LM2585 datasheet(21/25 Pages) NSC

21 / 25 page

Application Hints (Continued)

FLYBACK REGULATOR INPUT CAPACITORS

A flyback regulator draws discontinuous pulses of current

from the input supply. Therefore, there are two input capaci-

tors needed in a flyback regulator; one for energy storage

and one for filtering (see

Figure 39). Both are required due to

the inherent operation of a flyback regulator. To keep a

stable or constant voltage supply to the LM2585, a storage

capacitor (

≥100 µF) is required. If the input source is a recti-

fied DC supply and/or the application has a wide tempera-

ture range, the required rms current rating of the capacitor

might be very large. This means a larger value of capaci-

tance or a higher voltage rating will be needed of the input

capacitor. The storage capacitor will also attenuate noise

which may interfere with other circuits connected to the

same input supply voltage.

In addition, a small bypass capacitor is required due to the

noise generated by the input current pulses. To eliminate the

noise, insert a 1.0 µF ceramic capacitor between V

IN and

ground as close as possible to the device.

SWITCH VOLTAGE LIMITS

In a flyback regulator, the maximum steady-state voltage ap-

pearing at the switch, when it is off, is set by the transformer

turns ratio, N, the output voltage, V

OUT, and the maximum in-

put voltage, V

IN (Max):

SW(OFF) = VIN (Max) + (VOUT +VF)/N

where V

F is the forward biased voltage of the output diode,

and is 0.5V for Schottky diodes and 0.8V for ultra-fast recov-

ery diodes (typically). In certain circuits, there exists a volt-

age spike, V

LL, superimposed on top of the steady-state volt-

age (see

Figure 5, waveform A). Usually, this voltage spike is

caused by the transformer leakage inductance and/or the

output rectifier recovery time. To “clamp” the voltage at the

switch from exceeding its maximum value, a transient sup-

pressor in series with a diode is inserted across the trans-

former primary (as shown in the circuit on the front page and

other flyback regulator circuits throughout the datasheet).

The schematic in

Figure 39 shows another method of clamp-

ing the switch voltage. A single voltage transient suppressor

(the SA51A) is inserted at the switch pin. This method

clamps the total voltage across the switch, not just the volt-

age across the primary.

If poor circuit layout techniques are used (see the “Circuit

Layout Guideline” section), negative voltage transients may

appear on the Switch pin (pin 4). Applying a negative voltage

(with respect to the IC’s ground) to any monolithic IC pin

causes erratic and unpredictable operation of that IC. This

holds true for the LM2585 IC as well. When used in a flyback

regulator, the voltage at the Switch pin (pin 4) can go nega-

tive when the switch turns on. The “ringing” voltage at the

switch pin is caused by the output diode capacitance and the

transformer leakage inductance forming a resonant circuit at

the secondary(ies). The resonant circuit generates the “ring-

ing” voltage, which gets reflected back through the trans-

former to the switch pin. There are two common methods to

avoid this problem. One is to add an RC snubber around the

output rectifier(s), as in

Figure 39. The values of the resistor

and the capacitor must be chosen so that the voltage at the

Switch pin does not drop below −0.4V. The resistor may

range in value between 10

Ω and1kΩ, and the capacitor will

vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly)

reduce the efficiency of the overall circuit.

The other method to reduce or eliminate the “ringing” is to in-

sert a Schottky diode clamp between pins 4 and 3 (ground),

also shown in

Figure 39. This prevents the voltage at pin 4

from dropping below −0.4V. The reverse voltage rating of the

diode must be greater than the switch off voltage.

OUTPUT VOLTAGE LIMITATIONS

The maximum output voltage of a boost regulator is the

maximum switch voltage minus a diode drop. In a flyback

regulator, the maximum output voltage is determined by the

turns ratio, N, and the duty cycle, D, by the equation:

OUT ≈ NxVIN xD/(1−D)

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 LM2585 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 LM2585 if the input voltage has an unusual large amount

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

The circuit in

Figure 40 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.

DS012515-57

FIGURE 40. Input Line Filter

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