LTC3855_15 Datasheet, PDF(17/44 Page) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3855_15 Datasheet, PDF (17/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

◁

LTC3855

Applications Information

VIN

INTVCC

VIN

INTVCC

BOOST

LTC3855

PGND

SENSE+

3855 F02a

SENSEâ

SGND

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE RESISTOR

PLUS PARASITIC

INDUCTANCE

RS ESL

VOUT

CF â¢ 2RF â¤ ESL/RS

POLE-ZERO

CANCELLATION

OPTIONAL

TEMP COMP

NETWORK

RNTC

BOOST

LTC3855

ITEMP

PGND

SENSE+

SENSEâ

SGND

R1**

C1* R2

INDUCTOR

L DCR

VOUT

**PLACE R1 NEXT TO *PLACE C1 NEAR SENSE+,

INDUCTOR

SENSEâ PINS

R1||R2 Ã C1 =

DCR

RSENSE(EQ) = DCR

R1 + R2

3855 F02b

(2a) Using a Resistor to Sense Current

(2b) Using the Inductor DCR to Sense Current

Figure 2. Two Different Methods of Sensing Current

the resulting waveform looks resistive again, as shown

in Figure 4. For applications using low maximum sense

voltages, check the sense resistor manufacturerâs data

sheet for information about parasitic inductance. In the

absence of data, measure the voltage drop directly across

the sense resistor to extract the magnitude of the ESL

step and use the equation above to determine the ESL.

However, do not over-filter. Keep the RC time constant less

than or equal to the inductor time constant to maintain a

high enough ripple voltage on VRSENSE.

VSENSE

20mV/DIV

VESL(STEP)

500ns/DIV

3855 F03

Figure 3. Voltage Waveform Measured

Directly Across the Sense Resistor.

VSENSE

20mV/DIV

500ns/DIV

3855 F04

Figure 4. Voltage Waveform Measured After the

Sense Resistor Filter. CF = 1000pF, RF = 100â¦.

The above generally applies to high density/high current

applications where I(MAX) >10A and low values of induc-

tors are used. For applications where I(MAX) <10A, set RF

to 10 Ohms and CF to 1000pF. This will provide a good

starting point.

The filter components need to be placed close to the IC.

The positive and negative sense traces need to be routed

as a differential pair and Kelvin connected to the sense

resistor.

Inductor DCR Sensing

For applications requiring the highest possible efficiency at

high load currents, the LTC3855 is capable of sensing the

voltage drop across the inductor DCR, as shown in Figure

2b. The DCR of the inductor represents the small amount

of DC winding resistance of the copper, which can be less

than 1mâ¦ for todayâs low value, high current inductors.

In a high current application requiring such an inductor,

conduction loss through a sense resistor would cost sev-

eral points of efficiency compared to DCR sensing.

If the external R1|| R2 â¢ C1 time constant is chosen to be

exactly equal to the L/DCR time constant, the voltage drop

across the external capacitor is equal to the drop across

the inductor DCR multiplied by R2/(R1 + R2). R2 scales the

voltage across the sense terminals for applications where

the DCR is greater than the target sense resistor value.

To properly dimension the external filter components, the

DCR of the inductor must be known. It can be measured

using a good RLC meter, but the DCR tolerance is not

3855f

▷