LTC3890-2 Datasheet, PDF (17/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-2
APPLICATIONS INFORMATION
Low Value Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 4a. R_SENSE is chosen based on the required
output current.
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3890-2
BG
R_SENSE
V_OUT
SENSE⁺
SENSE^–
SGND
R1*
C1* ^{PLACE CAPACITOR NEAR}
SENSE PINS
*R1 AND C1 ARE OPTIONAL.
38902 F04a
(4a) Using a Resistor to Sense Current
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3890-2
BG
INDUCTOR
L DCR
SENSE⁺
SENSE^–
SGND
*PLACE C1 NEAR
SENSE PINS
R1
C1* R2
(R1||R2) t C1 = ^L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
(4b) Using the Inductor DCR to Sense Current
V_OUT
38902 F04b
Figure 4. Current Sensing Methods
The current comparator has a maximum threshold
V_SENSE(MAX) determined by the I_LIM setting. The current
comparator threshold voltage sets the peak of the induc-
tor current, yielding a maximum average output current,
I_MAX, equal to the peak value less half the peak-to-peak
ripple current, ΔI_L. To calculate the sense resistor value,
use the equation:
R_SENSE
=
V_{SENSE(MAX )}
I_MAX
+
ΔI_L
2
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimum value for the Maximum Current Sense Threshold
(V_SENSE(MAX)) in the Electrical Characteristics table (30mV,
50mV or 75mV, depending on the state of the I_LIM pin).
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability cri-
terion for buck regulators operating at greater than 50%
duty factor. A curve is provided in the Typical Performance
Characteristics section to estimate this reduction in peak
inductor current depending upon the operating duty factor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3890-2 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 4b. The DCR of the inductor represents the small
amount of DC resistance of the copper wire, which can be
less than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
power loss through a sense resistor would cost several
points of efficiency compared to inductor DCR sensing.
38902f
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