LTC7803 datasheet(20/44 Pages) AD

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LTC7817

Rev 0

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7817 F02a

LTC7817

INTVCC

BOOST

SENSE1,2+

(SENSE3–)

SENSE1, 2–

(SENSE3+)

VOUT1,2

(VIN3)

VIN1,2

(VOUT3)

ESL

PLACE RF AND CF NEAR SENSE PINS

RSENSE

SENSE RESISTOR

WITH PARASITIC

INDUCTANCE

RF *CF = ESL/RSENSE

POLE-ZERO

CANCELLATION

7817 F02b

LTC7817

INTVCC

BOOST

SENSE1, 2+

(SENSE3–)

SENSE1, 2–

(SENSE3+)

GND

VIN1,2

(VOUT3)

VOUT1,2

(VIN3)

C1*

*PLACE C1 NEAR SENSE PINS

RSENSE(EQ) = DCR(R2/(R1+R2))

DCR

INDUCTOR

(R1||R2) • C1 = L/DCR

2b. Using the Inductor DCR to Sense Current

2a. Using a Resistor to Sense Current

Figure 2. Current Sensing Methods

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 for the bucks or

at 50% duty cycle for the boost 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 (<3uH) or higher current (>5A)

applications. For a buck converter, this error is propor-

tional to the 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 approximated 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 LTC7817 is capable of sensing

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

inductors. In a high current application requiring such

an inductor, power loss through a sense resistor would

cost several points of efficiency compared to inductor

DCR sensing.

APPLICATIONS INFORMATION

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