ltc3787gn Linear Technology Corporation, ltc3787gn Datasheet - Page 15

• Manufacturer Part Number: ltc3787gn
• Description: Ltc3787 - Polyphase Synchronous Boost Controller
• Manufacturer: Linear Technology Corporation

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
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
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
The SENSE
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.
applicaTions inForMaTion
3787 F02a
+
and SENSE
LTC3787
SENSE
+
SENSE
(2a) Using a Resistor to Sense Current
and SENSE
SENSE
SENSE
INTV
BOOST
VBIAS
SGND
) for current sensing. The choice between
SW
BG
TG
CC
(if R
+
–
–
Pins
SENSE
–
(OPTIONAL)
pins are the inputs to the cur-
is used) and inductor value.
Figure 2. Two Different Methods of Sensing Current
V
V
IN
OUT
PLACE C1 NEAR SENSE PINS
The SENSE
parator. It draws ~200μA during normal operation. There
is a small base current of less than 1μA that flows into
the SENSE
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.
3787 F02b
LTC3787
(2b) Using the Inductor DCR to Sense Current
SENSE
SENSE
INTV
BOOST
–
Figure 1. Sense Lines Placement with
Inductor or Sense Resistor
V
VBIAS
+
SGND
IN
pin. The high impedance SENSE
SW
pin also provides power to the current com-
BG
TG
CC
+
–
NEXT TO THE CONTROLLER
INDUCTOR OR R
TO SENSE FILTER,
C1
(R1 || R2)
R2
• C1 =
SENSE
DCR
R1
L
DCR
R
L
3787 F01
SENSE(EQ)
LTC3787
INDUCTOR
–
input to the
= DCR •
15
V
V
IN
OUT
R1 + R2
3787fa
R2