Datasheet

LTC3856
15
3856f
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
SENSE
(if R
SENSE
is used) and inductor value.
Next, the power MOSFETs are selected. 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.
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. All SENSE
+
pins are
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
–
pins
and DIFFP should be connected directly to V
OUT
when DCR
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.
Figure 1. Sense Lines Placement with Sense Resistor
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R
SENSE
3856 F01
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3856
V
OUT
3856 F02a
C
F
• 2R
F
≤ ESL/R
S
POLE-ZERO
CANCELLATION
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R
S
ESL
C
F
R
F
R
F
(2a) Using a Resistor to Sense Current (2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE
+
,
SENSE
–
PINS
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3856
V
OUT
3856 F02b
R1
R2C1*
R1
||
R2 × C1 =
L
DCR
R
SENSE(EQ)
= DCR
R2
R1 + R2
ITEMP
OPTIONAL
TEMP COMP
NETWORK
R
P
R
NTC
R
S