LTC3858-2 datasheet(16/40 Pages) LINER

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LTC3858-2

38582f

APPLICATIONS INFORMATION

Figure 3. Sense Lines Placement with Inductor or Sense Resistor

The Typical Application on the first page is a basic

LTC3858-2 application circuit. LTC3858-2 can be config-

ured 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 resistors 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 requirement, and begins with the selection of

RSENSE (if RSENSE is used) and inductor value. Next, the

power MOSFETs and Schottky diodes are selected. Finally,

input and output capacitors are selected.

Current Limit Programming

The ILIMpinisatri-levellogicinputwhichsetsthemaximum

current limit of the converter. When ILIM is grounded, the

maximum current limit threshold voltage of the current

comparator is programmed to be 30mV. When ILIM is

floated, the maximum current limit threshold is 50mV.

When ILIM is tied to INTVCC, the maximum current limit

threshold is set to 75mV.

SENSE+ and SENSE– Pins

The SENSE+ and SENSE– pins are the inputs to the current

comparators. The common mode voltage range on these

pins is 0V to 28V (Abs Max), enabling the LTC3858-2 to

regulate output voltages up to a nominal 24V (allowing

plenty of margin for tolerances and transients).

The SENSE+ pin is high impedance over the full common

mode range, drawing at most ±1μA. This high impedance

allows the current comparators to be used in inductor

DCR sensing.

The impedance of the SENSE– pin changes depending on

the common mode voltage. When SENSE– is less than

INTVCC – 0.5V, a small current of less than 1μA flows out

of the pin. When SENSE– is above INTVCC + 0.5V, a higher

current (~550μA) flows into the pin. Between INTVCC –

0.5V and INTVCC + 0.5V, the current transitions from the

smaller current to the higher current.

Filter components mutual to the sense lines should be

placed close to the LTC3858-2, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 3). 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 inductor DCR

sensing is used (Figure 4b), resistor R1 should be placed

close to the switching node, to prevent noise from coupling

into sensitive small-signal nodes.

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

3858 F03

Low Value Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 4a. RSENSE is chosen based on the required

output current.

The current comparator has a maximum threshold

VSENSE(MAX) determined by the ILIM setting. The current

comparator threshold voltage sets the peak of the induc-

tor current, yielding a maximum average output current,

IMAX, equal to the peak value less half the peak-to-peak

ripple current, ΔIL. To calculate the sense resistor value,

use the equation:

RSENSE =

VSENSE(MAX)

IMAX +

ΔIL

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due to

the internal compensation required to meet stability cri-

terion for buck regulators operating at greater than 50%

duty factor. A curve is provided in the Typical Performance

Characteristics section to estimate this reduction in peak

output current depending upon the operating duty factor.

LTC3858-2 Datasheet(PDF) 16 Page - Linear Technology

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