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LTC3613

3613fa

APPLICATIONS INFORMATION

The inductor DCR is sensed by connecting an RC filter

across the inductor. This filter typically consists of one

or two resistors (R1 and R2) and one capacitor (C1) as

shown in Figure 4. If the external R1||R2 • C1 time constant

is chosen to be exactly equal to the L/DCR time constant,

the voltage drop across the external capacitor is equal

to the voltage drop across the inductor DCR multiplied

by R2/(R1 + R2). Therefore, R2 may be used to scale

the voltage across the sense terminals when the DCR is

greater than the target sense resistance. With the ability

to program current limit through the VRNG pin, R2 may

be optional. C1 is usually selected to be in the range of

0.01μF to 0.47μF. This forces R1|| R2 to around 2k to 4k,

reducing error that might have been caused by the sense

pins’ input bias currents.

The first step in designing DCR current sensing is to

determine the DCR of the inductor. Where provided, use

the manufacturer’s maximum value, usually given at 25°C.

Increase this value to account for the temperature coef-

ficient of resistance, which is approximately 0.4%/°C. A

conservative value for inductor temperature TL is 100°C.

The DCR of the inductor can also be measured using a good

RLC meter, but the DCR tolerance is not always the same

and varies with temperature; consult the manufacturers’

datasheets for detailed information.

From the DCR value, VSENSE(MAX) is calculated as:

VSENSE(MAX)=DCRMAX at 25°C• 1+0.4% TL(MAX)–25°C

()

⎡

⎣

⎤

⎦

• I

OUT(MAX) –ΔIL /2

⎡⎣

⎤⎦

If VSENSE(MAX) is within the maximum sense voltage of

the LTC3613 as programmed by the VRNG pin (30mV to

100mV), then the RC filter only needs R1. If VSENSE(MAX) is

higher, then R2 may be used to scale down the maximum

sense voltage so that it falls within range.

The maximum power loss in R1 is related to duty cycle,

and will occur in continuous mode at the maximum input

voltage:

PLOSS R1

()=

VIN(MAX)–VOUT

()•V

OUT

Ensure that R1 has a power rating higher than this value.

If high efficiency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing or

RSENSE sensing. Light load power loss can be modestly

higher with a DCR network than with a sense resistor due

to the extra switching losses incurred through R1. However,

DCR sensing eliminates a sense resistor, reduces conduc-

tion losses and provides higher efficiency at heavy loads.

Peak efficiency is about the same with either method.

To maintain a good signal-to-noise ratio for the current

sense signal, use a minimum ΔVSENSE of 10mV. For a

DCR sensing application, the actual ripple voltage will be

determined by:

ΔV

SENSE =

VIN –VOUT

•C1

•

VOUT

VIN • f

Operating Multiple Units in Parallel

The LTC3613’s current mode control architecture makes

it straightforward to parallel multiple units for higher

output current. Figure 13 shows an example circuit of two

LTC3613s placed in parallel to provide 30A at 1.2V from

a 6V to 24V input. The signals at MODE/PLLIN are 180°

out of phase, to reduce stress on the input and output

capacitors.

Since the ITH pin voltage determines the cycle-by-cycle

valley inductor current, sharing is achieved by connecting

the ITH pins together. Because the ITH pin is sensitive to

noise, a small 22pF to 47pF decoupling capacitor should

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