Transistor latch improves on/off circuitry

Figure 1 shows an example of on/off circuitry commonly used in battery-operated devices. The P-channel MOSFET, Q_{1, serves as a power switch. When you push the on button, S1, Q1 ' s gate goes low. Q1 turns on and supplies battery voltage to the DC-to converter. Depending on the battery voltage in the device, the DC-to converter might convert the voltage either up or down. In either case, it supplies VCC to Theµc. Theµc goes through its power-up software sequence and programs one of its general-purpose I/O pins, setting it to logic O Ne. This operation, in turn, causes saturation of the NPN transistor, Q2, which "confirms" the power-up state. Later, when Theµc decides to power itself off, Theµc simply sets its I/O output to logic zero, and Q 1returns to its Off state. The circuit are simple and reliable but have a significant disadvantage. It usually takes a fraction of a second for the DC + converter to reach its stable output voltage. Then, Theµc's Reset pulse usually lasts to msec. After the release of Reset, Theµc must go through its "housekeeping" start-up code before it had a chance to set its I/O Pin to logic one. This delay in some portable systems is user-unfriendly, because if you don ' t depress the button long enough, the SY Stem won't power up. The circuit in Figure 2 eliminates this uncertainty.}

The circuit includes a simple two-transistor latch, which the "on" button switches to the "on" state. As in Figure 1, the P-channel MOSFET, Q_{1, serves as a power switch. When you push the on button, S1, it causes saturation of the NPN transistor, Q4, via the base-current-limiting resistor, R5. The collector current of Q4 flows through r1 and the Base-emitter junction of PnP transistor q3, thereby saturating q3. Q3 redirects some current into the base-emitter junction of Q 4 and finishes the latching process. At this point, both Q3 and q4 are saturated, and the Voltage on the gate of Q1 is a function of the voltage drop across the Base-emitter juncti On Q3 and the saturation voltage of q4. This voltage is approximately 0.9V. Theµc need not confirm the in state of the latch. When theµc powers up and finishes its housekeeping start-up code, it programs the I/O pin to logic zero.}

Later, when Theµc decides to power itself off, it programs the I/O pin to logic one and stops. Q_{2 Turns off Q4, resetting the latch to their initial off state. R4 lowers the equivalent input impedance of Q3. This function improves EMI and ESD noise immunity and prevents the circuit from turning itself on in the presence O F Strong electromagnetic fields. Capacitor C1 in combination with R5 protects Q4 and Q2 from direct ESD into the pushbutton. Some Portable devices use undervoltage-lockout circuitry. This circuitry usually uses a voltage comparator with a built-in voltage reference. If the battery voltage drops below the threshold, the output of the comparator (usually an open-drain type) switches low. If your portable system uses this type of circuitry, you can connect the Open-drain output of the comparator in parallel w ITH Q2, thus preventing the latch from turning on if the battery voltage are too low.}

Transistor latch improves on/off circuitry