn

pin is shared between LTM4681s, only one LTM4681 con-

trollers can be programmed with a frequency output. All

the other LTM4681s should be programmed to disable

the SYNC_nn output. However their frequency should be

programmed to the nominal desired value.

INPUT CURRENT SENSE AMPLIFIER

The LTM4681 input current sense amplifier can sense the

supply current into the VIN01 and VIN23 power stages pins

using an external sense resistor as shown in the Figure 2

Block Diagram. The RSENSEn value can be programmed

using the MFR_IIN_CAL_GAIN command. Kelvin sensing

is recommended across the RSENSE resistor to eliminate

errors. The MFR_PWM_CONFIG [6:5] sets the input cur-

rent sense amplifier gain. See the MFR_PWM_CONFIG

section. The IIN_OC_WARN_LIMIT command sets the

value of the input current measured by the ADC, in

amperes, that causes a warning indicating the input cur-

rent is high. The READ_IIN value will be used to determine

if this limit has been exceeded. The READ_IIN command

returns the input current, in Amperes, as measured across

the input current sense resistor.

There is an IR voltage drop from the supply to the SVIN_nn

pin due to the current flowing into the SVIN_nn pin. To

compensate for this voltage drop, the MFR_RVIN will

be automatically set to the 1Ω internal sense resistor in

the Figure 2 Block Diagram. The LTM4681 will multiply

the MFR_READ_ICHIP measurement value by this 1Ω

resistor and add this voltage to the measured voltage at

the SVIN_nn pin. Therefore, READ_VIN = VSVIN_PIN +

(MFR_READ_ICHIP • 1Ω) The MFR_READ_ICHIP com-

mand is used to measure the internal controller current.

Using the READ_PIN command allows for reading calcu-

lated input power.

PROGRAMMABLE LOOP COMPENSATION

The LTM4681 offers programmable loop compensation

to optimize the transient response without any hardware

change. The error amplifier gain gm varies from 1.0mS

to 5.76mS, and the compensation resistor RCOMPn varies

from 0kΩ to 62kΩ inside the controller. Two compensa-

tion capacitors, COMPna and COMPnb, are required in

the design and the typical ratio between COMPna and

COMPnb is 10. Also see Figure 2 Block Diagram and

Figure 27.

By adjusting the gm and RCOMPn only, the LTM4681 can

provide a flexible Type II compensation network to opti-

mize the loop over a wide range of output capacitors.

Adjusting the gm will change the gain of the compensation

over the whole frequency range without moving the pole

and zero location, as shown in Figure 28.

Adjusting the RCOMP will change the pole and zero loca-

tion, as shown in Figure 29. It is recommended that the

user determines the appropriate value for the gm and

RCOMPn using the LTPowerCAD tool.

Figure 27. Programmable Loop Compensation

Figure 28. Error Amp gm Adjust

INCREASE gm

FREQUENCY

4681 F28

GAIN

TYPE II COMPENSATION

–

VREF

4681 F27

CCOMPH

CCOMPL

COMPnb

RCOMPn

COMPna

LTM4664 Datasheet(PDF) 63 Page - Analog Devices

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