Short circuit frequency shift circuit for switching regulators

A controller for use in a switching power supply having a power switch, the controller comprising:

• an oscillator for generating pulse width modulation (PWM) pulses having a frequency for use in activating the power switch in the switching power supply, the oscillator including a Schmitt trigger having its input connected to a first current source and to a first capacitor, the first current source capable of generating a first current for charging the first capacitor at a charging rate in order to increase a voltage at the input of the Schmitt trigger in order to trigger the Schmitt trigger;
• a frequency shift circuit for shifting the frequency of the PWM pulses at a shift rate in response to the voltage at an output of the switching power supply decreasing below a threshold level, the frequency shift circuit including a first transistor connected to the input of the Schmitt trigger to divert a portion of the first current to ground to prevent the portion of the first current from charging the first capacitor in order to reduce the charging rate of the first capacitor in order to shift the frequency of the PWM pulses, the frequency shift circuit also including current mirror circuitry for switching the first transistor into a conducting state when the output voltage of the switching power supply decreases below the threshold level;
• and
• programming circuitry for programming the shift rate at which the frequency of the PWM pulses is shifted;
• wherein the frequency shift circuit further comprises:
• a reference voltage source;
• and
• a second transistor having its base connected to the reference voltage source and its collector connected to the current mirror circuitry;
• wherein the programming circuitry includes:
• a first resistor for setting the shift rate at which the frequency of the PWM pulses is shifted, the first resistor being connected between the emitter of the second transistor and the output of the switching power supply;
• wherein, the second transistor is switched into a conducting state when the output voltage of the switching power supply decreases below the threshold level.