Datasheet

LM2588

SNVS117D –APRIL 1998–REVISED APRIL 2013

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where V

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

, superimposed on top of the

steady-state voltage (see Figure 21, 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 suppressor in series with a diode is inserted across the transformer primary (as

shown in the circuit in Figure 20 and other flyback regulator circuits throughout the datasheet). The schematic in

Figure 58 shows another method of clamping 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 voltage

across the primary.

If poor circuit layout techniques are used (see the CIRCUIT LAYOUT GUIDELINES 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 negative 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 “ringing”

voltage, which gets reflected back through the transformer 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 58. 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 10Ω and 1 kΩ, 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 insert a Schottky diode clamp between pins 5 and 4

(ground), also shown in Figure 58. 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.

Figure 59. Input Line Filter

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

≈ N × V

× D/(1 − D) (4)

The duty cycle of a flyback regulator is determined by the following equation:

(5)

Theoretically, the maximum output voltage can be as large as desired—just keep increasing the turns ratio of the

transformer. However, there exists some physical limitations that prevent the turns ratio, and thus the output

voltage, from increasing 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).

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