LM2575 datasheet(17/28 Pages) MOTOROLA

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LM2575

MOTOROLA ANALOG IC DEVICE DATA

GENERAL RECOMMENDATIONS

Output Voltage Ripple and Transients

Source of the Output Ripple

Since the LM2575 is a switch mode power supply

regulator, its output voltage, if left unfiltered, will contain a

sawtooth ripple voltage at the switching frequency. The

output ripple voltage value ranges from 0.5% to 3% of the

output voltage. It is caused mainly by the inductor sawtooth

ripple current multiplied by the ESR of the output capacitor.

Short Voltage Spikes and How to Reduce Them

The regulator output voltage may also contain short

voltage spikes at the peaks of the sawtooth waveform (see

Figure 24). These voltage spikes are present because of the

fast switching action of the output switch, and the parasitic

inductance of the output filter capacitor. There are some

other important factors such as wiring inductance, stray

capacitance, as well as the scope probe used to evaluate

these transients, all these contribute to the amplitude of these

spikes. To minimise these voltage spikes, low inductance

capacitors should be used, and their lead lengths must be

kept short. The importance of quality printed circuit board

layout design should also be highlighted.

Figure 24. Output Ripple Voltage Waveforms

HORTIZONTAL TIME BASE: 10

µs/DIV

UNFILITERED

OUTPUT

VOLTAGE

VERTICAL

RESOLUTION:

20 mV/DIV

FILITERED

OUTPUT

VOLTAGE

Voltage spikes caused by switching action of the output

switch and the parasitic inductance of the output capacitor

Minimizing the Output Ripple

In order to minimise the output ripple voltage it is possible

to enlarge the inductance value of the inductor L1 and/or to

use a larger value output capacitor. There is also another way

to smooth the output by means of an additional LC filter

(20

µH, 100 µF), that can be added to the output (see

Figure 33) to further reduce the amount of output ripple and

transients. With such a filter it is possible to

reduce the

output ripple voltage transients 10 times or more. Figure 24

shows the difference between filtered and unfiltered output

waveforms of the regulator shown in Figure 33.

The upper waveform is from the normal unfiltered output of

the converter, while the lower waveform shows the output

ripple voltage filtered by an additional LC filter.

Heatsinking and Thermal Considerations

The Through–Hole Package TO–220

The LM2575 is available in two packages, a 5–pin

TO–220(T, TV) and a 5–pin surface mount D2PAK(D2T).

There are many applications that require no heatsink to keep

the LM2575 junction temperature within the allowed

operating range. The TO–220 package can be used without

a heatsink for ambient temperatures up to approximately

°C (depending on the output voltage and load current).

Higher ambient temperatures require some heatsinking,

either to the printed circuit (PC) board or an external heatsink.

The Surface Mount Package D2PAK and its Heatsinking

The other type of package, the surface mount D2PAK, is

designed to be soldered to the copper on the PC board. The

copper and the board are the heatsink for this package and

the other heat producing components, such as the catch

diode and inductor. The PC board copper area that the

package is soldered to should be at least 0.4 in2 (or

100 mm2) and ideally should have 2 or more square inches

(1300 mm2) of 0.0028 inch copper. Additional increasing of

copper area beyond approximately 3.0 in2 (2000 mm2) will

not improve heat dissipation significantly. If further thermal

improvements are needed, double sided or multilayer PC

boards with large copper areas should be considered.

Thermal Analysis and Design

The following procedure must be performed to determine

whether or not a heatsink will be required. First determine:

1. PD(max) maximum regulator power dissipation in the

application.

2. TA(max)

maximum ambient temperature in the

application.

3. TJ(max)

maximum allowed junction temperature

(125

°C for the LM2575). For a conservative

design, the maximum junction temperature

should not exceed 110

°C to assure safe

operation. For every additional 10

°C

temperature rise that the junction must

withstand, the estimated operating lifetime of

the component is halved.

4. R

θJC

package thermal resistance junction–case.

5. R

θJA

package thermal resistance junction–ambient.

(Refer to Absolute Maximum Ratings in this data sheet or

θJC and RθJA values).

The following formula is to calculate the total power

dissipated by the LM2575:

PD = (Vin x IQ) + d x ILoad x Vsat

where d is the duty cycle and for buck converter

ton

(quiescent current) and Vsat can be found in the

LM2575 data sheet,

Vin

is minimum input voltage applied,

is the regulator output voltage,

ILoad is the load current.

The dynamic switching losses during turn–on and turn–off

can be neglected if proper type catch diode is used.

Packages Not on a Heatsink (Free–Standing)

For a free–standing application when no heatsink is used,

the junction temperature can be determined by the following

expression:

TJ = (R

θJA) (PD) + TA

where (R

θJA)(PD) represents the junction temperature rise

caused by the dissipated power and TA is the maximum

ambient temperature.

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