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LTC3890-2

38902f

APPLICATIONS INFORMATION

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 drop across

the inductor DCR multiplied by R2/(R1 + R2). R2 scales the

voltage across the sense terminals for applications where

the DCR is greater than the target sense resistor value.

To properly dimension the external filter components, the

DCR of the inductor must be known. It can be measured

using a good RLC meter, but the DCR tolerance is not

always the same and varies with temperature; consult

the manufacturers’ data sheets for detailed information.

Using the inductor ripple current value from the Inductor

Value Calculation section, the target sense resistor value is:

RSENSE(EQUIV) =

VSENSE(MAX)

IMAX +

ΔI

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimum value for the Maximum Current Sense Threshold

(VSENSE(MAX)) in the Electrical Characteristics table (30mV,

50mV or 75mV, depending on the state of the ILIM pin).

Next, determine the DCR of the inductor. When provided,

use the manufacturer’s maximum value, usually given at

20°C. Increase this value to account for the temperature

coefficient of copper resistance, which is approximately

0.4%/°C. A conservative value for TL(MAX) is 100°C.

To scale the maximum inductor DCR to the desired sense

resistor value (RD), use the divider ratio:

RD =

RSENSE(EQUIV)

DCRMAX at TL(MAX)

C1 is usually selected to be in the range of 0.1μF to 0.47μF.

This forces R1|| R2 to around 2k, reducing error that might

have been caused by the SENSE+ pin’s ±1μA current.

The equivalent resistance R1|| R2 is scaled to the room

temperature inductance and maximum DCR:

R1|| R2

DCR at 20°C

() •C1

The sense resistor values are:

R1|| R2

; R2

R1• RD

1– RD

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

sense resistors. 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.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because of

MOSFET switching and gate charge losses. In addition to

this basic trade-off, the effect of inductor value on ripple

current and low current operation must also be considered.

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