Switching regulator having fast startup time and low standby power
A switching regulator having fast start-up time and low standby power is disclosed. In an exemplary embodiment, an apparatus includes a transistor that generates a charging current at a first current level from a base current received at a base terminal. The apparatus also includes a capacitor that charges in response to the charging current at the first current level to generate a voltage signal that increases at a first rate. The apparatus also includes a charge pump having an output coupled to the base terminal. The charge pump outputs a charge pump current when the voltage signal exceeds a first voltage level. The base current is increased by charge pump current to cause the transistor to generate the charging current at a second current level, and the capacitor charges in response to the charging current at the second current level to generate the voltage signal that increases at a second rate.
1. An apparatus comprising:
a transistor that generates a charging current at a first current level from a base current received at a base terminal;
a capacitor that charges in response to the charging current at the first current level to generate a voltage signal that increases at a first rate; and
a charge pump having an output coupled to the base terminal, the charge pump outputs a charge pump current when the voltage signal exceeds a first voltage level, wherein the base current is increased by the charge pump current and the transistor generates the charging current at a second current level, and wherein the capacitor charges in response to the charging current at the second current level to generate the voltage signal that increases at a second rate.
2. The apparatus of claim 1 , further comprising a resistor connected to the base terminal, wherein the resistor receives an input voltage and generates the base current.
3. The apparatus of claim 1 , further comprising a threshold circuit that sets an enable signal to a first state when the voltage signal exceeds the first voltage level, and wherein the first state of the enable signal enables the charge pump to generate the charge pump current.
4. The apparatus of claim 3 , wherein the threshold circuit sets the enable signal to a second state when the voltage signal exceeds a second voltage level, and wherein the second state of the enable signal disables the charge pump.
5. The apparatus of claim 1 , further comprising a switch having a closed position that couples the charging current to the capacitor when the voltage signal is less than a second voltage level, and wherein the switch has an open position that disconnects the charging current from the capacitor when the voltage signal is equal to or greater than a second voltage level.
6. The apparatus of claim 1 , further comprising a diode that has an input terminal connected to receive an auxiliary voltage signal and an output terminal that is connected to the capacitor.
7. The apparatus of claim 1 , wherein the second current level determines the second rate.
8. The apparatus of claim 7 , wherein the second rate determines a start-up time.
9. The apparatus of claim 1 , wherein the transistor comprises an NPN power transistor that outputs the charging current at an emitter terminal.
10. The apparatus of claim 1 , wherein the transistor generates the charging current by multiplying current at the base terminal by (β+1), wherein β is a gain factor of the transistor.
11. A method comprising:
generating a charging current at a first current level from an input current;
charging a capacitor in response to the charging current at the first current level to generate a voltage signal that increases at a first rate;
generating a charge pump current when the voltage signal exceeds a first voltage level;
generating the charging current at a second current level from the input current and the charge pump current; and
charging the capacitor using the charging current at the second current level to generate the voltage signal to increase at a second rate.
12. The method of claim 11 , further comprising generating the input current using a resistor connected to an input voltage.
13. The method of claim 11 , further comprising decreasing the charging current from the second current level to the first current level when the voltage signal exceeds a second voltage level.
14. The method of claim 11 , further comprising setting the charge pump current to obtain a selected startup time that begins when the voltage signal exceeds the first reference level and ends when the voltage signal exceeds a second reference level.
15. A system comprising:
a transformer having a primary winding;
a transistor connected to the primary winding, wherein the transistor generates a charging current at a first current level from a base current received at a base terminal;
a capacitor that charges in response to the charging current at the first current level to generate a voltage signal that increases at a first rate; and
a charge pump having an output connected to the base terminal, the charge pump outputs a charge pump current when the voltage signal exceeds a first voltage level, wherein the base current is increased by charge pump current and the transistor generates the charging current at a second current level, and wherein the capacitor charges in response to the charging current at the second current level to generate the voltage signal to increase at a second rate.
16. The system of claim 15 , further comprising a threshold circuit that sets an enable signal to a first state when the voltage signal exceeds the first voltage level, and wherein the first state of the enable signal enables the charge pump to generate the charge pump current.
17. The system of claim 16 , wherein the threshold circuit sets the enable signal to a second stage when the voltage signal exceeds a second voltage level, and wherein the second state of the enable signal disables the charge pump.
18. The apparatus of claim 15 , wherein the charge pump generates the charge pump current at a selected current level to determine the second rate.
19. The apparatus of claim 18 , wherein the second rate determines a start-up time.
20. The apparatus of claim 15 , wherein the transistor generates the charging current by multiplying the base current by (β+1), wherein β is a gain factor of the transistor.