LTC3890
applicaTions inForMaTion
TheTypicalApplicationonthefirstpageisabasicLTC3890
application circuit. LTC3890 can be configured to use
either DCR (inductor resistance) sensing 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 selection of
Filter components mutual to the sense lines should be
placed close to the LTC3890, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 3). Sensing cur-
rent 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 inductor DCR
sensing is used (Figure 4b), sense resistor R1 should be
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C
OUT
R
(if R
is used) and inductor value. Next, the
SENSE
SENSE
3890 F03
powerMOSFETsandSchottkydiodesareselected. Finally,
input and output capacitors are selected.
INDUCTOR OR R
SENSE
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
Current Limit Programming
V
V
IN
IN
INTV
CC
The ILIM pin is a tri-level logic input which sets the maxi-
mumcurrentlimitofthecontroller.WhenILIM isgrounded,
the maximum current limit threshold voltage of the cur-
rent comparator is programmed to be 30mV. When ILIM
is floated, the maximum current limit threshold is 75mV.
When ILIM is tied to INTVCC, the maximum current limit
threshold is set to 50mV.
BOOST
TG
R
SENSE
SW
V
OUT
LTC3890
BG
R1*
+
SENSE
PLACE CAPACITOR NEAR
SENSE PINS
C1*
–
SENSE
SGND
+
–
SENSE and SENSE Pins
+
–
*R1 AND C1 ARE OPTIONAL.
3890 F04a
The SENSE and SENSE pins are the inputs to the cur-
rent comparators. The common mode voltage range on
these pins is 0V to 28V (abs max), enabling the LTC3890
to regulate output voltages up to a nominal 24V (allowing
margin for tolerances and transients).
(4a) Using a Resistor to Sense Current
V
V
IN
IN
INTV
CC
INDUCTOR
BOOST
TG
+
The SENSE pin is high impedance over the full common
L
DCR
mode range, drawing at most 1µA. This high impedance
allows the current comparators to be used in inductor
DCR sensing.
SW
V
OUT
LTC3890
BG
R1
C1* R2
–
+
The impedance of the SENSE pin changes depending on
SENSE
–
the common mode voltage. When SENSE is less than
–
SENSE
INTV – 0.5V, a small current of less than 1µA flows out
CC
SGND
–
of the pin. When SENSE is above INTV + 0.5V, a higher
CC
3890 F04b
R2
R1 + R2
L
||
(R1 R2) • C1 =
*PLACE C1 NEAR
SENSE PINS
R
= DCR
SENSE(EQ)
current (~700µA) flows into the pin. Between INTV
–
DCR
CC
0.5V and INTV + 0.5V, the current transitions from the
CC
(4b) Using the Inductor DCR to Sense Current
Figure 4. Current Sensing Methods
smaller current to the higher current.
3890fb
15
Linear Systems [ Linear Systems ]
