LTC3850-1_15 Datasheet, PDF (13/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic LTC3850
application circuit. LTC3850 can be configured to use either
DCR (inductor resistance) sensing or low value resistor
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.
Current Limit Programming
The I_LIM pin is a tri-level logic input which sets the maxi-
mum current limit of the controller. When I_LIM is either
grounded, floated or tied to INTV_CC, the typical value for
the maximum current sense threshold will be 30mV, 50mV
or 75mV, respectively.
Which setting should be used? For the best current limit
accuracy, use the 75mV setting. The 30mV setting will allow
for the use of very low DCR inductors or sense resistors,
but at the expense of current limit accuracy. The 50mV
setting is a good balance between the two. For single output
dual phase applications (see Figure 21), use the 50mV or
75mV setting for optimal current sharing.
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, 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
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R_SENSE
C_OUT
38501 F01
Figure 1. Sense Lines Placement
with Inductor or Sense Resistor
V_IN
V_IN
INTV_CC
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3850
BG
PGND
SENSE⁺
SENSE^–
SGND
38501 F02a
R_F
C_F
R_F
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
V_OUT
C_F • 2_RF ≤ ESL/R_S
POLE-ZERO
CANCELLATION
BOOST
TG
SW
LTC3850
BG
PGND
SENSE⁺
SENSE^–
SGND
38501 F02b
*PLACE C1 NEAR SENSE⁺,
SENSE^– PINS
R1
C1*
R2
R1||R2 × C1 =
L
DCR
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
38501fc
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