LTC3850-2_15 Datasheet, PDF (12/36 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850-2
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic LTC3850-
2 application circuit. LTC3850-2 can be configured to use
either DCR (inductor resistance) sensing or low value resis-
tor 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 saves expensive current sensing resis-
tors 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 require-
ment, and begins with the selection of R_SENSE (if R_SENSE is
used) and inductor value. Next, the power MOSFETs are se-
lected. Finally, input and output capacitors are selected.
SENSE⁺ and SENSE^– Pins
The SENSE⁺ and SENSE^– pins are the inputs to the current
comparators. The common mode input voltage range of
the current comparators is 0V to 5V. Both SENSE pins
are high impedance inputs with small base currents of
less than 1μA. When the SENSE pins ramp up from 0V
to 1.4V, the small base currents flow out of the SENSE
pins. When the SENSE pins ramp down from 5V to 1.1V,
the small base currents flow into the SENSE pins. The
high impedance inputs to the current comparators allow
accurate DCR sensing. However, care must be taken not
to float these pins during normal operation.
Filter components mutual to the sense lines should be placed
close to the LTC3850-2, and the sense lines should run close
together to a Kelvin connection underneath the current sense
element (shown in Figure 1). Sensing current 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 unpre-
dictable. 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. The
capacitor C1 should be placed close to the IC pins.
Low Value Resistors 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). The input common mode range of the current
comparator is 0V to 5V. The current comparator threshold
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R_SENSE
C_OUT
38502 F01
Figure 1. Sense Lines Placement
with Inductor or Sense Resistor
V_IN
INTV_CC
BOOST
TG
SW
LTC3850-2
BG
PGND
SENSE⁺
SENSE^–
SGND
38502 F02a
R_F
C_F
R_F
FILTER COMPONENTS
PLACED NEAR SENSE PINS
V_IN
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
V_OUT
C_F • 2_RF ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_IN
INTV_CC
BOOST
TG
SW
LTC3850-2
BG
PGND
SENSE⁺
SENSE^–
SGND
38502 F02b
*PLACE C1 NEAR SENSE⁺,
SENSE^– PINS
R1
C1*
R2
R1||R2 × C1 =
L
DCR
V_IN
INDUCTOR
L DCR
V_OUT
R_SENSE(EQ) = DCR
R2
R1 + R2
(2a) Using a Resistor to Sense Current
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
38502f
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