LM2588 datasheet(25/29 Pages) NSC

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Application Hints (Continued)

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 typically 0.5V for Schottky diodes and 0.8V for

ultra-fast recovery diodes. In certain circuits, there exists a

voltage spike, V

LL, superimposed on top of the steady-state

voltage (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 volt-

age at the switch from exceeding its maximum value, a tran-

sient suppressor in series with a diode is inserted across the

transformer primary (as shown in the circuit in

Figure 4 and

other flyback regulator circuits throughout the datasheet).

The schematic in

Figure 46 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 5). 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 LM2588 IC as well. When used in a flyback

regulator, the voltage at the Switch pin (pin 5) 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 46. 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 5 and 4 (ground),

also shown in

Figure 46. This prevents the voltage at pin 5

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 LM2588 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 LM2588 if the input voltage has an unusually large

amount of transient noise, such as with an input switch that

bounces. The circuit in

Figure 47 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 schematic are good enough for most applications, but

some readjusting might be required for a particular applica-

tion. If efficiency is a major concern, replace the resistor with

a small inductor (say 10 µH and rated at 200 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.

DS012420-58

FIGURE 47. Input Line Filter

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