ltc3891 Linear Technology Corporation, ltc3891 Datasheet - Page 14

• Manufacturer Part Number: ltc3891
• Description: Ltc3891 - Low Iq, 60v Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3891
The Typical Application on the first page is a basic LTC3891
application circuit. LTC3891 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 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 selection of
R
power MOSFETs and Schottky diodes are selected. Finally,
input and output capacitors are selected.
Current Limit Programming
The I
current limit of the controller. When I
maximum current limit threshold voltage of the current
comparator is programmed to be 30mV. When I
floated, the maximum current limit threshold is 75mV.
When I
threshold is set to 50mV.
SENSE
The SENSE
rent comparators. The common mode voltage range on
these pins is 0V to 28V (abs max), enabling the LTC3891
to regulate output voltages up to a nominal 24V (allowing
margin for tolerances and transients).
The SENSE
mode range, drawing at most ±1μA. This high impedance
allows the current comparators to be used in inductor
DCR sensing.
The impedance of the SENSE
the common mode voltage. When SENSE
INTV
of the pin. When SENSE
current (~700μA) flows into the pin. Between INTV
and INTV
current to the higher current.
14
SENSE
LIM
CC
+
LIM
– 0.5V, a small current of less than 1μA flows out
(if R
pin is a tri-level logic input which sets the maximum
and SENSE
CC
is tied to INTV
+
+
+ 0.5V, the current transitions from the smaller
SENSE
pin is high impedance over the full common
and SENSE
is used) and inductor value. Next, the
–
Pins
–
–
is above INTV
CC
pins are the inputs to the cur-
, the maximum current limit
–
pin changes depending on
LIM
CC
is grounded, the
+ 0.5V, a higher
–
is less than
CC
– 0.5V
LIM
is
Filter components mutual to the sense lines should be
placed close to the LTC3891, 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 inductor 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.
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
*PLACE C1 NEAR
SENSE PINS
*R1 AND C1 ARE OPTIONAL
LTC3891
(2b) Using the Inductor DCR to Sense Current
LTC3891
SENSE
SENSE
INTV
BOOST
SGND
SENSE
SENSE
INTV
BOOST
SW
V
(2a) Using a Resistor to Sense Current
TG
BG
SGND
CC
IN
Figure 2. Current Sensing Methods
+
–
SW
V
BG
TG
CC
IN
+
–
(R1 || R2)
NEXT TO THE CONTROLLER
C1*
INDUCTOR OR R
• C1 =
C1*
TO SENSE FILTER,
R1*
R2
PLACE CAPACITOR NEAR
SENSE PINS
R1
DCR
L
R
SENSE
SENSE(EQ)
3891 F01
V
INDUCTOR
IN
L
= DCR
V
C
R
IN
OUT
SENSE
DCR
R1 + R2
R2
3891 F02a
V
OUT
3891 F02b
V
OUT
3891f