LTC3788-1 Datasheet, PDF (14/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic
LTC3788-1 application circuit. LTC3788-1 can be config-
ured 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.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
V_IN
INDUCTOR OR R_SENSE
37881 F01
Figure 1. Sense Lines Placement with
Inductor or Sense Resistor
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 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.
VBIAS
SENSE⁺
SENSE^–
INTV_CC
LTC3788-1
BOOST
(OPTIONAL)
V_IN
VBIAS
V_IN
SENSE⁺
C1 R2
DCR
SENSE^–
INTV_CC
LTC3788-1
INDUCTOR
R1
L
BOOST
TG
TG
37881 F02a
SW
BG
SGND
V_OUT
(2a) Using a Resistor to Sense Current
SW
BG
SGND
37881 F02b
PLACE C1 NEAR SENSE PINS
V_OUT
(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
37881f
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