The TL593 incorporates, on a single monolithic chip, all the functions required for a pulse-width modulation control circuit. The TL593 is similar to the TL594, from which it was derived, except that the TL593 includes a current-limit amplifier instead of a second error amplifier.
The current-limit amplifier of the TL593 has an offset voltage of approximately 80 mV in series with the inverting input (pin 15). This makes it easier to design the current-limit portion of the power supply and also requires fewer components. With 80mV on the inverting input, it is only necessary to apply an 80mV control voltage to the non-inverting input (pin 16). This is easily accomplished by taking the voltage across a resistor in series with the load.
The floppy disk power supply schematic is shown in Figure 6-9. The power supply uses a pair of TIP34 pnp transistors in a push-pull configuration. The oscillation frequency is set at 25 kHz and – 5 V at 500 mA by the .01µF capacitor on pin 5 and the 5 kΩ resistor on pin 6.
The center connection of the two 5.6 kΩ resistors on pins 13 and 14 establishes a 2.5 V reference voltage on pin 2, which is the inverting input of the voltage control error amplifier. The voltage feedback to pin 1, the noninverting input, comes from the center connection of the two 5.6 kΩ resistors located on the 5 V/2.5 A power supply output terminal. Because this voltage supplies the logic circuits, it requires closer regulation.
The 24V winding, on the other hand, is not critical as it furnishes voltage for the stepping motor. The – 5V supply is regulated separately with a uA7905 three- terminal regulator.
In choosing components for this circuit, the same precautions taken in the construction of any switching power supply should be observed; be careful of layout, ground loops, and heatsinking of the power transistors. In the output section, where high frequency rectifiers are needed, either Schottky or fast recovery diodes should be used. For output capacitors, low equivalent series
resistance (ESR) types should be considered. The output ripple depends more on this resistance than on the capacitor value.
The transformer for this circuit was wound on a toroid core. The core used was 3C8 ferrite material (F-42908-TC). The winding layout is shown in Figure 6-10.
Transformer Winding Data
- Primary A + B = 20 turns bifilar # 20 HNP
- Secondary C + 0 = 28 turns bifilar #20 HNP over A+B
- Secondary E + F = 6 turns bifilar #20 HNP over C+D
- Secondary G + H = 10 turns bifilar #26 HNP over E+F
NOTE: All windings to be center tapped.
- Winding 1 – 3 = 11Ω
- Winding 4 – 6 = 11Ω
- Winding 7 – 9 = 025Ω
- Winding 10 – 12 = 0.15Ω