Generating Symmetrical Power Supply with LM 2685
When it comes to small operational amplifier projects and analog circuits, having a symmetrical ±5 V power supply is crucial. An ideal solution for this requirement is the National Semiconductor LM 2685 IC. This component incorporates a switched capacitor voltage doubler, followed by a 5-V regulator. Additionally, the same IC features a voltage inverter that utilizes the switched-capacitor technique and operates from this output voltage.
Minimal External Components
The beauty of the LM 2685 lies in its simplicity. The external circuitry required is minimal, involving only two pump capacitors and three electrolytic storage capacitors. This streamlined setup makes it exceptionally convenient for various applications. The IC is designed to function within a broad input voltage range, spanning from +2.85 V to +6.5 V. This characteristic renders it highly suitable for battery-operated devices, providing efficient power management solutions for portable equipment.
Generating Dual Power Supplies with LM 2865
The process of generating dual power supplies begins with applying the input voltage to a voltage doubler operating at 130 kHz. The external capacitor, connected to pins 13 and 14, plays a crucial role in this operation. The output voltage from this doubler undergoes filtration through capacitor C3, linked to pin 12. If the input voltage falls within the range of +5.4 to +6.5 V, the voltage doubler deactivates and allows the input voltage to pass directly to the subsequent +5-V low-dropout regulator, capable of supplying up to 50 mA.
Capacitor C4 acts as the output filter capacitor in this configuration. To generate the -5 V output voltage, the +5 V voltage is inverted. This inversion is achieved through a clocked power-MOS circuit, which charges capacitor C2 (connected between pins 8 and 9) before reversing its polarity. The resulting chopped voltage requires filtration by C5 at the output. The unregulated -5 V output can provide a maximum current of 15 mA.
Control and Shutdown Features of LM 2865
The LM 2865 voltage converter IC is equipped with a chip-enable input (CE) and two control inputs, SDP (shut down positive) and SDN (shut down negative). When CE is set to Low, the entire IC is switched off (shut down), reducing its current consumption to approximately 6 µA. This feature allows convenient on/off switching of the connected circuit without the need to disconnect the battery. The SDP and SDN inputs control the VPSW and VNSW outputs, respectively, via low-resistance CMOS switches.
These switches enable independent shutdown of the negative output, leading to the deactivation of the voltage inverter. Switching off with SDP not only opens the output switch but also halts the oscillator. Consequently, there is no longer any input voltage for the -5 V inverter, causing the -5 V output to cease. For normal operation, the SDP and SDN inputs are set Low (< 0.8 V), while a High state (>2.4 V) is used to deactivate the associated voltage(s).
The positive output of the LM 2865 is short-circuited proof. However, a short circuit between the positive and negative outputs must always be avoided. The IC is protected against thermal destruction by an overtemperature monitor.
It switches off automatically at a chip temperature of around 150 oC. The full type number of the IC is LM 2685MTC. It comes in a TSSOP14 SMD package. National Semiconductor can be found on the Internet under www.national.com.