LTC3856_15 Datasheet, PDF (18/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
The LTC3856 also features a DCR temperature compensa-
tion circuit by using a NTC temperature sensor. See the
Inductor DCR Sensing Temperature Compensation and
the ITEMP Pin section for details.
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
( ) _{PLOSS R1=}
V_IN(MAX) − V_OUT
R1
• V_OUT
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor, due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method. To
maintain a good signal-to-noise ratio for the current sense
signal, use a minimum ∆V_SENSE of 10mV for duty cycles
less than 40%. For a DCR sensing application, the actual
ripple voltage will be determined by the equation:
∆V_SENSE
=
V_IN − V_OUT
R1• C1
V_OUT
V_IN • f_OSC
Inductor DCR Sensing Temperature Compensation
and the ITEMP Pin
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 the inductor, which is the
small amount of DC winding resistance of the copper,
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.
The LTC3856 offers a method to counter this inaccuracy
by allowing the user to place an NTC temperature sensing
resistor near the inductor to actively correct this error.
The ITEMP pin, when left floating, is at a voltage around
5V and DCR temperature compensation is disabled. The
ITEMP pin has a constant 10µA precision current flow-
ing out of the pin. By connecting an NTC resistor from
the ITEMP pin to SGND, the maximum current sense
threshold can be varied over temperature according the
following equation:
V_{SENSEMAX(ADJ)}
=
V_SENSE(MAX)
•
1.8
– V_ITEMP
1.3
where:
V_{SENSEMAX(ADJ)} is the maximum adjusted current sense
threshold at temperature.
V_SENSE(MAX) is the maximum current sense threshold
specified in the Electrical Characteristics table. It is
typically 75mV, 50mV or 30mV, depending on the set-
ting I_LIM pins.
V_ITEMP is the voltage of the ITEMP pin.
The valid voltage range for DCR temperature compensa-
tion on the ITEMP pin is between 0.5V to 0.2V, with 0.5V
or above being no DCR temperature correction and 0.2V
the maximum correction. However, if the duty cycle of the
controller is less than 25%, the ITEMP range is extended
from 0.5V to 0V.
The NTC resistor has a negative temperature coefficient,
meaning its value decreases as temperature rises. The
V_ITEMP voltage, therefore, decreases as temperature in-
creases and in turn, the V_{SENSEMAX(ADJ)} will increase to
compensate the DCR temperature coefficient. The NTC
resistor, however, is nonlinear and the user can linear-
ize its value by building a resistor network with regular
resistors. Consult the NTC manufacture data sheets for
detailed information.
Another use for the ITEMP pins, in addition to NTC com-
pensated DCR sensing, is adjusting V_SENSE(MAX) to values
between the nominal values of 30mV, 50mV and 75mV
for a more precise current limit. This is done by applying
a voltage less than 0.5V to the ITEMP pin. V_SENSE(MAX)
will be varied per the previous equation and the same
duty cycle limitations will apply. The current limit can be
adjusted using this method either with a sense resistor
or DCR sensing.
3856f
18