LTC7803 datasheet(21/44 Pages) AD

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LTC7817

Rev 0

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APPLICATIONS INFORMATION

The sense resistor values are:



R1 R2

; R2

R1• RD

� RD

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

For the buck controllers, the maximum power loss occurs

in continuous mode at the maximum input voltage:

PLOSS R1=

(VIN(MAX) � VOUT) • VOUT

For the boost controller, the maximum power loss occurs

in continuous mode at VIN = VOUT/2:

PLOSS R1=

(VOUT(MAX) � VIN) • VIN

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 mod-

estly higher with a DCR network than with a sense resis-

tor, due to the extra switching losses incurred through

R1. However, DCR sensing eliminates a sense resistor,

reduces conduction losses and provides higher efficiency

at heavy loads. Peak efficiency is about the same with

either method.

Setting the Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing gate charge and transition losses, but requires

larger inductance values and/or more output capacitance

to maintain low output ripple voltage.

In higher voltage applications transition losses contrib-

ute more significantly to power loss, and a good balance

between size and efficiency is generally achieved with a

switching frequency between 300kHz and 900kHz. Lower

voltage applications benefit from lower switching losses

and can therefore more readily operate at higher switching

frequencies up to 3MHz if desired.

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 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.

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 target equivalent resistance R1||R2 is calculated from

the nominal inductance, C1 value, and DCR:



R1 R2 =

(DCR at 20

°C) • C1

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