LTC3856_15 Datasheet, PDF (15/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
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.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R_SENSE
C_OUT
3856 F01
Figure 1. Sense Lines Placement with Sense Resistor
V_IN
INTV_CC
BOOST
TG
SW
LTC3856
BG
PGND
SENSE⁺
SENSE^–
SGND
3856 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 • 2R_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
OPTIONAL
TEMP COMP
NETWORK
V_IN
INTV_CC
LTC3856
BOOST
TG
SW
ITEMP BG
R_S
PGND
R_NTC
SENSE⁺
R_P
SENSE^–
SGND
R1
C1* R2
V_IN
INDUCTOR
L DCR
V_OUT
3856 F02b
*PLACE C1 NEAR SENSE⁺,
SENSE^– PINS
R1||R2 × C1 =
L
DCR
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
3856f
15