LTC3867
17
3867f
APPLICATIONS INFORMATION
20°C. Increase this value to account for the temperature
coefficient of resistance, which is approximately 0.4%/°C.
A conservative value for T
L(MAX)
is 100°C. To scale the
maximum inductor DCR to the desired sense resistor
value, use the divider ratio:
R
D
=
R
SENSE(EQUIV)
DCR
(MAX)
at T
L(MAX)
C1 is usually selected to be in the range of 0.047µF to
0.47µF. This forces R1|| R2 to around 2k, reducing error
that might have been caused by the SENSE
+
pin’s ±1µA
current. T
L(MAX)
is the maximum inductor temperature.
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
R1||R2=
L
(DCR at 20°C) • C1
The sense resistor values are:
R1=
R1||R2
R
D
, R2=
R1• R
D
1− R
D
The LTC3867 also features a DCR temperature compen-
sation circuit that uses an NTC temperature sensor. See
the Inductor DCR Sensing Temperature Compensation
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:
P
LOSS
R1=
V
IN(MAX)
− V
OUT
• 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 LTC3867 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 30µA precision current flowing out 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
2.8
1.3
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
typically 75mV, 60mV, 50mV, 40mV or 30mV depending
on the setting I
LIM
pins.
V
ITEMP
is the voltage of the ITEMP pin.
The valid voltage range for DCR temperature compensation
on the ITEMP pin is 1.4V to 0.6V, with 1.4V or above being
no DCR temperature correction and 0.6V the maximum
correction. However, if the duty cycle of the controller is less
than 25%, the ITEMP range is extended from 1.4V to 0V.
