Datasheet

LTC3866
16
3866fb
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:
P
LOSS
R
( )
=
V
IN(MAX)
– V
OUT
( )
• V
OUT
R
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
•
V
IN
– V
OUT
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.
INDUCTOR TEMPERATURE (°C)
10
RESISTANCE (kΩ)
100
1000
10000
–50 25 50 75 125100 150
1
–25 0
3866 F04
THERMISTOR RESISTANCE
R
O
= 100k
T
O
= 25°C
B = 4334 FOR 25°C TO 100°C
R
ITEMP
R
S
= 22.6k
R
P
= 90.9k
100k NTC
Figure 4. Resistance Versus Temperature for the ITEMP Pin
Network and the 100k NTC