LTC3787
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic LTC3787
application circuit. LTC3787 can be configured to use either
inductor DCR (DC resistance) sensing or a discrete sense
resistor (R
SENSE
) for current 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 does not require
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. Note that the two control-
ler channels of the LTC3787 should be designed with the
same components.
SENSE
+
and SENSE
–
Pins
The SENSE
+
and SENSE
–
pins are the inputs to the cur-
rent comparators. The common mode input voltage range
of the current comparators is 2.5V to 38V. The current
sense resistor is normally placed at the input of the boost
controller in series with the inductor.
V
IN
INDUCTOR OR R
SENSE
3787 F01
The SENSE
+
pin also provides power to the current com-
parator. It draws ~200μA during normal operation. There
is a small base current of less than 1μA that flows into
the SENSE
–
pin. The high impedance SENSE
–
input to the
current comparators allows accurate DCR sensing.
Filter components mutual to the sense lines should be
placed close to the LTC3787, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 1). 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 DCR sensing
is used (Figure 2b), sense resistor R1 should be placed
close to the switching node, to prevent noise from coupling
into sensitive small-signal nodes.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
Figure 1. Sense Lines Placement with
Inductor or Sense Resistor
VBIAS
SENSE
+
VBIAS
SENSE
+
(OPTIONAL)
SENSE
–
INTV
CC
LTC3787
BOOST
TG
SW
BG
SGND
3787 F02a
V
IN
V
IN
C1
R2
DCR
SENSE
LTC3787
–
INTV
CC
R1
L
BOOST
TG
V
OUT
SW
BG
SGND
3787 F02b
INDUCTOR
V
OUT
PLACE C1 NEAR SENSE PINS
(R1||R2) • C1 =
L
DCR
R
SENSE(EQ)
= DCR •
R2
R1 + R2
(2a) Using a Resistor to Sense Current
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
3787fc
15
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
