# Patent

## Title
Enabling circuit for avoiding negative voltage transients

## Abstract

An enabling circuit for enabling a DC—DC converter having a capacitor coupled to an output terminal of the DC—DC converter to be controlled by a control signal. The enabling circuit may include a comparison circuit configured to compare a feedback signal representative of a charge on the capacitor with a signal representative of a reference charge and to provide an output to in response to the comparison to enable the DC—DC converter to be controlled by the control signal if the charge on the capacitor is less than the reference charge. The enabling circuit may also include a discharge path configured to discharge the charge on the capacitor if the charge is greater than the reference charge. A related system and method are also provided.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of prior application Ser. No. 10/738,058 filed Dec. 17, 2003 now U.S. Pat. No. 6,906,497, which itself is a continuation of prior application Ser. No. 10/176,141, filed Jun. 20, 2002, now U.S. Pat. No. 6,756,769, the teachings of which are both incorporated herein by reference.


FIELD OF THE INVENTION
This invention relates to an enabling circuit for avoiding negative voltage transients from an associated regulating circuit, and more particularly to such an enabling circuit for enabling a synchronous rectifier converter to switch from a first state to second state if the charge on an energy storage element of the synchronous rectifier converter is less than a reference charge.
BACKGROUND OF THE INVENTION
A variety of circuits have energy storage element such as capacitors, inductors, and transformers that transfer energy from an input to an output of such circuits. If such energy storage elements are not properly discharged in some instances, unwanted power disturbances, e.g., negative voltage transients, may occur in the output signal causing damage to nearby sensitive components.
For instance, such a regulating circuit may be a DC—DC converter. DC—DC converters generally accept a DC input at one voltage level and convert it to a DC output at a higher or lower voltage level. Such DC—DC converters may be used in a wide variety of electronic devices in conjunction with a variety of systems. One such system may be used to provide a battery charging function for portable electronic devices such as laptop computers, cell phones, pagers, personal digital assistants, and the like.
One type of DC—DC converter is a synchronous rectifier converter (SRC). An SRC does not use any Schottky diodes, but rather uses transistors referred to as “synchronous rectifiers.” Such transistors may be a variety of transistors such as MOS or MOSFET transistors. An SRC may also have a variety of internal components that typically include an energy storage element, e.g., a capacitor, inductor, or transformer, with one or more transistors controlled by various control techniques, e.g., pulse width modulation where the switch frequency is constant and the duty cycle varies with the load.
When an SRC is used in conjunction with a battery power management system, the SRC may accept an input voltage from a number of different power sources and convert it to an appropriate output voltage to, among other things, provide an appropriate charging current to an associated rechargeable battery. In such a battery power management system, there is typically an associated controller used to control the battery charging process. Such controller may be an integrated circuit (IC) having a plurality of input terminals or pins, some of which are connected to the output of the SRC. For instance, two such terminals may be coupled to either side of a sense resistor. The sense resistor may be in series with the output of the SRC such that it provides a signal representative of the charging current provided at the output of the SRC.
If a soft start occurs when the energy storage element, e.g., a capacitor, of the SRC is charged at a significant value, e.g., over several volts, negative voltage transients may appear on either terminal of the sense resistor potentially causing catastrophic failure of the associated controller IC. Accordingly, there is a need for an enabling circuit and method that overcomes the above deficiencies in the prior art and is capable of avoiding negative voltage transients from an associated regulating circuit by enabling the regulating circuit only when the charge on the energy storage element is below a reference charge.
BRIEF SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided an enabling circuit for enabling a DC—DC converter having a capacitor coupled to an output terminal of the DC—DC converter to be controlled by a control signal. The enabling circuit may include a comparison circuit configured to compare a feedback signal representative of a charge on the capacitor with a signal representative of a reference charge and to provide an output to in response to the comparison to enable the DC—DC converter to be controlled by the control signal if the charge on the capacitor is less than the reference charge. The enabling circuit may also include a discharge path configured to discharge the charge on the capacitor if the charge is greater than the reference charge.
According to another aspect of the invention, there is provided a system. The system may include a DC—DC converter configured to accept an input power level from a power source and provide a regulated output power level to a load. The DC—DC converter may have a capacitor coupled to an output terminal of the DC—DC converter. The system may further include an enabling circuit for enabling the DC—DC converter to be controlled by a control signal. The enabling circuit may include a comparison circuit configured to compare a feedback signal representative of a charge on the capacitor with a signal representative of a reference charge and to provide an output to in response to the comparison to enable the DC—DC converter to be controlled by the control signal if the charge on the capacitor is less than the reference charge. The enabling circuit may also include a discharge path configured to discharge the charge on the capacitor if the charge is greater than the reference charge.
According to yet another aspect of the invention there is provided a method. The method may include monitoring a charge on a capacitor of a DC—DC converter coupled to an output terminal of said DC—DC converter; preventing the DC—DC converter from being controlled by a control signal if the charge is greater than the reference charge; and enabling the DC—DC converter to be controlled by the control signal if the charge is less than the reference charge.



BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts:
FIG. 1 is a block diagram of a system including an enabling circuit consistent with the present invention for enabling an associated regulating circuit to switch from a first state to a second state;
FIG. 2 is a block diagram of an exemplary enabling circuit consistent with the invention for enabling an associated synchronous rectifier converter to switch from one state to another state;
FIG. 2A is a circuit diagram illustrating an exemplary embodiment of the enabling circuit of FIG. 2; and
FIG. 3 is a block diagram of a battery management system utilizing the enabling circuit of FIG. 2.



DETAILED DESCRIPTION
Turning to FIG. 1, an exemplary system 100 including an enabling circuit 104 and an associated regulating circuit 102 is illustrated. The regulating circuit 102 may be any variety of circuits, e.g., a synchronous rectifier converter, containing an energy storage element 106, e.g., a capacitor. In general, the enabling circuit 104 monitors the charge on the energy storage element 106 and enables the regulating circuit 102 to switch from a first state to a second state when the charge on the energy storage element is below a reference charge. The first state may be any variety of states such as a power off state, and the second state may also be any variety of states such as an operating state where the regulating circuit 102 is controlled by a particular control technique. The reference charge should be chosen based on the particular system and sensitivity of associated components. In one embodiment, the reference charge may be 3.0 volts.
The enabling circuit 104 has one input terminal 107 configured to accept a signal, Vdsch, representative of an acceptable reference charge level for the energy storage element 106. The enabling circuit 104 may have another input terminal 109 configured to accept a logic control signal chginh. Such logic control signal, chginh, has a predetermined state, e.g., low state, when at least one non energy storage element condition related to operation of the regulating circuit 102 is satisfied. Such a condition or conditions may be any variety of other conditions known to those skilled in the art unrelated to the charge on the energy storage element 106. For instance, one condition may be the proper coupling of an input power source at a proper power level to the input of the regulating circuit 102.
The enabling circuit 104 accepts a feedback signal from the regulating circuit 102 along path 119. Such feedback signal is representative of the charge on the energy storage element 106. The enabling circuit compares the charge on energy storage element 106 with a reference charge and may output an enabling signal if the charge level is less than the reference charge level. The enabling circuit 104 may also include various discharging means as further described in reference to FIG. 2 in order to discharge the energy storage element 106 below the reference charge level should the charge be greater than the reference charge level.
In one embodiment, once the charge on the energy storage element 106 is below the reference charge, the enabling circuit 104 sends an enabling signal along path 115 to the controller 120. The controller 120 is responsive to the enabling signal to then enable the regulating circuit to switch from a first state, e.g., a non-operating state or a predetermined suitable operating state, to a second state, e.g., another operating state. As such, an enabling circuit 104 consistent with the invention may advantageously delay operation of the regulating circuit 102 in the second state while maintaining operation of the regulating circuit 102 in the first state until the energy storage element 106 is discharged below a reference charge.
In another embodiment, the enabling circuit 104 may not send an enabling signal to the controller 120 until both the energy storage element 106 is discharged below the reference charge level and a logic control signal, e.g., signal chginh, input to the enabling circuit is at a predetermined state. Such predetermined state is representative of at least one satisfactory non energy storage element condition. Such predetermined state may also be representative of a satisfactory condition for all other non energy storage element conditions. In this case, an enabling circuit 104 consistent with the invention delays switching the regulating circuit 102 to operation in the second state while maintaining operation of the regulating circuit 102 in the first state, e.g., a non-operating state or a predetermined suitable operating state, until the energy storage element 106 is discharged below a predetermined charge level and a signal representing at least one satisfactory non energy storage element condition related to operation of the regulating circuit is satisfied.
Turning to FIG. 2, one exemplary system 200 having an enabling circuit 204 consistent with the invention and a synchronous rectifier converter (SRC) 202 is illustrated. The SRC accepts an input voltage signal at input terminal 226 and provides an output voltage signal at output terminal 211. The SRC may include an inductor 208, a capacitor 206 and switches K1, K2. Switches K1, K2 may be any type of transistors and for simplicity are drawn to represent MOS type transistors. Such switches K1, K2 may be controlled by a variety of control techniques such as pulse width modulation (PWM) where the switch frequency is constant and the duty cycle varies with load, pulse-frequency modulation, or current-limited pulse-frequency modulation as those control techniques are known to those skilled in the art.
The enabling circuit 204 may include a comparator circuit 232, an output decision circuit 234, and discharge path including resistors R1, R2 and switches K3, K4. The comparator circuit 232 may be any variety of circuits for comparing the charge on the capacitor 206 to a predetermined reference value charge represented by control signal Vdsch input to the comparator circuit. The comparator circuit 232 is configured to provide an output signal to maintain the SRC 202 in the first state, e.g., a non-operating state or a predetermined suitable operating state, if the charge on the capacitor 206 is higher than the reference charge. The comparator circuit 232 is also configured to provide output signal to switch the SRC 202 from the first state to a second state, e.g., another operating state where switches K1, K2 are under control of the controller 220, if the charge on the capacitor 206 is less than the reference charge.
The output decision circuit 234 may be any variety of circuits for performing a desired logic function. The output decision circuit 234 accepts the output signal dsch from the comparator circuit 232 and may further accept a logic signal chginh from a separate source. The output decision circuit 234 may be configured to output an enabling signal on path 215 to the controller 220 if signal dsch indicates the charge on the capacitor 206 is less than a reference charge level. The output decision circuit 234 may alternately require signal dsch to indicate the charge on the capacitor 206 is less than a reference charge level, and require signal chginh to be in a predetermined state, e.g., low state, when at least one non energy storage element condition is satisfied. The output decision circuit 234 is not a necessary part of the enabling circuit 204 if the logic signal chginh is not input to the enabling circuit. In this case, the comparator circuit 232 would provide the enabling signal if the charge on the capacitor 206 is less than the reference charge.
The enabling circuit 204 may also include a discharge path including resistors R1, R2, and switches K3, K4 to discharge the capacitor 206 below a reference level. Switches K3, K4 may be any type of transistors and for simplicity are drawn to represent MOS type transistors. Detailed operation of an enabling circuit and the discharge path is made later with reference to the exemplary enabling circuit of FIG. 2A.
Turning to FIG. 2A, a circuit diagram of one exemplary enabling circuit 204a is illustrated. Those skilled in the art will recognize a variety of circuit configurations which may be utilized in an enabling circuit consistent with the present invention. In the exemplary embodiment of FIG. 2A, the comparator circuit 232 of FIG. 2 includes a comparator 232a having its positive input terminal accepting the input signal representative of the charge on the capacitor 206 and its negative input terminal accepting the other input control signal, Vdsch, representative of a reference value charge.
The output decision circuit 234 includes a NOR gate 234a. The NOR gate 234a accepts the output from the comparator 232a and the control signal chginh. The output of the NOR gate 234a is HIGH only if all inputs are LOW. Otherwise, the output of the NOR gate 234a is LOW. In this embodiment, the enabling signal sent by the enabling circuit 204 to the controller 220 to enable the regulating circuit 202 to switch from the first state, e.g. switches K1, K2 open, to the second state state, e.g., switches K1, K2 under control of controller 220, is sent when the output of the NOR gate is HIGH.
Operation of the exemplary enabling circuit 204a in conjunction with the system 200 is described further herein with reference to the Truth Table of Table 1. Table 1 details the status of the various control signals and switches relative to each other with the output decision circuit 234 functioning as the NOR gate 234a of FIG. 2A. The status of control signal chginh input to the enabling circuit 204, control signal dsch output from comparator 232a, control signal chgen output from the NOR gate 234a, switches K1, K2 of the SRC 202, and switches K3, K4 of the enabling circuit 204 are all detailed in Table 1.











TABLE 1







chginh
H
H
L
L


dsch
H
L
H
L


chgen
L
L
L
H


K1
OFF
OFF
OFF
PWM


K2
OFF
OFF
OFF
PWM


K3
ON
ON
ON
OFF


K4
ON
ON
ON
OFF









As illustrated and described more fully herein, the exemplary enabling circuit 204a advantageously does not enable switches K1, K2 of the SRC 204 to be controlled by the proper control technique, e.g., PWM control, and maintains switches K1, K2 in an OFF position, until the capacitor 206 of the SRC 202 is discharged below a reference charge (control signal dsch is L), and at least one other non capacitor charge related condition for operation of the regulating circuit (control signal chginh is L) is satisfied. As such, negative voltage transients that may otherwise occur at the output terminal 211 of the SRC 202 are avoided.
As illustrated in the first substantive column of Table 1, if the charge level on capacitor 206 is greater than a reference charge level as represented by control signal Vdsch input to the comparator 232a, then the control signal dsch output from the comparator 232a is HIGH. The output control signal dsch from the comparator 232a is then input to the NOR gate 234a. 
The other input to the NOR gate 234a may be the control signal chginh from a separate source. Such control signal chginh is representative of at least one non capacitor related condition pertinent to operation of the SRC 202. In this embodiment, if this chginh signal is LOW, at least one and perhaps all other non capacitor related conditions pertinent to operation of the SRC 202 are satisfactory. If this chginh signal is HIGH, such condition or conditions are unsatisfactory. Accordingly, if the output control signal dsch from the comparator 232a is HIGH and the chginh signal is also HIGH, the output signal from the NOR gate 234a is LOW. Thus, switches K1 and K2 remain OFF or open and the operation of the SRC 202 is delayed. As such, if the SRC is operating to provide a charging current to an associated rechargeable battery, such charging current would not be provided in this instance, nor would current flow from the capacitor 206 through the inductor 208 be possible in this instance.
Turning to the second substantive column of Table 1, if the capacitor 206 is discharged below the reference charge represented by control signal Vdsch, the output control signal dsch from the comparator 232a goes LOW indicating the charge level on the capacitor 206 is acceptable. However, if another non capacitor charge related condition is unsatisfactory, the control signal chginh remains HIGH. As such, the output of the NOR gate 234a remains LOW and the operation of the SRC 202 is still delayed.
Turning to the third substantive column of Table 1, if the control signal chginh is LOW representing a satisfactory starting condition for at least one non-charge related condition, but the output control signal dsch from the comparator 232a is HIGH, then the output signal chgen from the NOR gate 234a remains LOW. As such, the operation of the SRC 202 is still delayed.
As illustrated in the fourth substantive column of Table 1, it is not until the capacitor 206 is discharged below the reference charge level (control signal dsch output from the comparator 232a is LOW), and at least one if not all other non-charge related conditions for operation of the regulating circuit (control signal chginh is LOW) are satisfied, that the control signal chgen output of the NOR gate 234a is HIGH. Once the control signal chgen is HIGH, switches K3 and K4 of the enabling circuit 204 open or are in an OFF position. The HIGH control signal chgen enables the controller 220 to drive the SRC 202. Hence, switches K1, K2 are controlled by an appropriate control technique, e.g., PWM.
If the enabling signal is not present in this embodiment, switches K3, K4 are closed or in an ON position. A discharge path for the capacitor 206 is then created through the switches K3, K4. Switch K3 is further coupled in series to resistor R1, while switch K4 is further coupled in series to resistor R2. Resistor R1 has a resistive value that is higher than the resistive value for resistor R2. As such, when switches K3, K4 are closed because no enabling signal from the NOR gate 234a is present, a discharge path is created and resistor R2 serves to discharge the capacitor 206.
Turning to FIG. 3, an exemplary battery charging system 300 generally including a power source 344, an SRC 302, an enabling circuit 304 consistent with the invention, a rechargeable battery 340, and a battery charging controller 320 is illustrated. Such a battery charging system 300 may be used in a variety of portable electronic devices such as laptop computers, cell phones, pagers, personal digital assistants, and the like to provide and control power flow to a rechargeable battery 340, e.g., a lithium, nickel-cadmium, or nickel-metal hydride battery.
A sensor such as sense resistor 346 may be used in order to provide a sensed signal to the controller 320 indicative of the charging current Ichg to the battery. Such a controller 320 is typically an integrated circuit (IC) and may be sensitive to negative voltage transients that may otherwise occur at either terminal 349, 351 if the energy storage element 306 is not properly discharged below a reference charge value. Such negative voltage transients may appear on either terminal 349, 351 due to oscillation induced by an inductor and capacitor group of the SRC 302.
Once a power source 344 is properly coupled to the system 300, it provides an input DC voltage signal to the SRC 302. The power source 344 may be an AC/DC adapter configured to receive conventional AC voltage from a power outlet and convert it to an applicable DC voltage, or a DC/DC adapter such as a “cigarette lighter” type adapter configured to plug into that type of socket, or other types of power sources.
Advantageously, the SRC 302 accepts power input from the power source 344 and converts it to a proper output voltage and current level for providing a charging current Ichg to the battery 340 only if the controller 320 receives an enabling signal from the enabling circuit 304 along path 315. Otherwise, the controller 320 delays providing a charging current Ichg to the battery 340, while keeping the SRC 302 in a predetermined suitable state. This predetermined suitable state may be any variety of states as determined by the position of various switches in the SRC 302. In the previous embodiment of FIG. 2, switches K1, K2 of SRC 202 were chosen to be in an open state. The SRC 302 is controlled by the controller 320 by any variety of control techniques, e.g., PWM, known by those skilled in the art.
Advantageously therefore, the enabling circuit 304 delays providing of the charging current Ichg to the battery 340 until the energy storage element 306 is discharged below a reference charge. The enabling circuit 304 may also further delay the charging current Ichg to the battery 340 until another control signal, e.g., signal chginh, from another source indicates that at least one non-charge related condition pertinent to operation of the SRC 302 is satisfactory.
The embodiments that have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of the invention.

## Claims

1. An enabling circuit for enabling a DC—DC converter having a capacitor coupled to an output terminal of said DC—DC converter to be controlled by a control signal, said enabling circuit comprising:
a comparison circuit configured to compare a feedback signal representative of a charge on said capacitor with a signal representative of a reference charge and to provide an output to in response to said comparison to enable said DC—DC converter to be controlled by said control signal if said charge on said capacitor is less than said reference charge; and
a discharge path configured to discharge said charge on said capacitor if said charge is greater than said reference charge.

2. The enabling circuit of claim 1, wherein said discharge path comprises at least one switch, said at least one switch configured to close to create a path from said capacitor to ground if said charge on said capacitor is greater than said reference charge.

3. The enabling circuit of claim 2, wherein said discharge path further comprises at least one resistor in series with said at least one switch.

4. The enabling circuit of claim 1, wherein said comparison circuit comprises a comparator.

5. The enabling circuit of claim 1, wherein said control signal comprises a pulse width modulated (PWM) signal.

6. The enabling circuit of claim 1, wherein said output from said comparison circuit prevents said DC—DC converter from being controlled by said control signal if said charge on said capacitor is greater than said reference charge.

7. A system comprising:
a DC—DC converter configured to accept an input power level from a power source and provide a regulated output power level to a load, said DC—DC converter having a capacitor coupled to an output terminal of said DC—DC converter; and
an enabling circuit for enabling said DC—DC converter to be controlled by a control signal, said enabling circuit comprising:
a comparison circuit configured to compare a feedback signal representative of a charge on said capacitor with a signal representative of a reference charge and to provide an output to in response to said comparison to enable said DC—DC converter to be controlled by said control signal if said charge on said capacitor is less than said reference charge; and
a discharge path to discharge said charge on said capacitor if said charge is greater than said reference charge.

8. The system of claim 7, wherein said discharge path comprises at least one switch, said at least one switch configured to close to create a path from said capacitor to ground if said charge on said capacitor is greater than said reference charge.

9. The system of claim 8, wherein said discharge path further comprises at least one resistor in series with said at least one switch.

10. The system of claim 7, wherein said comparison circuit comprises a comparator.

11. The system of claim 7, wherein said control signal comprises a pulse width modulated (PWM) signal.

12. The system of claim 7, wherein said output from said comparison circuit prevents said DC—DC converter from being controlled by said control signal if said charge on said capacitor is greater than said reference charge.

13. A method comprising:
monitoring a charge on a capacitor of a DC—DC converter coupled to an output terminal of said DC—DC converter;
comparing said charge to a reference charge;
preventing said DC—DC converter from being controlled by a control signal if said charge is greater than said reference charge; and
enabling said DC—DC converter to be controlled by said control signal if said charge is less than said reference charge.

14. The method of claim 13, further comprising discharging said capacitor if said charge is greater than said reference charge.

15. The method of claim 14, wherein said discharge operation further comprises closing a switch to create a discharge path from said capacitor to ground.

16. The method of claim 5, wherein said control signal comprises a pulse width modulated (PWM) signal.