LM2587 datasheet(23/26 Pages) NSC

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Application Hints (Continued)

The duty cycle of a flyback regulator is determined by the fol-

lowing equation:

Theoretically, the maximum output voltage can be as large

as desired — just keep increasing the turns ratio of the trans-

former. However, there exists some physical limitations that

prevent the turns ratio, and thus the output voltage, from in-

creasing to infinity. The physical limitations are capacitances

and inductances in the LM2587 switch, the output diode(s),

and the transformer — such as reverse recovery time of the

output diode (mentioned above).

NOISY INPUT LINE CONDITION)

A small, low-pass RC filter should be used at the input pin of

the LM2587 if the input voltage has an unusual large amount

of transient noise, such as with an input switch that bounces.

The circuit in

Figure 43 demonstrates the layout of the filter,

with the capacitor placed from the input pin to ground and

the resistor placed between the input supply and the input

pin. Note that the values of R

IN and CIN shown in the sche-

matic are good enough for most applications, but some read-

justing might be required for a particular application. If effi-

ciency is a major concern, replace the resistor with a small

inductor (say 10 µH and rated at 100 mA).

STABILITY

All current-mode controlled regulators can suffer from an in-

stability, known as subharmonic oscillation, if they operate

with a duty cycle above 50%. To eliminate subharmonic os-

cillations, a minimum value of inductance is required to en-

sure stability for all boost and flyback regulators. The mini-

mum inductance is given by:

where V

SAT is the switch saturation voltage and can be

found in the Characteristic Curves.

CIRCUIT LAYOUT GUIDELINES

As in any switching regulator, layout is very important. Rap-

idly switching currents associated with wiring inductance

generate voltage transients which can cause problems. For

minimal inductance and ground loops, keep the length of the

leads and traces as short as possible. Use single point

grounding or ground plane construction for best results.

Separate the signal grounds from the power grounds (as in-

dicated in

Figure 44). When using the Adjustable version,

physically locate the programming resistors as near the

regulator IC as possible, to keep the sensitive feedback wir-

ing short.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, no heat sink is required to keep the LM2587

junction temperature within the allowed operating range. For

each application, to determine whether or not a heat sink will

be required, the following must be identified:

1) Maximum ambient temperature (in the application).

2) Maximum regulator power dissipation (in the application).

3) Maximum allowed junction temperature (125˚C for the

LM2587). For a safe, conservative design, a temperature ap-

proximately 15˚C cooler than the maximum junction tem-

perature should be selected (110˚C).

4) LM2587 package thermal resistances

JA and θJC (given

in the Electrical Characteristics).

Total power dissipated (P

D) by the LM2587 can be estimated

as follows:

Boost:

IN is the minimum input voltage, VOUT is the output voltage,

N is the transformer turns ratio, D is the duty cycle, and I

LOAD

is the maximum load current (and

∑I

LOAD is the sum of the

maximum load currents for multiple-output flyback regula-

tors). The duty cycle is given by:

DS012316-29

FIGURE 44. Circuit Board Layout

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