LM2585 datasheet(21/32 Pages) TI

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LM2585

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SNVS120F – APRIL 2000 – REVISED APRIL 2013

In a flyback regulator application (Figure 53), using the standard transformers, the LM2585 will survive a short

circuit to the main output. When the output voltage drops to 80% of its nominal value, the frequency will drop to

25 kHz. With a lower frequency, off times are larger. With the longer off times, the transformer can release all of

its stored energy before the switch turns back on. Hence, the switch turns on initially with zero current at its

collector. In this condition, the switch current limit will limit the peak current, saving the device.

Figure 53. Flyback Regulator

FLYBACK REGULATOR INPUT CAPACITORS

A flyback regulator draws discontinuous pulses of current from the input supply. Therefore, there are two input

capacitors needed in a flyback regulator; one for energy storage and one for filtering (Figure 53). 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 rectified DC supply and/or the

application has a wide temperature range, the required rms current rating of the capacitor might be very large.

This means a larger value of capacitance 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 VIN and ground as close as possible to the device.

SWITCH VOLTAGE LIMITS

In a flyback regulator, the maximum steady-state voltage appearing at the switch, when it is off, is set by the

transformer turns ratio, N, the output voltage, VOUT, and the maximum input voltage, VIN (Max):

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

where

•

VF is the forward biased voltage of the output diode and is 0.5V for Schottky diodes and 0.8V for ultra-fast

recovery diodes (typically).

(3)

In certain circuits, there exists a voltage spike, VLL, superimposed on top of the steady-state voltage (Figure 22,

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 on the front

page and other flyback regulator circuits throughout the datasheet). The schematic in Figure 53 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.

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