LTC3866 Datasheet, PDF (15/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

Applications Information
R_ITEMP
R_S
22.6k
R_NTC R_P
100k 90.9k
V_IN
INTV_CC
BOOST
TG
LTC3866
SW
ITEMP
BG
PGND
SNSD⁺
C1
SNS^–
C2
SNSA⁺
SGND
PLACE C1, C2 NEXT TO IC
PLACE R1, R2 NEXT TO INDUCTOR
R1C1 = 5 • R2C2
V_IN
INDUCTOR
L DCR
R1 R2
Figure 3. Inductor DCR Current Sensing
LTC3866
V_OUT
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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.
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