LTC3787
16
3787fc
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, ∆I
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 LTC3787 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 1m
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)
).
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of resistance, which is approximately 0.4%/°C.
A conservative value for the maximum inductor temperature
(T
L(MAX)
) is 100°C.
