LTC3867
14
3867f
APPLICATIONS INFORMATION
The Typical Application on the first page of this data sheet
is a basic LTC3867 application circuit. The LTC3867 can be
configured to use either DCR (inductor resistance) sens-
ing or low value resistor sensing. The choice between the
two current sensing schemes is largely a design trade-off
between cost, power consumption and accuracy. DCR
sensing is becoming popular because it saves expensive
current sensing resistors and is more power efficient,
especially in high current applications. However, current
sensing resistors provide the most accurate current limits
for the controller. Other external component selection is
driven by the load requirement, and begins with the se-
lection of R
SENSE
(if R
SENSE
is used) and inductor value.
Next, the power MOSFETs are selected. Finally, input and
output capacitors are selected.
Current Limit Programming
The I
LIM
pin is a 5-level logic input which sets the maxi-
mum current limit of the controller. When I
LIM
is either
grounded, floated or tied to INTV
CC
, the typical value
for the maximum current sense threshold will be 30mV,
50mV or 75mV, respectively. Set I
LIM
between 1.1V and
1.9V (typically 1.5V) for a 40mV maximum current sense
threshold. For the 60mV setting, set I
LIM
between 3.3V
and 4.1V, typically 3.7V. These numbers are relative to a
5.3V INTV
CC
. Setting I
LIM
using a resistor divider off of
INTV
CC
will allow the maximum current sense threshold
setting to not change when the 5.3V LDO is in dropout
at start-up. Please note that the I
LIM
pin has an internal
500k pull-down to SGND and a 500k pull-up to INTV
CC
.
Which setting should be used? For the best current limit
accuracy, use the 75mV setting. The 30mV setting will allow
for the use of very low DCR inductors or sense resistors,
but at the expense of current limit accuracy.
SENSE
+
and SENSE
–
Pins
The SENSE
+
and SENSE
–
pins are the inputs to the current
comparators. The common mode input voltage range of
the current comparators is 0V to 14V. All SENSE pins are
high impedance inputs with small currents of less than
1µA. The high impedance inputs to the current compara-
tors allow accurate DCR sensing. The SENSE
–
pin should
be connected to V
OUT
directly when DCR sensing is used.
Care must be taken not to float these pins during normal
operation. Filter components mutual to the sense lines
should be placed close to the LTC3867, and the sense
lines should run close together to a Kelvin connection
underneath the current sense element (shown in Figure 2).
Sensing current elsewhere can effectively add parasitic
inductance and capacitance to the current sense element,
degrading the information at the sense terminals and
making the programmed current limit unpredictable. If
DCR sensing is used (Figure 3b), resistor R1 should be
placed close to the switching node, to prevent noise from
coupling into sensitive small-signal nodes. The capacitor
C1 should be placed close to the IC pins.
Figure 2. Sense Lines Placement with Sense Resistor
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 3a. R
SENSE
is chosen based on the required
output current. The current comparator has a maximum
threshold V
SENSE(MAX)
determined by the I
LIM
setting. The
input common mode range of the current comparator
is 0V to 14V. The current comparator threshold sets the
peak of the inductor current, yielding a maximum average
output 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
+
∆I
L
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ∆V
SENSE
= ∆I
L
• R
SENSE
also needs to be checked in the design to get a good
signal-to-noise ratio. In general, for a reasonably good
PCB layout, a 10mV ∆V
SENSE
voltage is recommended as
a conservative number to start with, either for R
SENSE
or
DCR sensing applications. For previous generation current
mode controllers, the maximum sense voltage was high
enough (e.g., 75mV for the LTC1628/LTC3728 family)
that the voltage drop across the parasitic inductance of
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R
SENSE
3867 F02
