SC1480ITSTRT datasheet(13/22 Pages) SEMTECH

13 / 22 page

 2006 Semtech Corp.

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SC1480

POWER MANAGEMENT

Applications Information (Cont.)

The error comparator offset is trimmed so that it trips

when VOUT is 1.25 volts +/-1% at room temperature.

This offset does not drift significantly with supply and

temperature. Thus, the error comparator contributes 1%

or less to DC system inaccuracy.

The on pulse in the SC1480 is calculated to give a pseudo

fixed frequency. Nevertheless, some frequency variation

with line and load can be expected. This variation changes

the output ripple voltage. Because constant on regulators

regulate to the valley of the output ripple, ½ of the output

ripple appears as a DC regulation error. For example, if

REFOUT=1.25 volts, then the valley of the output ripple

will be 1.25 volts. If the ripple is 20mv with VIN=6, then

the DC output voltage will be 1.26 volts. If the ripple is

40mv with VIN=25 volts, then the DC output voltage will

be 1.27 volts. The best way to minimize this effect is to

minimize the output ripple.

To compensate for valley regulation it is usually desirable

to use passive droop. Take the feedback directly from

the output side of the inductor incorporating a small

amount of trace resistance between the inductor and

output capacitor. This trace resistance should be

optimized so that at full load the output droops to near

the lower regulation limit. Passive droop minimizes the

required output capacitance because the voltage

excursions due to load steps are reduced. Passive droop

also improves stability so it should be used when possible.

Thermal Considerations

The junction temperature of the device may be calculated

as follows:

•

Where:

A = ambient temperature (°C)

D = power dissipation in (W)

JA = thermal impedance junction to ambient from

absolute maximum ratings (°C/W)

The power dissipation may be calculated as follows:

VCCA

•

Where:

VCCA = chip supply voltage (V)

VCCA = operating current (A)

g = gate drive voltage, typically 5V (V)

g = FET gate charge, from the FET datasheet (C)

f = switching frequency (kHz)

Inserting the following values as an example:

A = 85°C

JA = 100°C/W

VCCA = 5V

VCCA = 1100µA (data sheet maximum)

g = 5V

g = 60nC

f = 300kHz

gives us:

()

100

300

1100

•

−

As can be seen, the heating effects due to internal power

dissipation are practically negligible, thus requiring no

special consideration thermally during layout.

SC1480ITSTRT Datasheet(PDF) 13 Page - Semtech Corporation

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