LTC3853 Datasheet, PDF (13/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
OPERATION
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_FB2 to V_IN through a 200k resistor switches the controller
from triple to dual (2 + 1) operation. Do not exceed the
absolute maximum current rating for the V_FB2 pin.
In dual (2 + 1) mode, phase 1 and phase 2 are 180 degrees
apart (instead of 120 degrees) with phase 3 remaining at
240 degrees from phase 1. The I_TH1 and I_TH2 pins must be
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.
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic LTC3853
application circuit. LTC3853 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 tradeoff between cost, power
consumption, and accuracy. DCR sensing is becoming
popular because it saves expensive current sensing resis-
tors 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 require-
ment, and begins with the selection of R_SENSE (if R_SENSE is
used) and inductor value. Next, the power MOSFETs are se-
lected. 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. For single output
dual phase applications ((2 + 1) mode), use the 50mV or
75mV setting for optimal current sharing.
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 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 flow out of the SENSE pins. When
the SENSE pins ramp down from the maximum common
mode voltage to 1.1V, the small base currents flow into
the SENSE pins. The high impedance inputs to the cur-
rent comparators allow accurate DCR sensing. However,
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 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
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R_SENSE
C_OUT
3853 F01
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
3853f
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