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LTC3890-2

38902f

APPLICATIONS INFORMATION

The Typical Application on the first page is a basic LTC3890-

2 application circuit. LTC3890-2 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 and Schottky diodes 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 grounded,

the maximum current limit threshold voltage of the cur-

rent comparator is programmed to be 30mV. 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 50mV.

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 LTC3890-2 to

regulate output voltages up to a nominal 24V (allowing

margin for tolerances and transients). This common mode

range is independent of the state of the VFB pin.

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 (~700μA) 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 LTC3890-2, 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), sense 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

38902 F03

Figure 3. Sense Lines Placement with Inductor or Sense Resistor

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