LTC3786_15 Datasheet, PDF (14/34 Pages) Linear Technology – Low IQ Synchronous Boost Controller

LTC3786
Applications Information
The Typical Application on the first page is a basic LTC3786
application circuit. LTC3786 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.
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 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 LTC3786, 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.
Sense Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R_SENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
V_SENSE(MAX) of 75mV. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average inductor current, I_MAX, equal to the peak value
TO SENSE FILTER,
NEXT TO THE CONTROLLER
V_IN
INDUCTOR OR R_SENSE
3786 F01
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
14
VBIAS
V_IN
SENSE⁺
SENSE^–
(OPTIONAL)
INTV_CC
LTC3786
BOOST
TG
VBIAS
SENSE⁺
SENSE^–
INTV_CC
LTC3786
BOOST
TG
V_IN
C1 R2
DCR
INDUCTOR
R1
L
SW
BG
SGND
V_OUT
3786 F02a
(2a) Using a Resistor to Sense Current
SW
V_OUT
BG
SGND
PLACE C1 NEAR SENSE PINS
(R1||R2)
•
C1
=
L
DCR
3786 F02b
R_SENSE(EQ)
= DCR
•
R2
R1 + R2
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
3786fa