LTC2977 datasheet(70/138 Pages) AD

70 / 138 page

LTM4664A

Rev. 0

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to avoid this problem. Multiple LTM4664As PSM sections

are required to share one SYNC pin in PolyPhase config-

urations. For other configurations, connecting the SYNC

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

pin is shared between LTM4664As PSM sections, only

one LTM4664A section can be programmed with a fre-

quency output. All the other LTM4664As sections should

be programmed to disable the SYNC output. However

their frequency should be programmed to the nominal

desired value. See application schematic in Figure 51.

INPUT CURRENT SENSE AMPLIFIER

The LTM4664A input current sense amplifier can sense

the supply current into the VINS3_Cn power stages pins

using an external sense resistor as shown in the Figure 2

Block Diagram. The RSENSE 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 VINS3

controller pin due to the current flowing into the VINS3 con-

troller 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 LTM4664A

PSM will multiply the MFR_READ_ICHIP measurement

value by this 1Ω resistor and add this voltage to the

measured voltage at the VINS3 controller pin. Therefore,

READ_VIN = VIN_CNTLPIN + (MFR_READ_ICHIP • 1Ω)

The MFR_READ_ICHIP command is used to measure the

internal controller current. Using the READ_PIN com-

mand allows for reading calculated input power.

PROGRAMMABLE LOOP COMPENSATION

The LTM4664A offers programmable loop compensation

to optimize the transient response without any hardware

change. The error amplifier gain gm varies from 1.0m

to 5.73m , and the compensation resistor RCOMP varies

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

tion capacitors, COMP_na and COMP_nb, are required in

the design and the typical ratio between COMP_

na and

COMP_nb is 10. Also see Figure 2 Block Diagram, and

Figure 31.

By adjusting the gm and RCOMP only, the LTM4664A PSM

can provide a flexible Type II compensation network to

optimize 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 32.

Adjusting the RCOMP will change the pole and zero loca-

tion, as shown in Figure 33. It is recommended that the

user determines the appropriate value for the gm and

RCOMP using the LTpowerCAD tool.

–

VREF

4664A F31

COMP_

RCOMP

Figure 31. Programmable Loop Compensation

Figure 32. Error Amp gm Adjust

INCREASE gm

FREQUENCY

4664A F32

GAIN

TYPE II COMPENSATION

DUAL 25A/30A PSM APPLICATIONS INFORMATION

LTC2977 Datasheet(PDF) 70 Page - Analog Devices

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