ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 12

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

LTC3850-2
APPLICATIONS INFORMATION
The Typical Application on the fi rst page is a basic LTC3850-
2 application circuit. LTC3850-2 can be confi gured 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 effi cient, 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
used) and inductor value. Next, the power MOSFETs are se-
lected. Finally, input and output capacitors are selected.
SENSE
The SENSE
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 fl ow out of the SENSE
pins. When the SENSE pins ramp down from 5V to 1.1V,
the small base currents fl ow into the SENSE pins. The
high impedance inputs to the current comparators allow
accurate DCR sensing. However, care must be taken not
to fl oat these pins during normal operation.
12
38502 F02a
LTC3850-2
+
SGND
and SENSE
SENSE
SENSE
INTV
BOOST
+
PGND
(2a) Using a Resistor to Sense Current
and SENSE
SW
V
BG
TG
PLACED NEAR SENSE PINS
CC
IN
+
–
FILTER COMPONENTS
–
C
Pins
F
–
R
R
F
F
pins are the inputs to the current
Figure 2. Two Different Methods of Sensing Current
V
SENSE RESISTOR
C
PLUS PARASITIC
CANCELLATION
IN
F
INDUCTANCE
R
• 2
POLE-ZERO
SENSE
S
RF
≤ ESL/R
ESL
(if R
S
V
SENSE
OUT
is
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
output current.
The current comparator has a maximum threshold
V
comparator is 0V to 5V. The current comparator threshold
*PLACE C1 NEAR SENSE
38502 F02b
SENSE(MAX)
SENSE
LTC3850-2
–
SGND
PINS
SENSE
SENSE
(2b) Using the Inductor DCR to Sense Current
INTV
BOOST
PGND
SW
V
BG
TG
CC
IN
. The input common mode range of the current
+
–
Figure 1. Sense Lines Placement
with Inductor or Sense Resistor
+
,
SENSE
C1*
INDUCTOR OR R
R1 || R2 × C1 =
is chosen based on the required
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R2
SENSE
DCR
R1
L
R
C
SENSE(EQ)
OUT
38502 F01
INDUCTOR
L
V
= DCR
IN
DCR
R1 + R2
R2
V
OUT
38502f