LTC4366 datasheet(15/32 Pages) AD

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LTC7862

Rev 0

For more information www.analog.com

The Typical Application on the first page is a basic

LTC7862 application circuit. LTC7862 can be configured

to use either DCR (inductor resistance) sensing or low

value resistor sensing. The choice between the two cur-

rent 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, a current sensing

resistor provides the most accurate current limit 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.

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 LTC7862 to regulate an output clamp voltage up to

a nominal set point of 60V (allowing margin for toler-

ances and transients). The SENSE+ pin is high impedance,

drawing less than ≈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, it is high impedance, drawing less than

≈1µA. When SENSE– is above INTVCC + 0.5V, a higher

current (≈900µA) 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 LTC7862, 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

APPLICATIONS INFORMATION

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

7862 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 out-

put current.

The current comparator has a maximum threshold

VSENSE(MAX) = 50mV typical. The current comparator

threshold voltage sets the peak of the inductor current,

yielding a maximum average output current, IMAX, equal

to the peak value less half the peak-to-peak ripple cur-

rent, ∆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

compensation required to meet stability criterion operat-

ing at greater than 50% duty factor. The LTC7862, how-

ever, uses a proprietary circuit to nullify the effect of slope

compensation on the current limit performance.

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