Abstract:
Methods and apparatus for the soft start of linear regulators for controlling inrush current. In linear regulators having a pass transistor controlled by a regulator control circuit, the regulator control circuit is disabled until the regulator output reaches a predetermined threshold level. On startup, an additional transistor is coupled with a resistor and capacitor to the control terminal of the pass transistor in such a way as to provide for the slow turn-on of the pass transistor. During this time, the control circuit for the pass transistor is held inoperative. After the regulator output reaches a predetermined threshold, the pass transistor control circuit becomes operative and the slow start circuitry becomes inoperative.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of linear regulators. 
     2. Prior Art 
     Linear voltage regulators are well known in the prior art, being commonly used to receive an unregulated input voltage and to provide a regulated output voltage somewhat lower than the input voltage. Such regulators comprise a pass transistor coupled between the input to the regulator and the output of the regulator, and a pass transistor control circuit controlling the control terminal of the pass transistor based upon the comparison of a reference voltage with the output voltage of the regulator, typically as divided down by a resistor divider. 
     The foregoing types of linear regulators work well and are widely used. Such regulators are widely commercially available in integrated circuit form, the lower power regulators including the pass transistor as part of the integrated circuit and the higher power regulators using an external discrete pass transistor. However, unless some provision is made for the soft start of such regulators, the pass transistor will be turned on hard when power is first applied to the regulator, drawing an initial high current spike from the power supply. In that regard, substantial energy may be initially required at the output of the regulator if there is a substantial capacitive load thereon, whether because of the circuitry being driven by the regulator, or merely the presence of the typical smoothing capacitor normally provided on the output of the regulator. Substantial energy also may be initially required at the output of the regulator due to nonlinear loads where the nonlinearity is a function of voltage. 
     To limit the inrush current on turn-on, a resistor is commonly coupled in series with the regulator circuit, with the pass transistor control circuit sensing the voltage drop across the resistor and controlling the control terminal of the pass transistor to limit that current to a predetermined maximum value. That maximum current, of course, must be higher than the maximum expected load on the regulator in normal operation. Accordingly, when a system using such regulators is first turned on, there will be a momentary load on the power supply exceeding the largest load expected during normal operation of the system, caused by the simultaneous extraordinary inrush currents of all the circuits in the system. Further, the system itself may have various circuits, not all of which could operate at their maximum power requirements at the same time. Accordingly, the maximum normal operating power requirements from the power supply may be much less than the momentary power requirement on first turn-on of the system. Consequently, the inrush current requirements of a system can often determine the minimum power supply size, weight and cost, even though the normal operation of the system would only require a smaller, lighter and less costly supply. 
     In addition, in many systems it is desired to be able to replace a printed circuit board without shutting off power to the system, typically referred to as “hot swapping.” In computer systems, hot swapping will allow the replacement of a board or the addition of a new board without loss of information in volatile memory, without requiring rebooting the system, etc. In systems such as communication systems and the like, wherein a plurality of boards of similar function are plugged into a motherboard, boards may be hot swapped or additional boards added without shutting down the system. This allows maintenance and upgrading without interfering with communications or other functions in channels serviced by the remaining boards in the system. In hot swapping applications, however, unless inrush currents are adequately limited, the addition of a board to a system in operation can cause a momentary power glitch which may disturb other circuits in the system. 
     A similar effect is encountered when a circuit is shut down. In this case, when an existing electrical load of the circuit is suddenly removed, an over-voltage condition may be imposed on the other circuits in the system, causing a temporary or permanent disruption in their operation. Also, if the load on a switching power supply if drastically reduced, the switching power supply may latch in a shutdown condition. 
     BRIEF SUMMARY OF THE INVENTION 
     Methods and apparatus for the soft start and/or soft turnoff of the pass transistor of linear regulators for controlling inrush current are described. On startup, an additional transistor is coupled with a resistor and capacitor to the control terminal of the pass transistor in such a way as to provide for the slow turn-on of the pass transistor. During this time, the control circuit for the pass transistor is held inoperative. After the regulator output reaches a predetermined threshold, the pass transistor control circuit becomes operative and the slow start circuitry becomes inoperative. On shutdown, the reverse process occurs, providing a slow turn-off of the pass transistor. While exemplary embodiments using P-MOS transistors for regulating the positive power supply connection is disclosed, the circuit may be readily converted for use in regulating a negative power supply connection. Also, other transistor types may be used, such as bipolar junction transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram for an exemplary embodiment of the present invention having a soft turn-on capability. 
     FIG. 2 is a circuit diagram for an alternate embodiment of the present invention also having a soft turn-on capability. 
     FIG. 3 is a circuit diagram for a still further alternate embodiment of the present invention having a soft turn-on and a soft turn-off capability. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now referring to FIG. 1, an exemplary embodiment of the present invention having a soft start capability may be seen. As shown therein, a pass transistor controller U 1  receives a feedback signal VFB equal to the output voltage V OUT  divided down by a resistor divider comprised of resistors R 2  and R 3 . This feedback voltage is compared with the reference voltage, in the embodiment shown in FIG. 1 generated within the pass transistor controller U 1 , with the pass transistor controller controlling the gate of transistor Q 1  to maintain a match between the feedback voltage VFB and the reference voltage, internally generated or otherwise. In the specific embodiment shown in FIG. 1, the pass transistor Q 1  is a PMOS device coupled between the input VIN and the output V OUT , the ground terminal being common to both the input and output circuits. It should be noted, however, that in other embodiments, other types of transistors may be used, such as junction transistors. Alternatively the pass device may be on the negative side of the circuit, so that the higher voltage power supply terminal is common between the regulator input and output, with the pass transistor being in the lower voltage connection between the input and output. By way of a more specific example, the regulator may be used to regulate a negative voltage relative to ground, wherein typically an NMOS device or an npn transistor would be used. 
     In addition, resistor R 1  is coupled between the input V IN  and the source of transistor Q 1 , with the voltage across resistor R 1  providing the current sense signal I SENSE  to the pass transistor controller U 1  to allow the pass transistor controller to limit the maximum current drawn by the regulator. The pass transistor controller, the current sense resistor R 1  and pass transistor, such as transistor Q 1 , are generally found in linear regulators, either as part of a single integrated regulator circuit for low power applications, or alternatively, the pass transistor controller may be an integrated circuit, with the sense resistor and pass transistor being discrete components. 
     In the specific exemplary embodiment shown in FIG. 1, the pass transistor controller U 1  is an integrated circuit controller manufactured by National Semiconductor. This controller, as is typical of many integrated circuit controllers for controlling a discrete pass transistor, has an I SENSE  input for sensing the current (voltage drop) across a current sensing resistor (R 1  in FIG. 1) and a feedback voltage input VFB for receiving a feed back of a fraction of the output voltage V OUT  determined by user selected resistors R 2  and R 3  and for providing a gate control signal GATE for controlling the gate of the pass transistor (transistor Q 1  in FIG. 1) responsive thereto. The integrated circuit pass transistor controller, as is also typical of such integrated circuit controllers, includes an on/{overscore (off)} or {overscore (standby)} input signal, making the controller active when the on/{overscore (off)} or {overscore (standby)} signal is high, and disabling the gate control output of the controller when the signal is low. The integrated circuit controller used with the exemplary embodiment has a gate control circuit controlling the gate control signal GATE which, when inactive, provides a 500K pull-up resistor within the integrated circuit controller to pull the gate voltage of transistor Q 1  high to hold the transistor off. 
     In the exemplary embodiment of the present invention shown in FIG. 1, resistor R 4 , capacitor C 1 , transistor Q 1  and comparator U 2  have been added to provide the desired soft start. With the addition of these components, the operation of the circuit may be described as follows. When the input voltage V IN  is off, the voltages at the various nodes of the circuit will generally be at ground potential. Then, immediately after the circuit is turned on, the output voltage V OUT  will initially again be at ground potential, with comparator U 2  comparing the output voltage V OUT  with the threshold voltage V TH , providing a low comparator output to hold the pass transistor controller U 1  inactive. The source of transistor Q 2 , connected to the current sense resistor R 1 , will follow the input voltage V IN . The gate of transistor Q 2 , connected to the output voltage V OUT , will of course also initially be at ground potential, turning on transistor Q 2  to couple the input voltage V IN  through resistor R 1  and transistor Q 2  to capacitor C 1 . This initially drives node  1  high, holding the gate of transistor Q 1  high in cooperation with the pull-up resistor within the integrated circuit pass transistor controller U 1 . However, the capacitor C 1  will now begin to charge with an RC time constant determined by resistor R 4 , capacitor C 1  and the pull-up resistor within the integrated circuit pass transistor controller, which may or may not be large in comparison to the resistor R 4 . As capacitor C 1  charges, the voltage on node  1  decreases, slowly turning on transistor Q 1 , causing the output voltage V OUT  to increase at a rate responsive to the load thereon, the RC time constant of resistor R 4 , capacitor C 1 , the internal resistor of the controller and the turn-on characteristics of transistor Q 1 . 
     When the output voltage V OUT  reaches the threshold voltage V TH  on the negative terminal of comparator U 2 , the output of the comparator will go high, activating the pass transistor controller U 1 . Now the pass transistor controller takes over, driving the gate control signal GATE controlling the gate of transistor Q 1  to bring the regulator into regulation. As the output voltage V OUT  further increases toward regulation, transistor Q 2  turns off because of the decreasing source-gate voltage on the transistor, so that the integrated circuit pass transistor controller may have full control of the gate control signal GATE for transistor Q 1  unaffected by the capacitor C 1 . The output impedance of the circuit driving the gate control signal GATE when the integrated circuit pass transistor is active is low compared to the resistance of resistor R 4 . This allows the controller to control the gate of transistor Q 1  substantially independent of the presence of the additional resistor R 4 . 
     If the input voltage V IN  is now turned off, both the input voltage V IN  and the output voltage V OUT  will drop, so that transistor Q 2  may remain off during the shutdown of the circuit. Capacitor C 1 , being charged to a substantial voltage, will tend to retain that charge. However, transistor Q 2  is a PMOS transistor having its body connected to its source. As the circuit is shut off, the voltage on capacitor C 1  will forward bias the pn junction between the drain of transistor Q 2  and the body thereof, discharging capacitor C 1  through that pn junction. Therefore the voltage on capacitor C 1  which can be maintained with V IN  and V OUT  both at ground potential cannot exceed one forward bias pn junction voltage drop. Consequently, the circuit will reset itself for immediate functioning again in the event power (V IN ) is provided and momentarily lost, such as can occur when inserting a board into an already hot system. 
     Referring again to FIG. 1, it may be seen that capacitor C 1  is decoupled from the control of the gate of transistor Q 1  during normal operation of the regulator by the turn-off of transistor Q 2 . Accordingly, the threshold of transistor Q 2  should be chosen to be greater than the maximum difference between the input voltage V IN  and the output voltage V OUT . This normally is not a problem, as linear regulators are normally used in applications wherein the unregulated input voltage V IN  is some percentage range higher than the desired regulated output voltage V OUT , not a number of times the desired regulated output voltage V OUT . At the other extreme, the threshold of transistor Q 2  should be substantially less than the input voltage V IN  itself, to be sure that the transistor initially turns on, as desired, to pull node  1  high and allow the same to decrease in voltage at a controlled rate by the charge of capacitor C 1  through resistor R 4 . Alternatively, as shown in FIG. 2, the gate of transistor Q 2  may be coupled to the output V OUT  through a resistor divider R 5 ,R 6 , either fixed or adjustable such as a potentiometer, to provide a circuit adjustment for specific transistor thresholds or variations in transistor threshold. 
     In the circuit shown in FIG. 1, the threshold voltage V TH  used by comparator U 2  does not determine the accuracy of regulation of the regulator, but rather merely determines when the pass transistor control circuit will take control of the pass transistor. As such, the threshold voltage V TH  need not be a particularly accurate voltage and could be generated various ways, such as, by way of example, with a resistor and zener diode connected to VIN, or even a resistor divider connected to VIN, provided the various circuit and operating parameters assure that the pass transistor control circuit will take control before the output voltage V OUT  reaches or exceeds the regulated output voltage. 
     Now referring to FIG. 3, the exemplary circuit of FIG. 2 further including additional exemplary circuitry for accomplishing a soft shutdown may be seen. The soft shutdown presumes that power to the circuit is maintained during the shutdown, with the shutdown being controlled by a Shutdown signal applied to the base of transistor Q 4  through resistor R 9 . 
     In addition to transistor Q 4  and resistor R 9 , the exemplary soft shutdown circuitry includes p-channel transistor Q 3 , resistors R 7  and R 8  and capacitor C 2 . Before power to a printed circuit board containing the circuitry of FIG. 3 is applied, the shutdown signal will be low, so that transistor Q 4  will be off. Resistor R 7  will result in capacitor C 2  being discharged. When power to the printed circuit board is applied, generally the shutdown signal will be low, holding transistor Q 4  off. Capacitor C 2  and resistor R 7  will pull the gate of transistor Q 3  to the source voltage of transistor Q 3 , holding the transistor off. Consequently, the soft shutdown circuitry is inactive, and the soft start circuitry will operate as previously described. 
     When the Shutdown signal is driven high, transistor Q 4  will turn on. Now capacitor C 2  will start to charge through resistor R 8 . By proper selection of resistors R 7  and R 8 , the voltage on the gate of transistor Q 3  may be made to decrease sufficiently in comparison to the source of transistor Q 3  to turn on the transistor. This pulls the gate of p-channel transistor Q 1  to its source voltage, overriding (disabling) the pass transistor controller U 1  and turning off transistor Q 1  to shut down the circuitry connected to V OUT . The rate at which transistor Q 3  is turned on and thus the rate at which transistor Q 1  is turned off will depend on the time constant of the R-C circuit comprising capacitor C 2  and resistors R 7  and R 8 . Similarly, on removal of the shutdown signal (change of the signal to the opposite state), the rate at which transistor Q 3  is turned off and thus the rate at which transistor Q 1  is turned on to the regulating state will also depend on the time constant of the R-C circuit comprising capacitor C 2  and resistors R 7  and R 8 . 
     Even if a board using the circuit of FIG. 3 is plugged into a hot motherboard connector having the shutdown signal high, the circuitry will perform properly. In particular, the soft start circuitry will begin to operate, though when the soft shutdown circuitry begins to turn on transistor Q 3 , that transistor will begin to override (disable) any other drive provided through passive elements to the gate of transistor Q 1 , including the pass transistor controller U 1 , to force the soft shutdown regardless of the state of the soft startup. If desired, by selection of the relative parameters determining the characteristics of the soft startup and the soft shutdown, the shutdown could prevent any circuit startup from beginning to occur. 
     In use of the present invention, different boards in a system might use different time constants for resistor R 4  and capacitor C 1 , so that the inrush current when the entire system is turned on is further limited. Similarly, different boards in a system might use different time constants for the circuit of capacitor C 2  and resistors R 7  and R 8  to provide varied soft shutdown times. Also, while an exemplary embodiment using an integrated circuit controller for a discrete p-channel transistor for regulating the positive power supply connection is disclosed, the circuit may be readily converted for use in regulating a negative power supply connection using complementary transistors, or using a controller fabricated using discrete components. Also, other transistor types may be used, such as bipolar junction transistors. Thus, while certain preferred embodiments of the present invention have been disclosed and described herein, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.