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LTC3895

3895fa

For more information www.linear.com/LTC3895

TheTypicalApplicationonthefirstpageisabasicLTC3895

application circuit. LTC3895 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 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 are selected. Finally, input and output

capacitors are selected.

Current Limit Programming

The ILIM pin is a three-state logic input which sets the

maximum current limit of the controller. When ILIM is

grounded, the maximum current limit threshold voltage of

the current comparator is programmed to be 50mV. When

ILIM is floated, the maximum current limit threshold is

75mV. When ILIM is tied to INTVCC, the maximum current

limit threshold is set to 100mV.

SENSE+ and SENSE– Pins

The SENSE+ and SENSE– pins are the inputs to the cur-

rent comparator. The common mode voltage range on

these pins is 0V to 65V (absolute maximum), enabling

the LTC3895 to regulate output voltages up to a nominal

set point of 60V (allowing margin for tolerances and tran-

sients). 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 (≈850μA) flows into the

pin. Between INTVCC–0.5VandINTVCC+0.5V,thecurrent

transitions from the smaller current to the higher current.

applicaTions inForMaTion

Filter components mutual to the sense lines should be

placed close to the LTC3895, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 2). 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 3b), resistor R1 should be placed close to

the switching node, to prevent noise from coupling into

sensitive small-signal nodes.

Figure 2. Sense Lines Placement with Inductor or Sense Resistor

3895 F02

TO SENSE FILTER

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

CURRENT FLOW

COUT

Low Value Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 3a. 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, ∆IL. To calculate the sense resistor value,

use the equation:

RSENSE =

VSENSE(MAX)

IMAX +

ΔIL

Normally in high duty cycle conditions, the maximum

output current level will be reduced due to the internal

compensationrequiredtomeetstabilitycriterionoperating

at greater than 50% duty factor. The LTC3895, however,

uses a proprietary circuit to nullify the effect of slope

compensation on the current limit performance.

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