LTC3784_15 Datasheet, PDF (15/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Applications Information
The Typical Application on the first page is a basic LTC3784
application circuit. The LTC3784 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. Note
that the two controller channels of the LTC3784 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 2.3V to 60V. 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 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 LTC3784, 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
V_IN
INDUCTOR OR R_SENSE
3784 F01
Figure 1. Sense Lines Placement with
Inductor or Sense Resistor
VBIAS
V_IN
SENSE⁺
SENSE^–
(OPTIONAL)
INTV_CC
LTC3784
BOOST
VBIAS
SENSE⁺
SENSE^–
INTV_CC
LTC3784
BOOST
V_IN
C1 R2
DCR
INDUCTOR
R1
L
TG
SW
BG
SGND
V_OUT
3784 F02a
(2a) Using a Resistor to Sense Current
TG
SW
V_OUT
BG
SGND
3784 F02b
PLACE C1 NEAR SENSE PINS
(R1||R2)
•
C1
=
L
DCR
R_SENSE(EQ)
=
DCR
•
R2
R1 + R2
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
For more information www.linear.com/LTC3784
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