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LTC7812

7812fc

For more information www.linear.com/LTC7812

APPLICATIONS INFORMATION

Using the inductor ripple current value from the Inductor

ValueCalculationsection,thetargetsenseresistorvalueis:

R(EQUIV) =

VSENSE(MAX)

IMAX +

DIL

To ensure that the application will deliver full load cur-

rent over the full operating temperature range, determine

RSENSE(EQUIV), keeping in mind that the maximum current

sense threshold (VSENSE(MAX)) for the LTC7812 is fixed

at 50mV.

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

use the manufacturer’s maximum value, usually given at

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

coefficient of 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, use the divider ratio:

RD =

RSENSE(EQUIV)

DCRMAX atTL(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=

R1 R2

; R2 =

R1• RD

1−RD

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

For the buck controllers, the maximum power loss will

occur in continuous mode at the maximum input voltage:

PLOSS R1=

(VIN(MAX) − VOUT) • VOUT

For the boost controller, the maximum power loss in R1

will occur in continuous mode at VIN = 1/2•VOUT:

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 modestly

higher with a DCR network than with a sense resistor,

due to the extra switching losses incurred through R1.

However,DCRsensingeliminatesasenseresistor,reduces

conduction losses and provides higher efficiency at heavy

loads.Peakefficiencyisaboutthesamewitheithermethod.

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 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.

The inductor value has a direct effect on ripple current.

The inductor ripple current DIL decreases with higher

inductance or frequency. For the buck controller, DIL

increases with higher VIN:

DIL =

(f)(L)

VOUT 1−

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

For the boost controller, the inductor ripple current DIL

increases with higher VOUT:

DIL =

(f)(L)

VIN 1−

VIN

VOUT

⎛

⎝

⎜

⎞

⎠

⎟

Accepting larger values of DIL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is DIL = 0.3(IMAX). The maximum

DIL occurs at the maximum input voltage for the buck and

VIN = 1/2•VOUT for the boost.

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