ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 15

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

The Typical Application on the fi rst page is a basic LTC3890
application circuit. LTC3890 can be confi gured 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 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 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
mum current limit of the controller. When I
the maximum current limit threshold voltage of the cur-
rent comparator is programmed to be 30mV. When I
is fl oated, 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 LTC3857
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) fl ows into the pin. Between INTV
and INTV
current to the higher current.
APPLICATIONS INFORMATION
SENSE
CC
LIM
+
LIM
– 0.5V, a small current of less than 1μA fl ows out
(if R
and SENSE
pin is a tri-level logic input which sets the maxi-
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
CC
LIM
+ 0.5V, a higher
–
is grounded,
is less than
CC
– 0.5V
LIM
Filter components mutual to the sense lines should be
placed close to the LTC3890, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 3). 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 4b), sense resistor R1 should be
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
*PLACE C1 NEAR
SENSE PINS
LTC3890
LTC3890
SENSE
SENSE
INTV
BOOST
SGND
(4b) Using the Inductor DCR to Sense Current
SENSE
SENSE
SW
INTV
V
BOOST
TG
BG
CC
SGND
IN
+
–
(4a) Using a Resistor to Sense Current
SW
V
TG
BG
CC
IN
Figure 4. Current Sensing Methods
+
–
(R1 || R2) • C1 =
NEXT TO THE CONTROLLER
C1*
INDUCTOR OR R
PLACE CAPACITOR NEAR
SENSE PINS
TO SENSE FILTER,
R2
R1
DCR
L
R
SENSE
SENSE(EQ)
3890 F03
V
INDUCTOR
IN
L
= DCR
V
C
R
IN
OUT
SENSE
LTC3890
DCR
R1 + R2
R2
3890 F04a
15
V
3890 F04b
OUT
V
OUT
3890f