LTC3784_15 Datasheet, PDF (16/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Applications Information
Sense Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R_SENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
V_SENSE(MAX). When the ILIM pin is grounded, floating or
tied to INTV_CC, the maximum threshold is set to 50mV,
75mV or 100mV, respectively. The current comparator
threshold sets the peak of the inductor current, yielding
a maximum average inductor 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
The actual value of I_MAX for each channel depends on the
required output current I_OUT(MAX) and can be calculated
using:
I_MAX
=
⎛
⎝⎜
I_OUT(MAX)
2
⎞
⎠⎟
•
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
When using the controller in low V_IN and very high voltage
output applications, the maximum inductor current and
correspondingly the maximum output current level will
be reduced due to the internal compensation required to
meet stability criterion for boost 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 level depending
upon the operating duty factor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3784 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 2b. The DCR of the inductor can be less than 1mΩ
for high current inductors. In a high current application
requiring such an inductor, conduction loss through a
sense resistor could reduce the efficiency by a few percent
compared to DCR 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
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
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.
Using the inductor ripple current value from the induct-
or value calculation section, the target sense resistor
value is:
R_SENSE(EQUIV)
=
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)).
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