Regulating current output from a buck converter without external current sensing
A step-down (buck) switching regulator regulates output current without sensing a current external to a converter integrated circuit. The regulator generates a set current that is indicative of a predetermined current level to which the output current is regulated. The regulator generates a sense current whose magnitude is proportional to an inductor current flowing through a power switch during an on time. During a first time period, the sense current is less than the set current. During a second time period, the sense current is greater than the set current. The output current of the regulator is maintained at the predetermined current level such that the first time period is equal to the second time period when the output current equals the predetermined current level. The set current is compared to the sense current in circuitry inside a bootstrap power generator whose voltage fluctuates with the voltage across the inductor.
1. A method comprising:
generating a reference current, wherein a switching regulator includes an inductor and a power switch, and wherein an inductor current flows through the inductor during an on time of the power switch;
generating a sense current whose magnitude is proportional to the inductor current flowing through the power switch during the on time, wherein the sense current increases during the on time;
determining a trip time at which the increasing sense current equals the reference current; and
maintaining an output current of the switching regulator at a predetermined current level such that, when the output current equals the predetermined current level, a first time period before the trip time during which the sense current is less than the reference current is a fixed proportion of a second time period after the trip time during which the sense current is greater than the reference current.
2. The method of claim 1 , wherein the switching regulator has a converter integrated circuit, wherein the power switch is inside the converter integrated circuit, and the inductor is outside the converter integrated circuit, and wherein the maintaining is performed without sensing a current external to the converter integrated circuit.
3. The method of claim 1 , wherein the switching regulator has a converter integrated circuit, wherein the power switch is inside the converter integrated circuit, and the inductor is outside the converter integrated circuit, and wherein the reference current is generated using a resistor outside the converter integrated circuit.
4. The method of claim 1 , wherein the switching regulator has a converter integrated circuit, wherein the power switch is inside the converter integrated circuit, and the inductor is outside the converter integrated circuit, and wherein the reference current is generated by programming a register inside the converter integrated circuit.
5. The method of claim 1 , wherein the first time period is equal to the second time period.
6. The method of claim 1 , wherein neither the first time period nor the second time period is as long as the on time.
7. The method of claim 1 , wherein both the first time period and the second time period have non-zero durations.
8. The method of claim 1 , further comprising:
flowing the output current through a light emitting diode (LED).
9. The method of claim 8 , further comprising:
performing pulse-width-modulation (PWM) dimming of the LED.
10. The method of claim 1 , further comprising:
charging a battery using the output current.
11. The method of claim 1 , further comprising:
generating an output voltage of the switching regulator, wherein the reference current varies independently of the output voltage.
12. The method of claim 1 , further comprising:
maintaining an output voltage of the switching regulator at a predetermined voltage level when the output current is less than the predetermined current level.
13. The method of claim 1 , wherein the switching regulator maintains the first time period at the fixed proportion of the second time period using a single signal indicative of the relative proportions of the first time period and the second time period.
14. The method of claim 1 , wherein the determining the trip time at which the increasing sense current equals the reference current is performed by comparing the reference current directly to the sense current without using a comparator.
15. The method of claim 1 , further comprising:
deasserting a signal at the trip time.
16. The method of claim 1 , further comprising:
comparing the first time period to the second time period.
17. A method comprising:
generating a reference current, wherein a switching regulator includes an inductor and a power switch, and wherein an inductor current flows through the inductor during an on time of the power switch;
generating a sense current whose magnitude is proportional to the inductor current flowing through the power switch during the on time; and
maintaining an output current of the switching regulator at a predetermined current level such that, when the output current equals the predetermined current level, a first time period during which the sense current is less than the reference current is a fixed proportion of a second time period during which the sense current is greater than the reference current, wherein the first time period is half of the second time period.
18. A power converter, comprising:
an inductor; and
a converter integrated circuit with a power switch and a replica switch, wherein the power converter generates a reference current, wherein an inductor current with a magnitude flows through the inductor, wherein a sense current flowing through the replica switch has a magnitude that is proportional to the magnitude of the inductor current during an on time of the power switch, wherein the sense current increases during the on time, wherein the converter integrated circuit determines a trip time at which the increasing sense current equals the reference current, and wherein the power converter maintains an output current at a predetermined current level such that, when the output current equals the predetermined current level, a first time period before the trip time during which the sense current is less than the reference current is a fixed proportion of a second time period after the trip time during which the sense current is greater than the reference current.
19. The power converter of claim 18 , wherein the power converter maintains the output current at the predetermined current level without sensing a current external to the converter integrated circuit.
20. The power converter of claim 18 , further comprising:
a resistor outside the converter integrated circuit, wherein the reference current is generated using the resistor.
21. The power converter of claim 20 , wherein the resistor is a thermistor, wherein the output current varies inversely with a temperature, and wherein the temperature is taken from the group consisting of: a temperature of a load being charged by the output current, an ambient temperature around the power converter, and a temperature of a light emitting diode (LED) through which the output current flows.
22. The power converter of claim 18 , wherein the converter integrated circuit includes a register, and wherein the reference current is generated by programming the register.
23. The power converter of claim 18 , wherein the output current of the power converter flows through a light emitting diode (LED).
24. The power converter of claim 23 , wherein power converter performs pulse-width-modulation (PWM) dimming of the LED.
25. The power converter of claim 18 , wherein the power converter outputs an output voltage, and wherein the reference current varies independently of the output voltage.
26. The power converter of claim 18 , wherein the converter integrated circuit includes a latch that outputs a switching signal that turns off the power switch, and wherein the inductor current stops increasing through the inductor when the power switch is turned off.
27. The power converter of claim 18 , wherein the converter integrated circuit compares the reference current directly to the sense current without using a comparator to compare the reference current to the sense current.
28. A power converter, comprising:
an inductor, wherein a ripple current having a peak magnitude and a valley magnitude flows through the inductor and is output from the power converter, wherein an average output current is the average of the peak magnitude and the valley magnitude, and wherein a ripple magnitude is the difference between the peak magnitude and the valley magnitude;
a replica switch through which a sense current flows, wherein the sense current contributes to the ripple current; and
means for maintaining the average output current at a fixed current level without sensing a current external to the means, wherein the fixed current level does not substantially change as the ripple magnitude varies, wherein the sense current increases throughout an on time, wherein the means generates a set current indicative of the fixed current level, wherein the means determines a trip time at which the increasing sense current equals the set current, wherein a second time occurs when a predetermined portion of the on time has elapsed, and wherein the means regulates the average output current such that the trip time occurs at the second time.
29. The power converter of claim 28 , wherein the average output current charges a load, wherein the load has a load temperature, wherein the set current is generated using a thermistor, and wherein the average output current varies inversely with the load temperature.
30. The power converter of claim 28 , wherein the power converter is part of a car cigarette lighter adapter.
31. A power converter, comprising:
an inductor, wherein a ripple current having a peak magnitude and a valley magnitude flows through the inductor and is output from the power converter, wherein an average output current is the average of the peak magnitude and the valley magnitude, and wherein a ripple magnitude is the difference between the peak magnitude and the valley magnitude; and
means for maintaining the average output current at a fixed current level without sensing a current external to the means, wherein the fixed current level does not substantially change as the ripple magnitude varies, wherein both a main current that flows through a power switch and a sense current that flows through a replica switch contribute to the ripple current, wherein the means generates a set current indicative of the fixed current level, wherein the sense current ramps up to reach the set current at a first time, wherein the sense current increases throughout an on time and wherein a second time occurs when a predetermined portion of the on time has elapsed, wherein the means regulates the average output current such that the second time occurs at the first time, and wherein the predetermined portion is two thirds of the on time.