LTC3856 LINER [Linear Technology], LTC3856 Datasheet - Page 15

• Manufacturer Part Number: LTC3856
• Description: 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp
• Manufacturer: LINER [Linear Technology]

3856 F02a
applicaTions inForMaTion
The Typical Application on the first page of this data sheet
is a basic LTC3856 application circuit. LTC3856 can be
configured to use either DCR (inductor resistance) sens-
ing 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 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
Next, the power MOSFETs are selected. Finally, input and
output capacitors are selected.
Current Limit Programming
The I
mum current limit of the controller. When I
grounded, floated or tied to INTV
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.
LTC3856
SGND
LIM
SENSE
SENSE
INTV
BOOST
PGND
SW
pin is a tri-level logic input which sets the maxi-
V
BG
TG
CC
PLACED NEAR SENSE PINS
IN
+
–
FILTER COMPONENTS
SENSE
(2a) Using a Resistor to Sense Current
C
F
(if R
R
R
F
F
SENSE
is used) and inductor value.
V
SENSE RESISTOR
C
PLUS PARASITIC
CC
CANCELLATION
F
IN
INDUCTANCE
R
• 2R
POLE-ZERO
S
, the typical value for
Figure 2. Two Different Methods of Sensing Current
F
≤ ESL/R
ESL
S
LIM
V
OUT
is either
SENSE
The SENSE
comparators. The common mode input voltage range of
the current comparators is 0V to 5V. All SENSE
high impedance inputs with small currents of less than
1µA. The positive high impedance input to the current
comparators allows accurate DCR sensing. All SENSE
and DIFFP should be connected directly to V
sensing is used. 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 LTC3856, and
the sense lines should run close together to a Kelvin con-
nection 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 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. The
capacitor C1 should be placed close to the IC pins.
3856 F02b
R
NTC
TEMP COMP
OPTIONAL
NETWORK
Figure 1. Sense Lines Placement with Sense Resistor
R
+
S
and SENSE
(2b) Using the Inductor DCR to Sense Current
*PLACE C1 NEAR SENSE
SENSE
R
+
P
and SENSE
–
ITEMP
SGND
PINS
LTC3856
SENSE
SENSE
INTV
BOOST
PGND
–
SW
V
BG
TG
CC
IN
+
–
Pins
+
–
,
R
pins are the inputs to the current
SENSE
R1 || R2
C1*
TO SENSE FILTER,
NEXT TO THE CONTROLLER
×
C1 =
R2
R1
DCR
L
C
OUT
R
LTC3856
SENSE(EQ)
3856 F01
OUT
INDUCTOR
L
V
= DCR
IN
when DCR
+
DCR
pins are

R1 + R2
R2
–
pins
V
3856f
OUT