LTC7802EUFDM datasheet(14/34 Pages) AD

14 / 34 page

LTC7802

Rev. 0

For more information www.analog.com

The Typical Application on the first page is a basic

LTC7802 application circuit. External component selection

is largely driven by the load requirement and begins with

the selection of the inductor, current sense components,

operating frequency, and light load operating mode. The

remaining power stage components, consisting of the

input and output capacitors, and power MOSFETs can

then be chosen. Next, feedback resistors are selected

to set the desired output voltage. Then, the remaining

external components are selected, such as for soft-start,

biasing, and loop compensation.

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 addi-

tion 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 maximum average inductor current IL(MAX) is equal

to the maximum output current. The peak current is equal

to the average inductor current plus half of the inductor

ripple current, ΔIL, which decreases with higher induc-

tance or higher frequency and increases with higher VIN:

ΔIL =

(f)(L)

VOUT 1−

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of ΔIL allows the use of low induc-

tances, but results in higher output voltage ripple and

greater core losses. A reasonable starting point for setting

ripple current is ΔIL = 0.3 • IL(MAX). The maximum ΔIL

occurs at the maximum input voltage.

The inductor value also has secondary effects. The tran-

sition to Burst Mode operation begins when the average

inductor current required results in a peak current below

25% of the current limit determined by RSENSE. Lower

inductor values (higher ΔIL) will cause this to occur at

lower load currents, which can cause a dip in efficiency

in the upper range of low current operation.

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efficiency regulators generally can-

not afford the core loss found in low cost powdered

iron cores, forcing the use of more expensive ferrite or

molypermalloy cores. Actual core loss is very dependent

on inductance value selected. As inductance increases,

core losses go down. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Ferrite designs have very low core loss and are preferred

for high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates hard, which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Current Sense Selection

The LTC7802 can be configured to use either DCR (induc-

tor resistance) sensing or low value resistor sensing.

The choice between the two current sensing schemes

is largely a design trade-off between cost, power con-

sumption and accuracy. DCR sensing has become popular

because it saves expensive current sensing resistors and

is more power efficient, particularly in higher current and

lower frequency applications. However, current sensing

resistors provide the most accurate current limits for the

controller. Other external component selection is driven

by the load requirement and begins with the selection of

RSENSE (if RSENSE is used) and inductor value.

The SENSE+ and SENSE– pins are the inputs to the current

comparators. The common mode voltage range on these

pins is 0V to 40V (absolute maximum), enabling the

LTC7802 to regulate output voltages up to a maximum

of 40V. The SENSE+ pin is high impedance, drawing

less than ≈1μA. This high impedance allows the current

comparators to be used in inductor DCR sensing. The

impedance of the SENSE– pin changes depending on the

APPLICATIONS INFORMATION

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