LTC3876_15 Datasheet, PDF (22/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
V_SENSE
20mV/DIV
V_ESL(STEP)
500ns/DIV
3876 F04a
Figure 4a. Voltage Waveform Measured
Directly Across the Sense Resistor
V_SENSE
20mV/DIV
500ns/DIV
3876 F04b
Figure 4b. Voltage Waveform Measured After the
Sense Resistor Filter. C_F = 1000pF, R_F = 100Ω
Note that the SENSE1^– and SENSE2^– pins are also used
for sensing the output voltage for the adjustment of top
gate on time, t_ON. For this purpose, there is an additional
internal 500k resistor from each SENSE^– pin to SGND,
therefore there is an impedance mismatch with their cor-
responding SENSE⁺ pins. The voltage drop across the
R_F causes an offset in sense voltage. For example, with
R_F = 100Ω, at V_OUT = V_SENSE– = 5V, the sense-voltage
offset V_{SENSE(OFFSET)} = V_SENSE– • R_F/500k = 1mV. Such
small offset may seem harmless for current limit, but
could be significant for current reversal detection (I_REV),
causing excess negative inductor current at discontinuous
mode. Also, at V_SENSE(MAX) = 30mV, a mere 1mV offset
will cause a significant shift of zero-current ITH voltage
by (2.4V – 0.8V) • 1mV/30mV = 53mV. Too much shift
may not allow the output voltage to return to its regulated
value after the output is shorted due to ITH foldback.
Therefore, when a larger filter resistor R_F value is used,
it is recommended to use an external 500k resistor from
each SENSE⁺ pin to SGND, to balance the internal 500k
resistor at its corresponding SENSE^– pin.
22
The previous discussion generally applies to high density/
high current applications where I_OUT(MAX) > 10A and low
inductor values are used. For applications where I_OUT(MAX)
< 10A, set R_F to 10Ω and C_F to 1000pF. This will provide
a good starting point.
The filter components need to be placed close to the IC.
The positive and negative sense traces need to be routed
as a differential pair and Kelvin (4-wire) connected to the
sense resistor.
DCR Inductor Current Sensing
For applications requiring higher efficiency at high load
currents, the LTC3876 is capable of sensing the voltage
drop across the inductor DCR, as shown in Figure 5. The
DCR of the inductor represents the small amount of DC
winding resistance, which can be less than 1mΩ for today’s
low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost several
points of efficiency compared to DCR sensing.
The inductor DCR is sensed by connecting an RC filter
across the inductor. This filter typically consists of one or
two resistors (R1 and R2) and one capacitor (C1) as shown
in Figure 5. 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
voltage drop across the inductor DCR multiplied by R2/
(R1 + R2). Therefore, R2 may be used to scale the voltage
across the sense terminals when the DCR is greater than
LTC3876
SENSE⁺
SENSE^–
INDUCTOR
L DCR
L/DCR = (R1||R2) C1
R1
V_OUT
C_OUT
C1
R2
(OPT)
3876 F05
C1 NEAR SENSE PINS
Figure 5. DCR Current Sensing
3876f