LTC3875_15 Datasheet, PDF (18/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR
C_OUT
3875 F04a
Figure 4a. Sense Lines Placement with Inductor DCR
V_IN
INTV_CC
BOOST
R_ITEMP
TG
LTC3875
SW
TCOMP/ITEMP BG
R_S
22.6k
R_NTC R_P
100k 90.9k
PGND
SNSD⁺
C1
SNS^–
C2
SNSA⁺
SGND
V_IN
INDUCTOR
L DCR
R1 R2
PLACE C1, C2 NEXT TO IC
PLACE R1, R2 NEXT TO INDUCTOR
R1C1 = 5 • R2C2
Figure 4b. Inductor DCR Current Sensing
V_OUT
3875 F04b
output inductor and capacitors C1 and C2 are close to
the IC pins to prevent noise coupling to the sense signal.
For applications where the inductor DCR is large, the
LTC3875 could also be used like any typical current
mode controller with conventional DCR sensing by
disabling the SNSD⁺ pin, shorting it to ground. An
R_SENSE resistor or a DCR sensing 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, equals L/DCR of the output inductor. In these ap-
plications, the current limit, V_SENSE(MAX), will be five times
the value of V_SENSE(MAX) with DC loop enabled, and the
operating voltage range of SNSA⁺ and SNS^– is from 0V to
5V. An output voltage of 5V can be generated.
Low Inductor DCR Sensing and Current Limit
Estimation
The LTC3875 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 4b). The DCR is the inductor DC
winding resistance, which is often less than 1mΩ for high
current inductors. In high current and low output voltage
applications, conduction loss of a high DCR inductor or a
sense resistor will cause a significant reduction in power
efficiency. For a specific output requirement and induc-
tor, choose the current limit sensing level that provides
proper margin for maximum load current, 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) is the maximum sense voltage for a given
ILIM threshold.
I_MAX is the maximum load current.
∆I_L is the inductor ripple current.
L/DCR is the output inductor characteristics.
3875fa
18
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