LTC3860 datasheet(18/36 Pages) LINER

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LTC3860

3860f

applicaTions inFormaTion

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or transient current

handling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available in

surfacemountconfigurations.Newspecialpolymersurface

mount capacitors offer very low ESR also but have much

lower capacitive density per unit volume. In the case of

tantalum, it is critical that the capacitors are surge tested

for use in switching power supplies. Several excellent

output capacitor choices include the Sanyo POSCAP TPD,

TPE, TPF series, the Kemet T520, T530 and A700 series,

NEC/Tokin NeoCapacitors and Panasonic SP series. Other

capacitor types include Nichicon PL series and Sprague

595D series. Consult the manufacturer for other specific

recommendations.

Current Sensing

To maximize efficiency the LTC3860 is designed to sense

current through the inductor’s DCR, as shown in Figure 6.

The DCR of the inductor represents the small amount

of DC winding resistance of the copper, which for most

inductors applicable to this application, is between 0.3

and 1m

Ω. If the filter RC time constant is chosen to be

exactly equal to the L/DCR time constant of the inductor,

the voltage drop across the external capacitor is equal

to the voltage drop across the inductor DCR. Check the

manufacturer’s data sheet for specifications regarding the

inductor DCR in order to properly dimension the external

filter components. The DCR of the inductor can also be

measured using a good RLC meter.

Since the temperature coefficient of the inductor’s DCR is

3900ppm/°C, first order compensation of the filter time

constant is possible by using filter resistors with an equal

but opposite (negative) TC, assuming a low TC capacitor is

used. That is, as the inductor’s DCR rises with increasing

temperature, the L/DCR time constant drops. Since we

want the filter RC time constant to match the L/DCR time

constant, we also want the filter RC time constant to drop

with increasing temperature. Typically, the inductance will

also have a small negative TC.

The ISNSP and ISNSN pins are the inputs to the current

comparators. The common mode range of the current

comparators is –0.3V to VCC + 0.1V. Continuous linear

operation is provided throughout this range, allowing

output voltages between 0.6V (the reference input to the

error amplifiers) and VCC+0.1V.Themaximumdifferential

current sense input (VISNSP – VISNSN) is 50mV.

The high impedance inputs to the current comparators

allow accurate DCR sensing. However, care must be taken

not to float these pins during normal operation.

Filter components mutual to the sense lines should be

placed close to the LTC3860, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 5). Sensing cur-

rent elsewhere can effectively add parasitic inductance

and capacitance to the current sense element, degrading

the information at the sense terminals and making the

programmed current limit unpredictable. If low value

(<5mΩ) sense resistors are used, verify that the signal

across CF resembles the current through the inductor,

and reduce RF to eliminate any large step associated with

the turn-on of the primary switch. If DCR sensing is used

(Figure 6b), sense resistor R1 should be placed close to

the switching node, to prevent noise from coupling into

sensitive small-signal nodes. The capacitor C1 should be

placed close to the IC pins.

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

3860 F05

Figure 5. Sense Lines Placement with Inductor or Sense Resistor

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