LTC3890
applicaTions inForMaTion
placed close to the switching node, to prevent noise from
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.
coupling into sensitive small-signal nodes.
Low Value Resistor Current Sensing
If the external (R1||R2) • C1 time constant is chosen to be
exactly equal to the L/DCR time constant, the voltage drop
across the external capacitor is equal to the drop across
theinductorDCRmultipliedbyR2/(R1+R2).R2scalesthe
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
always the same and varies with temperature; consult
the manufacturers’ data sheets for detailed information.
A typical sensing circuit using a discrete resistor is shown
in Figure 4a. R
output current.
is chosen based on the required
SENSE
The current comparator has a maximum threshold
determined by the I setting. The current
V
SENSE(MAX)
LIM
comparator threshold voltage sets the peak of the induc-
tor current, yielding a maximum average output current,
I
, equal to the peak value less half the peak-to-peak
MAX
ripple current, ∆I . To calculate the sense resistor value,
L
use the equation:
Using the inductor ripple current value from the Inductor
ValueCalculationsection,thetargetsenseresistorvalueis:
VSENSE(MAX)
RSENSE
=
∆IL
2
IMAX
+
VSENSE(MAX)
RSENSE(EQUIV)
=
∆IL
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimumvaluefortheMaximumCurrentSenseThreshold
)intheElectricalCharacteristicstable(30mV,
50mV or 75mV, depending on the state of the I pin).
IMAX
+
2
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimumvaluefortheMaximumCurrentSenseThreshold
(V
SENSE(MAX)
LIM
(V
)intheElectricalCharacteristicstable(30mV,
SENSE(MAX)
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
inductorcurrentdependingupontheoperatingdutyfactor.
50mV or 75mV, depending on the state of the I pin).
LIM
Next, determine the DCR of the inductor. When provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of copper resistance, which is approximately
0.4%/°C. A conservative value for T
is 100°C.
L(MAX)
To scale the maximum inductor DCR to the desired sense
Inductor DCR Sensing
resistor value (R ), use the divider ratio:
D
For applications requiring the highest possible efficiency
at high load currents, the LTC3890 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
lessthan1mΩfortoday’slowvalue,highcurrentinductors.
RSENSE(EQUIV)
RD =
DCRMAX atT
L(MAX)
C1 is usually selected to be in the range of 0.1µF to 0.47µF.
ThisforcesR1||R2toaround2k, reducingerrorthatmight
+
have been caused by the SENSE pin’s 1µA current.
3890fb
16
Linear Systems [ Linear Systems ]
