LTC3860_15 Datasheet, PDF (17/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
APPLICATIONS INFORMATION
Manufacturers such as Sanyo, Panasonic and Cornell Du-
bilier should be considered for high performance through-
hole capacitors. The OS-CON semiconductor electrolyte
capacitor available from Sanyo has a good (ESR)(size)
product. An additional ceramic capacitor in parallel with
OS-CON capacitors is recommended to offset the effect
of lead inductance.
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
surface mount configurations. New special polymer surface
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 1mΩ. 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 V_CC + 0.1V. Continuous linear
operation is provided throughout this range, allowing
output voltages between 0.6V (the reference input to the
error amplifiers) and V_CC + 0.1V. The maximum differential
current sense input (V_ISNSP – V_ISNSN) 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
current elsewhere can effectively add parasitic induc-
tance 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 C_F resembles the current through the
inductor, and reduce R_F to eliminate any large step as-
sociated 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.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R_SENSE
C_OUT
3860 F05
Figure 5. Sense Lines Placement with Inductor or Sense Resistor
3860fc
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