LTM4677 datasheet(39/116 Pages) AD

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LTC3883/LTC3883-1

Rev. F

For more information www.analog.com

APPLICATIONS INFORMATION

TheTypicalApplicationonthebackpageisabasicLTC3883

application circuit. The LTC3883 can be configured to use

either DCR (inductor resistance) sensing or low value

resistor sensing. The choice between the two current

sensing schemes is largely a design trade-off between

cost, power consumption and accuracy. DCR sensing

is becoming popular because it saves expensive current

sensing resistors and is more power efficient, especially

in high current applications. The LTC3883 can nominally

accountforthetemperaturedependencyoftheDCRsensing

element. The accuracy of the current reading and current

limit are typically limited by the accuracy of the DCR resis-

tor (accounted for in the IOUT_CAL_GAIN parameter of

the LTC3883). Thus current sensing resistors provide the

most accurate current sense and limiting for the applica-

tion. Other external component selection is driven by the

load requirement, and begins with the selection of RSENSE

(if RSENSE is used) and inductor value. Next, the power

MOSFETs are selected. Then the input and output capaci-

tors are selected. Finally the current limit is selected. All of

thesecomponentsandrangesarerequiredtobedetermined

priortocalculatingtheexternalcompensationcomponents.

The current limit range is required because the two ranges

(25mV to 50mV vs 37.5mV to 75mV) have different EA

gains set with bit 7 of the MFR_PWM_MODE_LTC3883

command. The voltage RANGE bit also modifies the loop

gain and impacts the compensation network set with bits

5, 6 of MFR_PWM_CONFIG_LTC3883. All other program-

mable parameters do not affect the loop gain, allowing

parameters to be modified without impact to the transient

response to load.

CURRENT LIMIT PROGRAMMING

The LTC3883 has two ranges of current limit programming

and a total of eight levels within each range. Refer to the

IOUT_OC_FAULT_LIMITsectionofthePMBuscommands.

Within each range the error amp gain is fixed, resulting in

constant loop gain. The LTC3883 will account for the DCR

oftheinductorandautomaticallyupdatethecurrentlimitas

the inductor temperature changes. The temperature coef-

ficientoftheDCRisstoredintheMFR_IOUT_TCcommand.

For the best current limit accuracy, use the 75mV setting.

The 25mV setting will allow for the use of very low DCR

inductors or sense resistors, but at the expense of cur-

rent limit accuracy. Keep in mind this operation is on a

cycle-by-cycle basis and is only a function of the peak

inductorcurrent.Theaverageinductorcurrentismonitored

by the ADC converter and can provide a warning if too

much average output current is detected. The overcurrent

fault is detected when the ITH voltage hits the maximum

value. The digital processor within the LTC3883 provides

the ability to either ignore the fault, shut down and latch off

or shut down and retry indefinitely (hiccup). Refer to the

overcurrentportionoftheOperationsectionformoredetail.

ISENSE+ AND ISENSE– PINS

The ISENSE+ and ISENSE– pins are the inputs to the current

comparator and the A/D. The common mode input voltage

range of the current comparators is 0V to 5.5V. Both the

SENSE pins are high impedance inputs with small base

currents typically less than 1µA. When the ISENSE pin volt-

agesarebetween0Vand1.4V,thesmallbasecurrentsflow

out of the SENSE pins. When the ISENSE pin voltages are

greater than 1.4V, the base currents flow into the ISENSE

pins. The high impedance inputs to the current compara-

tors allow accurate DCR sensing. Do not float these pins

during normal operation.

Filter components mutual to the ISENSE lines should be

placed close to the IC. The positive and negative traces

should be routed differentially and Kelvin connected to

the current sense element, see Figure 17. A non-Kelvin

connection elsewhere can add parasitic inductance and

capacitance to the current sense element, degrading

the information at the sense terminals and making the

programmed current limit unpredictable. In a PolyPhase

system,poorplacementofthesensingelementwillresultin

sub-optimalcurrentsharingbetweenpowerstages.IfDCR

sensing is used (Figure 18a), sense resistor R1 should be

placed close to the switching node to prevent noise from

coupling into sensitive small-signal nodes. The capacitor

Figure 17. Optimal Sense Line Placement

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

3883 F17

LTM4677 Datasheet(PDF) 39 Page - Analog Devices

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