LTC3850 datasheet(13/36 Pages) LINER

13 / 36 page

LTC3850/LTC3850-1

38501fb

The Typical Application on the ﬁrst page is a basic LTC3850

application circuit. LTC3850 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 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 RSENSE (if RSENSE is

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

lected. 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

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 (see Figure 21), use the 50mV or

75mV setting for optimal current sharing.

APPLICATIONS INFORMATION

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 5V. 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 5V to 1.1V, the small base

currents ﬂow into the SENSE pins. The high impedance

inputs to the current 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 LTC3850, 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 DCR sensing

is used (Figure 2b), sense resistor R1 should be placed

Figure 2. Two Different Methods of Sensing Current

Figure 1. Sense Lines Placement

with Inductor or Sense Resistor

(2a) Using a Resistor to Sense Current

(2b) Using the Inductor DCR to Sense Current

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

38501 F01

VIN

INTVCC

BOOST

PGND

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE+

SENSE–

SGND

LTC3850

VOUT

38501 F02a

CF • 2RF ≤ ESL/RS

POLE-ZERO

CANCELLATION

SENSE RESISTOR

PLUS PARASITIC

INDUCTANCE

ESL

VIN

INTVCC

BOOST

PGND

*PLACE C1 NEAR SENSE+,

SENSE– PINS

INDUCTOR

DCR

SENSE+

SENSE–

SGND

LTC3850

VOUT

38501 F02b

C1*

R1||R2 × C1 =

DCR

RSENSE(EQ) = DCR

R1 + R2

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