Voltage dropout

The back-EMF appears when the current flowing an inductive load is turns off and it influence to the supplying voltage of the system, finally the supply voltage dropout causes unpredictable operating of the system. The back-EMF can be significantly reduced by connecting a diode such as fast recovery or Schottky diode in parallel with the air pump/motor coil and solenoid valve coil. 

For the cost down the air pressure in cuff is measured indirectly without using expensive air pressure sensor in this development. A current-sensing resistor is connected in series with the air pump/motor assembly drive circuit. The voltage dropout across this resistor is proportional to the driving current flow of the air pump/motor assembly. 

Figure 2-2 The pass unit s influence on the dropout voltage (a) NPN pass unit (b) PNP pass unit (low dropout).

This drive loss ean beeome signifieant. A driver transistor ean be added to the pass transistor to inerease the effeetive gain of the pass unit and thus deerease the drive eurrent, or a power MOSFET ean be used as a pass unit that uses magnitudes less de drive eurrent than the bipolar power transistor. Unfortunately, the MOSFET requires up to 10 VDC to drive the gate. This ean drasti-eally inerease the dropout voltage. In the vast majority of linear regulator applieations, there is little differenee in operation between a buffered pass unit and a MOSFET insofar as effieieney is eoneerned. Bipolar transistors are mueh less expensive than power MOSFET and have less propensity to oseillate. 

Blog Entry 4 The best way to do this is to put in a low dropout linear regulator on the output. Set the Boost regulator output for 12.5-12.7V, and then use an LDO to drop your voltage to 12 V. 

In some applications (e.g. battery powered portable electronic equipment), we may want the output rail to remain well-regulated even if the input voltage dips very low — say down to within 0.6 V or less of the set output level Vo- In such cases, the minimum possible headroom (or dropout ) achievable by the linear regulator stage may become an issue. 

Determine which type of voltage regulator is best suited for the appHcation of interest, based on the descriptions given previously. For example, negative, variable output, low dropout voltage, etc. 

Dropout Tolti The input-output voltage differential at which the circuit ceases to regulate against further reductions in input voltage. 

Low dropout voltage Refers to a class of voltage regulators designed to operate at especially low input-output voltage differentials. 

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