Abstract:
A power supply includes a power source having first and second terminals. A circuit is coupled to the source and is operable to maintain a first quantity between the terminals within a predetermined range of values until a second quantity between the terminals has a predetermined value. Such a power supply provides the ability to delay activating a load until the current that the supply can provide is at a level acceptable for proper load function.

Description:
GOVERNMENT LICENSE RIGHTS 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DAAH01-03-C-0010 awarded by US Army. 

   BACKGROUND 
   Thermal batteries are often used to power the circuitry in certain devices, such as missiles. Specifically, after activation, the battery powers, e.g., the electronic control circuitry and the motors that steer the missile. 
   A thermal battery is activated by igniting an internal portion of the battery. Upon ignition, the battery commences current and voltage production. 
   But unfortunately, upon activation, a thermal battery has a high equivalent series resistance (ESR)—typically on the order of tens of giga-ohms—which reduces the voltage level that the battery is able to generate across a load. Although the ESR reduces to a suitable value within a time typically on the order of ¼ to ¾ seconds, if the circuitry activates before the ESR is low enough—typically less than one ohm—and, thus, the output voltage high enough, the circuitry may initialize in an undesirable state or may otherwise malfunction. And if the circuitry malfunctions, it may cause a malfunction in the device, e.g., a missile, that incorporates the circuitry. 
     FIG. 1  is a schematic block diagram of a conventional device  10 , which is a vehicle, such as a missile, having at least one load, such as a motor  20  and electronic circuitry  30 , and a power source  35 , including a thermal battery  40  with associated ESR  50 . The motor  20  and electronic circuitry  30  are coupled to and receive a supply voltage Vs from the battery  40  via conductors  60  and  62 . The assembly for igniting the battery is omitted for clarity. 
   Typically, the electronic circuitry  30  operates in a reset mode when the supply voltage Vs is between a minimum operational level and a reset level, e.g., 0.5 Volts (V), and is fully operational when Vs is greater than the reset level. But if while the circuitry  30  is fully operational Vs falls below the reset level, then the circuitry re-enters the reset mode. Unfortunately, the circuitry  30  re-entering the reset mode may delay the start-up time for the missile  10 , or may cause the missile to malfunction. 
   More specifically, upon activation at missile-launch time, the battery  40  begins providing the supply voltage Vs to the motor  20  and electronic circuitry  30 , which typically requires minimal current (on the order of a few milliamps) to reset itself, exit the reset mode, and perform, for example, pre-launch system checks. Consequently, because the circuitry  30  presents a relatively small load to the battery  40 , Vs typically exceeds the circuitry&#39;s reset level relatively quickly, thus allowing the circuitry to become fully operational and perform the pre-launch routine within a few milliseconds after the battery  40  is activated. However, the motor  20 , when operating, draws a relatively large amount of current on the order of 10 Amps, and thus presents a relatively large load to the battery  40 . Therefore, if the circuitry  30  activates the motor  20  before the ESR  50  has fallen to a suitably small value, then the load presented by the motor  20  may cause Vs to fall below the reset level of the circuitry  30 , which, as discussed above, causes the circuitry  30  to re-enters its reset mode. Unfortunately, the circuitry  30  re-entering its reset mode may undesirably delay or abort the launch of the missile  10 . 
   SUMMARY 
   In an embodiment of the invention, a power supply includes a power source having first and second terminals. A circuit is coupled to the source and is operable to maintain a first quantity between the terminals within a predetermined range of values until a second quantity between the terminals has a predetermined value. 
   Such a power supply provides the ability to delay activating a load until the current that the supply can provide is at a level acceptable for proper load function. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of a conventional device; and 
       FIG. 2  is a schematic view of a device according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a schematic block diagram of a device  100 , such as a missile, which, according to an embodiment of the invention, includes a motor  110 , electronic control circuitry  120 , and a power supply  130  that includes a power source  140 , such as a thermal battery with associated ESR  150 , and a supply-activation circuit  160  such as a fuse. The motor  110  and electronic control circuitry  120  are coupled to and receive a supply voltage Vs from the power supply  130  via conductors  170  and  172 . 
   The supply-activation circuit  160  prohibits Vs from increasing above the reset voltage level of the control circuitry  120  until the ESR  150  reduces to a value ESRdesired that is low enough to allow the supply  130  to maintain Vs above the reset level while powering the control circuitry, the motor  110 , and any other load coupled to Vs. 
   According to Kirchov&#39;s voltage and current laws:
 
 Vs=Vi×R 160/( ESR+R 160)  (1)
 
 I=Vi /( ESR+R 160)  (2)
 
   where Vi is the internal voltage of the battery  140  and R 160  is the equivalent resistance of the circuit  160  and is small enough, e.g., often less than an ohm, such that the impedances presented by the control circuitry  120  and the motor  110  can be ignored. As the value of ESR  150  falls toward ESRdesired after the activation of the battery  140 , Vs and I both increase until:
 
 Vs   activate   =Vi×R   160 /( ESR   desired   +R   160 )  (3)
 
 I   activate   =Vi /( ESR   desired   +R   160 )  (4)
 
   Because Vi, ESRdesired, and R 160  are known quantities, then Iactivate and Vsactivate are also known. 
   Consequently, the circuit  160  is designed to transistion from a relatively low impedance to a relatively high impedance, e.g., on the order of Megaohms or Gigaohms, in response to I=Iactivate and/or Vs=Vactivate per equations (3) and (4). 
   For example, the circuit  160  may be a fuse designed to blow when the current through it equals Iactivate. Before the fuse blows, Vs has a level on the order of 0 V to a few hundred mV. Once the fuse blows, the value of ESR  150  is low enough such that Vs becomes greater than the reset voltage level of the circuitry  120  and remains greater than the reset voltage level even after the motor  110  is activated. Consequently, the circuitry  120  is significantly less likely to re-enter its reset mode when the motor  110  is activated, thus reducing the chances that the circuitry will delay or abort the launch of the missile  10 . 
   Still referring to  FIG. 2 , the operation of the missile  10  during a launch sequence is discussed where the circuit  160  is a fuse. 
   First, an igniter (omitted from  FIG. 2  for clarity) activates the battery  140 . Because the fuse  160  is electrically closed, Vs˜0 V. If Vs is large enough to power the circuitry  120  in its reset mode, then the circuitry  120  resets itself. If Vs is not large enough, then the circuity  120  is inactive. 
   Next, the value of the ESR  150  begins decreasing while the fuse  160  remains electrically closed and Vs remains ˜0 V. 
   Then, when the value of ESR  150  equals or falls below ESRdesired, the fuse  160  blows (i.e., electrically opens), and Vs rises to a level above the reset level of the circuitry  120 . If the circuitry  120  has not previously reset itself, then it does so now before becoming fully operational. 
   Next, the circuitry  120  executes its pre-launch routine, activates the motor  110 , and launches the missile  10 . 
   The power supply  130  comprises a housing  180  in which the battery  140  is disposed, and the circuit  160  is disposed within the housing  180 . Alternatively, the circuit  160  may be disposed outside of the housing  180  as indicated in  FIG. 2  by dashed lines. 
   Although described as being a fuse that is connected across the terminals  170  and  172  of the missile power supply  130 , other embodiments of the circuit  160  are contemplated. For example, the circuit  160  may include a one or more interconnected semiconductor components such as transistors, and may be reusable—a fuse, once blown, is typically not reusable. Furthermore, the circuit  160  may transition from a low to a high impedance in response to Vs in addition to or instead of in response to I. Moreover, the circuit  160  may be connected in a topology other than directly across the supply terminals  170  and  172 . In addition, the circuit  160  may be used in devices other than missiles. 
   The preceding discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. For example, it should be recognized that all operations described herein could be applied to any device employing a load that, to function properly, requires a minimum sustained voltage applied to the load. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.