Datasheet

LTC3866
15
3866fb
APPLICATIONS INFORMATION
The LTC3866 could also be used like any typical current
mode controller by disabling the SNSD
+
pin, shorting it
to ground. An R
SENSE
resistor or a RC filter can be used
to sense the output inductor signal and connects to the
SNSA
+
pin. If the RC filter is used, its time constant,
R • C, is equaled to L/DCR of the output inductor. In
these
applications, the current limit, V
SENSE (MAX)
, will be five
times larger for the specified ILIM, and the operating
voltage range of SNSA
+
and SNS
–
is from 0V to 5.25V.
Without using the internal differential amplifier, the output
voltage of 5V can be generated as shown in the Typical
Applications section.
Inductor DCR Sensing
The LTC3866 is specifically designed for high load current
applications requiring
the highest possible efficiency; it is
capable of sensing the signal of an inductor DCR in the
sub milliohm range (Figure 3). The DCR is the DC winding
resistance of the inductor’s copper, which is often less than
1mΩ for high current inductors. In high current and low
output voltage applications, a conduction loss of a high
DCR or a sense resistor will cause a significant
reduction
in power efficiency. For a specific output requirement,
chose the inductor with the DCR that satisfies the maxi-
mum desirable sense voltage, and uses the relationship
of the sense pin filters to output inductor characteristics
as depicted below.
DCR =
V
SENSE(MAX)
I
MAX
+
∆I
L
2
L/DCR = R1• C1 = 5 • R2 • C2
where:
V
SENSE(MAX)
: Maximum sense voltage for a given ILIM
threshold
I
MAX
: Maximum load current
∆I
L
: Inductor ripple current
L, DCR: Output inductor characteristics
R1, C1: Filter time constant of the SNSD
+
pin
R2, C2: Filter time constant of the SNSA
+
pin
To ensure that the load current will be delivered over the full
operating temperature range, the temperature coefficient of
DCR resistance, approximately 0.4%/°C, should be taken
into account. The LTC3866 features a DCR temperature
compensation circuit that uses an NTC temperature sensing
resistor for this purpose. See the Inductor DCR Sensing
Temperature Compensation section for details.
Figure 3. Inductor DCR Current Sensing
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
ITEMP
R
NTC
100k
INDUCTOR
DCRL
SNSD
+
SNSA
+
SNS
–
SGND
LTC3866
V
OUT
3866 F03
R1
C1
C2
PLACE C1, C2 NEXT TO IC
PLACE R1, R2 NEXT TO INDUCTOR
R1C1 = 5 • R2C2
R
S
22.6k
R
ITEMP
R
P
90.9k
R2