Datasheet

LTC3788-1
14
37881fc
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic
LTC3788-1 application circuit. LTC3788-1 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 selection of R
SENSE
(if R
SENSE
is used)
and inductor value. Next, the power MOSFETs are selected.
Finally, input and output capacitors are selected.
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.
The SENSE
+
pin also provides power to the current
comparator. 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 allow accurate DCR sensing.
Filter components mutual to the sense lines should be
placed close to the LTC3788-1, 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.
Figure 1. Sense Lines Placement with
Inductor or Sense Resistor
(2a) Using a Resistor to Sense Current (2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
V
IN
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R
SENSE
37881 F01
TG
SW
BG
INDUCTOR
DCR
L
LTC3788-1
INTV
CC
BOOST
SENSE
+
SENSE
–
R2C1
VBIAS
V
IN
V
OUT
PLACE C1 NEAR SENSE PINS
SGND
37881 F02b
(R1
||
R2) • C1 =
L
DCR
R
SENSE(EQ)
= DCR •
R2
R1 + R2
R1
TG
SW
BG
LTC3788-1
INTV
CC
BOOST
SENSE
+
SENSE
–
(OPTIONAL)
VBIAS
V
IN
V
OUT
SGND
37881 F02a