LTC7802 datasheet(16/34 Pages) AD

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LTC7802-3.3

Rev. 0

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impedance of the SENSE– pin changes depending on the

common mode voltage. When less than INTVCC – 0.5V,

these pins are relatively high impedance, drawing ≈ 1μA.

When above INTVCC + 0.5V, a higher current (≈650μA)

flows into each pin. Between INTVCC – 0.5V and INTVCC

+ 0.5V, the current transitions from the smaller current to

the higher current. Channel 1’s SENSE1– pin has an addi-

tional ≈ 50μA current when its voltage is above 3.2V to

bias internal circuitry from VOUT1 instead of VIN, thereby

reducing the input-referred supply current.

Filter components mutual to the sense lines should be

placed close to the LTC7802-3.3, and the sense lines

should run close together to a Kelvin connection under-

neath the current sense element (shown in Figure 1).

Sensing current elsewhere can effectively add parasitic

inductance and capacitance to the current sense ele-

ment, degrading the information at the sense terminals

and making the programmed current limit unpredictable.

If DCR sensing is used (Figure 2b), resistor R1 should be

placed close to the switching node, to prevent noise from

coupling into sensitive small signal nodes.

Using the maximum inductor current (IL(MAX)) and ripple

current (ΔIL) from the Inductor Value Calculation section,

the target sense resistor value is:

RSENSE(EQUIV) ≤

VSENSE(MAX)

IL(MAX)+

� IL

To ensure that the application will deliver full load cur-

rent over the full operating temperature range, choose

the minimum value for VSENSE(MAX) in the Electrical

Characteristics table.

To avoid potential jitter or instability due to PCB noise cou-

pling into the current sense signal, the AC current sensing

ripple of ΔVSENSE = ΔIL • RSENSE should also be checked

to ensure a good signal-to-noise ratio. In general, for a

reasonably good PCB layout, a target ΔVSENSE voltage of

10mV to 20mV at nominal input voltage is recommended

for both RSENSE and DCR sensing applications.

The parasitic inductance (ESL) of the sense resistor

introduces significant error in the current sense signal

for lower inductor value (<3μH) or higher current (>5A)

applications. This error is proportional to input voltage

and may degrade line regulation or cause loop instability.

An RC filter into the sense pins, as shown in Figure 2a, can

be used to compensate for this error. Set the RC filter time

constant RF • CF = ESL/RSENSE for optimal cancellation of

the ESL. In general, select CF to be in the range of 1nF to

10nF and calculate the corresponding RF. Surface mount

sense resistors in low ESL wide footprint geometries are

recommended to minimize this error. If not specified on

the manufacturer’s data sheet, the ESL can be approxi-

mated as 0.4nH for a resistor with a 1206 footprint and

0.2nH for a 1225 footprint.

Inductor DCR Current Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC7802-3.3 is capable of sens-

ing 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 1mΩ for today’s low value, high current

APPLICATIONS INFORMATION

Low Value Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 2a. RSENSE is chosen based on the required out-

put current. Each controller’s current comparator has a

maximum threshold VSENSE(MAX) of 50mV. The current

comparator threshold voltage sets the peak inductor

current.

780233 F01

TO SENSE FILTER

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

CURRENT FLOW

Figure 1. Sense Lines Placement with Inductor or

Sense Resistor

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