LTM4677 datasheet(40/116 Pages) AD

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LTC3883/LTC3883-1

Rev. F

For more information www.analog.com

VIN

INTVCC

BOOST

PGND

FILTER COMPONENTS

PLACED NEAR SENSE PINS

ISENSE+

ISENSE–

SGND

LTC3883

VOUT

3883 F018b

CF • 2RF ≤ ESL/RS

POLE-ZERO

CANCELLATION

SENSE RESISTOR

PLUS PARASITIC

INDUCTANCE

ESL

VIN

INTVCC

BOOST

PGND

*PLACE C1 NEAR SENSE+, SENSE– PINS

INDUCTOR

DCR

OPTIONAL

>1µF

ISENSE+

ISENSE–

SGND

LTC3883

VOUT

3883 F18a

C1*

[(R1+ R3)||R2]

• C1 = 2 •L

DCR

IOUT_CAL_GAIN = DCR

•

; R3 = R1

R1 + R2 + R3

APPLICATIONS INFORMATION

The current comparator has a maximum threshold

VSENSE(MAX) determined by the ILIMIT setting. The input

common mode range of the current comparator is 0V to

5.5V (if VIN is greater than 6V). The current comparator

threshold 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 current ∆IL.

To calculate the sense resistor value, use the equation:

RSENSE =

VSENSE(MAX)

IMAX +

∆IL

Due to possible PCB noise in the current sensing loop, the

AC current sensing ripple of ∆VSENSE = ∆IL • RSENSE also

needs to be checked in the design to get a good signal-to-

noise ratio. In general, for a reasonably good PCB layout,

a 15mV minimum ∆VSENSE voltage is recommended as

a conservative number to start with, either for RSENSE or

DCR sensing applications.

For previous generation current mode controllers, the

maximum sense voltage was high enough (e.g., 75mV for

the LTC1628/LTC3728 family) that the voltage drop across

the parasitic inductance of the sense resistor represented

a relatively small error. In the new highest current density

solutions; however, the value of the sense resistor can be

less than 1mΩ and the peak sense voltage can be less than

20mV.Inaddition,inductorripplecurrentsgreaterthan50%

with operation up to 1MHz are becoming more common.

Under these conditions, the voltage drop across the sense

resistor’s parasitic inductance is no longer negligible. A

typical sensing circuit using a discrete resistor is shown in

Figure 18b. In previous generations of controllers, a small

RC filter placed near the IC was commonly used to reduce

the effects of the capacitive and inductive noise coupled

in the sense traces on the PCB. A typical filter consists of

two series 100Ω resistors connected to a parallel 1000pF

capacitor, resulting in a time constant of 200ns.

This same RC filter with minor modifications, can be

used to extract the resistive component of the current

sense signal in the presence of parasitic inductance. For

example,Figure19illustratesthevoltagewaveformacross

a 2mΩ resistor with a 2010 footprint. The waveform is

the superposition of a purely resistive component and a

Figure 18a. Inductor DCR Current Sense Circuit

Figure 18b. Resistor Current Sense Circuit

C1 should be placed close to the IC pins. This impedance

difference can result in loss of accuracy in the current

reading of the ADC. The current reading accuracy can be

improved by matching the impedance of the two pins. To

accomplish this add a series resistor between VOUT and

ISENSE– equal to R1. A capacitor of 1µF or greater should

be placed in parallel with this resistor. If the peak voltage

is <75mV at room temperature, R2 is not required.

LOW VALUE RESISTOR CURRENT SENSING

A typical sensing circuit using a discrete resistor is shown

in Figure 18b. RSENSE is chosen based on the required

output current.

LTM4677 Datasheet(PDF) 40 Page - Analog Devices

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