The Power Integrations Topswitch family of integrated flyback-regulator ICs comes in small, low-pin-count packages. For the lowest-pin-count packages, the multi-function, or M, pin serves multiple purposes, including on/off control and under-voltage- and over-voltage-input detection. Other package types include an L pin, which also provides this function. The application notes and data sheets show how to implement the various features available at these pins. For example, to allow remote on/off control and still preserve under-voltage and over-voltage functions, the application drawings show an NPN transistor, QR, which connects between the M or L pin and the Control pin (figure 1). To turn off the regulator, QR must be biased on. To achieve this goal requires a base voltage of 2.6V dc or greater.

EDNAOL 2016MAY31 AN 02Fig1Figure 1: Adding transistor QR to the L pin of a Topswitch switching power controller enables an on/off-control feature.

The circuit in figure 2 provides a new feature that allows you to switch the regulator on or off using a grounded switch that is sometimes more convenient to implement than a switch that references to the Control pin. In the case of a mechanical switch, this circuit would require no external power to implement this function. This feature is important in applications in which the Topswitch power supply is the only source of power. This circuit does not disturb the functioning of the under-voltage and over-voltage functions of the M or L pin. To understand the functioning of the circuit in figure 2 requires an explanation of the internal workings of the M or L pin. This pin acts as a constant voltage source at approximately 2V dc and sinks current from the external circuit, which RLS supplies. The internal current-sense thresholds for under-voltage and over-voltage detection are roughly 50µA with 30µA of hysteresis for under-voltage and 225µA for over-voltage. That is, when the current into the M or L pin is less than 20µA, or 50–30µA, the regulator output switches off because of under-voltage. When the current into the M or L pin exceeds 225µA, the regulator output switches off because of over-voltage. When the current into the M or L pin is 50 to 225µA, the output is enabled.

EDNAOL 2016MAY31 AN 02Fig2Figure 2: Instead of using just the external transistor to switch a Topswitch on or off, a simple on/off switch provides manual on/off control.

The circuit of figure 2 works as follows: When the switch in the collector lead of Q1 is open, Q1 functions as a simple diode with a 0.6V drop from emitter to base. All the current that RLS supplies flows into the M or L pin through the base-emitter junction of Q1 and the 150-kΩ resistor. In this mode, the Topswitch IC senses the under-voltage and over-voltage thresholds. However, when the switch to ground closes, Q1 functions as a nonsaturated transistor with high gain. The circuit siphons off most of the current through RLS to ground as the collector current of Q1. Only a small base current from Q1 plus 4µA through the 150-kΩ resistor flows into the M or L pin. For the values in figure 2, this base current is less than 3.8µA, even when Q1 has minimum gain and input voltage is at a maximum of 450V dc. Therefore, 3.8+4µA, or 7.8µA, flows into the M or L pin. This low current flowing into the pin "fools" the regulator into "thinking" that the input voltage is under-voltage, and the regulator output switches off. If another voltage or current source is present, you could replace S1 with an open-collector switch that sinks current only. If the remote on/off driver can source and sink current, as the output of a logic gate can, then you should insert a diode in the collector lead of Q1, and the driver must drive the cathode of that diode above 2V dc to turn off the regulator (optional in figure 2). The M pin also allows current-limit-threshold adjustment.
This article is a Design Idea selected for re-publication by the editors. It was first published on June 21, 2007 in EDN.com.