LTC3866 Datasheet, PDF (16/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Applications Information
Typically, C1 and C2 are selected in the range of 0.047µF
to 0.47µF. If C1 and C2 are chosen to be 220nF, and an
inductor of 330nH with 0.32mΩ DCR is selected, R1 and
R2 will be 4.7k and 942Ω respectively. The bias current at
SNSD⁺ and SNSA⁺ is about 30nA and 500nA respectively,
and it causes some small error to the sense signal.
There will be some power loss in R1 and R2 that relates to
the duty cycle, and will be the most in continuous mode
at the maximum input voltage:
( ) _{PLOSS (R) =}
V_IN(MAX) – V_OUT
R
• V_OUT
Ensure that R1 and R2 have a power rating higher than this
value. However, DCR sensing eliminates the conduction
loss of a sense resistor; it will provide a better efficiency
at heavy loads. To maintain a good signal-to-noise ratio
for the current sense signal, using a minimum ∆V_SENSE of
2mV for duty cycles less than 40% is desirable. The actual
ripple voltage will be determined by the following equation:
∆V_SENSE
=
V_OUT
V_IN
• ^{VIN – VOUT}
R1C1• f_OSC
Inductor DCR Sensing Temperature Compensation
with NTC Thermistor
For DCR sensing applications, the temperature coefficient
of the inductor winding resistance should be taken into
account when the accuracy of the current limit is critical
over a wide range of temperature. The main element used
in inductors is Copper; that has a positive tempco of ap-
proximately 4000ppm/°C. The LTC3866 provides a feature
to correct for this variation through the use of the ITEMP
pin. There is a 10µA precision current source flowing out
of the ITEMP pin. A thermistor with a NTC (negative tem-
perature coefficient) resistance can be used in a network,
R_ITEMP (Figure 3) connected to maintain the current limit
threshold constant over a wide operating temperature.
The ITEMP voltage range that activates the correction is
from 0.7V or less. If floating this pin, its voltage will be at
INTV_CC potential, about 5.5V. When the ITEMP voltage is
higher than 0.7V, the temperature compensation is inactive.
The following guideline will help to choose components
for temperature correction. The initial compensation is for
25°C ambient temperature:
ITEMP • R_ITEMP = 0.7V for 25°C
R_ITEMP is a thermistor resistance network connected to
ITEMP pin.
Since ITEMP = 10µA, choose R_ITEMP network = 70kΩ at
25°C
TC_RITEMP = –(1.5/0.7) • TC_DCR
Typically TC_DCR = 4000ppm/°C, tempco of DCR which is
usually Copper. For ideal compensation, the tempco of
the R_ITEMP should be:
TC_RITEMP = –(1.5/0.7) • 4000 ppm/°C = –8570 ppm/°C
For example, a Murata NTC thermistor of 100k with B =
4334 that has a nonlinear temperature characteristic as
described in R[T] = R[T0] • EXP B (1/T – 1/T0) where T0
is the temperature at 300°K. Resistors R_S and R_P of 22.6k
and 90.9k respectively are used to linearize the network
as shown in Figure 4.The current limit threshold will be
compensated from 25°C to over 100°C of the inductor
temperature, Figure 5. Once the temperature compensation
is done, it will remain valid for all programmable current
sense limit scales.
10000
1000
THERMISTOR RESISTANCE
R_O = 100k
T_O = 25°C
B = 4334 FOR 25°C TO 100°C
100
R_ITEMP
R_S = 22.6k
10
R_P = 90.9k
100k NTC
1
–50 –25 0 25 50 75 100 125 150
INDUCTOR TEMPERATURE (°C)
3866 F04
Figure 4. Resistance Versus Temperature for the ITEMP Pin
Network and the 100k NTC
3866fa
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