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LTC3853

3853fc

For more information www.linear.com/LTC3853

Triple vs Dual (2 + 1) Operation

The LTC3853 can be used to regulate three different out-

puts. It can also be used as a dual output controller with

a high current 2-phase output and a single phase output.

Tying VFB2 to VIN through a 200k resistor switches the

controller from triple to dual (2 + 1) operation. Do not ex-

ceedtheabsolutemaximumcurrentratingfortheVFB2pin.

In dual (2 + 1) mode, phase 1 and phase 2 are 180 degrees

apart (instead of 120 degrees) with phase 3 remaining at

OPERATION

240 degrees from phase 1. The ITH1 and ITH2 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

TheTypicalApplicationonthefirstpageisabasicLTC3853

applicationcircuit.LTC3853canbeconfiguredtouseeither

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 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

externalcomponentselectionisdrivenbytheloadrequire-

ment, and begins with the selection of RSENSE (if RSENSE is

used) and inductor value. Next, the power MOSFETs are

selected. Finally, input and output capacitors are selected.

Current Limit Programming

The ILIM pin is a tri-level logic input which sets the maxi-

mum current limit of the controller. When ILIM is either

grounded, floated or tied to INTVCC, the typical value for

themaximumcurrentsensethresholdwillbe30mV,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

Figure 1. Sense Lines Placement with Inductor or Sense Resistor

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

3853 F01

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