LTC3875_15 Datasheet, PDF (15/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Operation
Inductor DCR Sensing Temperature Compensation
Inductor DCR current sensing provides a lossless method
of sensing the instantaneous current. Therefore, it can
provide higher efficiency for applications of high output
currents. However the DCR of a copper inductor typically
has a positive temperature coefficient. As the temperature
of the inductor rises, its DCR value increases. The current
limit of the controller is therefore reduced.
LTC3875 offers a method to counter this inaccuracy by
allowing the user to place an NTC temperature sensing
resistor near the inductor. The ENTMPB pin has to be
floating to enable the inductor DCR sensing temperature
compensation function. The TCOMP/ITEMP pin, when left
floating, is at a voltage around 5.5V and DCR temperature
compensation is also disabled. A constant 30µA precision
current flows out the TCOMP/ITEMP pin. By connecting a
linearized NTC resistor network from the TCOMP/ITEMP
pin to SGND, the maximum current sense threshold can be
varied over temperature according the following equation:
V_SENSEMAX(
ADJ)
=
V_SENSE(MAX
)
•
2.2
– V_ITEMP
1.5
where:
V_{SENSEMAX(ADJ)} is the maximum adjusted current sense
threshold.
V_SENSE(MAX) is the maximum current sense threshold
specified in the electrical characteristics table. It is typi-
cally 10mV, 15mV, 20mV, 25mV or 30mV depending on
the setting ILIM pins. V_ITEMP is the voltage of the TCOMP/
ITEMP pin.
The valid voltage range for DCR temperature compensa-
tion on the TCOMP/ITEMP pin is between 0.7V to SGND
with 0.7V or above being no DCR temperature correction.
An NTC resistor has a negative temperature coefficient,
meaning that its value decreases as temperature rises.
The V_ITEMP voltage, therefore, decreases as temperature
increases and in turn V_{SENSEMAX(ADJ)} will increase to
compensate the DCR temperature coefficient. The NTC
resistor, however, is nonlinear, but the user can linear-
ize its value by building a resistor network with regular
resistors. Consult the NTC manufacturer’s data sheets for
detailed information.
Another use for the TCOMP/ITEMP pins, in addition to
NTC compensated DCR sensing, is adjusting V_SENSE(MAX)
to values between the nominal values of 10mV,15mV,
20mV, 25mV and 30mV for a more precise current limit
setting. This is done by applying a voltage less than 0.7V
to the TCOMP/ITEMP pin. V_SENSE(MAX) will be varied per
the above equation. The current limit can be adjusted
using this method either with a sense resistor or DCR
sensing. The ENTMPB pin also needs to be floating to
use this function.
For more information see the NTC Compensated DCR
Sensing paragraph in the Applications Information section.
Frequency Selection and Phase-Locked Loop
(FREQ and MODE/PLLIN Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage. The switching
frequency of the LTC3875’s controllers can be selected
using the FREQ pin. If the MODE/PLLIN pin is not being
driven by an external clock source, the FREQ pin can be
used to program the controller’s operating frequency
from 250kHz to 720kHz. There is a precision 10µA current
flowing out of the FREQ pin, so the user can program the
controller’s switching frequency with a single resistor to
SGND. A curve is provided later in the application section
showing the relationship between the voltage on the FREQ
pin and switching frequency. A phase-locked loop (PLL)
is integrated on the LTC3875 to synchronize the internal
oscillator to an external clock source that is connected to
the MODE/PLLIN pin. The controller is operating in forced
continuous mode when it is synchronized. The PLL loop
filter network is also integrated inside the LTC3875. The
phase-locked loop is capable of locking any frequency
within the range of 250kHz to 720kHz. The frequency setting
resistor should always be present to set the controller’s
initial switching frequency before locking to the external
clock to minimize the transient.
For more information www.linear.com/LTC3875
3875fa
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