Low-drop regulator 1: If a stable 5-V direct voltage is to be derived from a low supply voltage, the 4805 from SGS Thomson is probably the most suitable regulator IC available at present. The popular 7805 does not work too well with supplies lower than about 8 V. The 4805, on the other hand, needs an input voltage that is only about 0.5 V higher than its output voltage. Its data sheets state that its output voltage remains stable so long as the input voltage does not drop below 5.7 V. That. voltage, by the way, is the worst-case voltage when the output current is 400 mA. In practice, therefore, the regulator; normally works fine with input voltages as low as 5.4 V.
What to do if the output current is more than 400 mA? Well. in that case, and for output currents up to 1 A. the LM2940T • appears to be just about the best available. This IC from National Semiconductor is available in three variations: 5 V, 8 V, and 10 V. The 5-V version, in which we are principally ‘interested here, is type-coded LM2940T-5. These 1Cs are pin-compatible with the 7805 and 4805, which makes it possible for an existing low-drop supply based on one of these devices to be upgraded fairly simply.
For completeness’ sake. the circuit diagram shows a simple 5-‘ design. The only really important parameter here is the capacitance of decoupling capacitor C4. According to the relevant data sheets, its value should be not lower than 22 pF to ensure correct stability.
If the input voltage does not rise above about 7.5 V, a heat sink is not necessary. If the regulator is required to operate at a higher output voltage and the full output current, a suitable heat sink is imperative. A heat sink rated at 6.5 °C W-1 makes the low-drop circuit suitable for input voltages up to 15 V at full output current or up to 25 V at 500 mA. The worst case input voltage is 5.8 V.
The LM2940T, like the 7805, is short-circuit-proof, but it does draw a rather high quiescent current as shown in the characteristic curves.
