ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 13

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

Triple vs Dual (2 + 1) Operation
The LTC3853 can be used to regulate three different outputs.
It can also be used as a dual output controller with a high
current 2-phase output and a single phase output. Tying
V
from triple to dual (2 + 1) operation. Do not exceed the
absolute maximum current rating for the V
In dual (2 + 1) mode, phase 1 and phase 2 are 180 degrees
apart (instead of 120 degrees) with phase 3 remaining at
The Typical Application on the fi rst page is a basic LTC3853
application circuit. LTC3853 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 tradeoff between cost, power
consumption, and accuracy. DCR sensing is becoming
popular because it saves expensive current sensing resis-
tors 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 require-
ment, and begins with the selection of R
used) and inductor value. Next, the power MOSFETs are se-
lected. Finally, input and output capacitors are selected.
Current Limit Programming
The I
mum current limit of the controller. When I
grounded, fl oated 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. For single output
dual phase applications ((2 + 1) mode), use the 50mV or
75mV setting for optimal current sharing.
OPERATION
APPLICATIONS INFORMATION
FB2
to V
LIM
IN
pin is a tri-level logic input which sets the maxi-
through a 200k resistor switches the controller
CC
, the typical value for
SENSE
FB2
LIM
(if R
pin.
is either
SENSE
is
240 degrees from phase 1. The I
shorted together externally and so must the TK/SS1 and
TK/SS2 pins for proper operating of the 2 phase portion
of the controller. RUN2 should be grounded. RUN1 will
now control both phases 1 and 2, while RUN3 continues
to control the turn on of phase 3.
Phase 3 is also capable of regulating up to a 13.5V output
in either mode, while phases 1 and 2 are limited to a 5.3V
output.
SENSE
The SENSE
comparators. The common mode input voltage range of
the current comparators is 0V to 5.3V for phases 1 and
2, and 0V to 13.5V for phase 3. Both SENSE pins are high
impedance inputs with small base currents of less than
1μA. When the SENSE pins ramp up from 0V to 1.4V, the
small base currents fl ow out of the SENSE pins. When
the SENSE pins ramp down from the maximum common
mode voltage to 1.1V, the small base currents fl ow into
the SENSE pins. The high impedance inputs to the cur-
rent comparators allow accurate DCR sensing. However,
care must be taken not to fl oat these pins during normal
operation.
Filter components mutual to the sense lines should be
placed close to the LTC3853, 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
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
+
and SENSE
+
and SENSE
INDUCTOR OR R
–
Pins
–
pins are the inputs to the current
TO SENSE FILTER,
NEXT TO THE CONTROLLER
SENSE
TH1
and I
C
OUT
LTC3853
3853 F01
TH2
pins must be
13
3853f