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.
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 xe2x80x9csynchronous rectifiers.xe2x80x9d 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.
An enabling circuit for enabling an associated regulating circuit having an energy storage element consistent with the invention includes: a comparison circuit configured to compare a feedback signal representative of a charge of the energy storage element with a signal representative of a reference charge and provide an output in response to the comparison; and an output decision circuit configured to receive at least the output from the comparison circuit and provide an enabling signal to enable the regulating circuit to switch from a first state to a second state if the charge of the energy storage element is less than the reference charge.
A battery charging system consistent with the invention includes: a rechargeable battery; a power source; a synchronous rectifier converter configured to accept an input power level from the power source and provide a regulated output power level to the battery, the synchronous rectifier converter having an energy storage element; and an enabling circuit for enabling the synchronous rectifier converter, the enabling circuit comprising: a comparison circuit configured to compare a feedback signal representative of a charge across the energy storage element with a signal representative of a reference charge and provide an output in response to the comparison; and an output decision circuit configured to receive at least the output from the comparison circuit and provide an enabling signal to enable the synchronous rectifier converter to switch from a first state to a second state if the charge of the energy storage element is less than the reference charge.
A method of avoiding negative voltage transients at the output of a regulating circuit having an energy storage element consistent with the invention includes the steps of: monitoring a charge on the energy storage element; maintaining the regulating circuit in a first state if the charge is above a reference level; and switching the regulating circuit to a second state if the charge is below the reference level.
A method of avoiding negative voltage transients at the output of a synchronous rectifier converter having a capacitor, wherein the synchronous rectifier converter provides a charging current to an associated rechargeable battery, wherein such a method consistent with the invention includes the steps of: monitoring a charge on the capacitor; maintaining the synchronous rectifier converter in a first state if the charge on the capacitor is above a reference charge; discharging the capacitor until the charge on the capacitor is less than the reference charge; and enabling the charging current to flow to the associated rechargeable battery once the capacitor is discharged to a charge value less than the reference charge.
Another enabling circuit for enabling an associated regulating circuit having an energy storage element consistent with the invention includes: a comparison circuit configured to compare a feedback signal representative of a charge of the energy storage element with a signal representative of a reference charge and provide an output in response to the comparison to enable the regulating circuit to switch from a first state to a second state if the charge of the energy storage element is less than the reference charge.
Another enabling circuit for enabling an associated regulating circuit having an energy storage element consistent with the invention includes: an output decision circuit configured to receive a signal representative of a comparison between a charge on the energy storage element and a reference charge level and provide an enabling signal to enable the regulating circuit to switch from a first state to a second state if the charge of the energy storage element is less than the reference charge.