Converter circuit and control method thereof
A converter circuit may require sensing an average current as feedback for regulating the average current supplied to a load. Sensing the average current may be inaccurate due to the non-ideal behavior of devices in the converter circuit. The disclosed circuits and methods help to improve the accuracy of the sensed average current by ignoring portions of a PWM cycle. Some of the ignored portions are based on a peak threshold for a rising current of a PWM cycle and a valley threshold for a falling current of a PWM cycle. The peak threshold and the valley threshold may be adjusted to control the average current and the switching frequency of the converter circuit.
1 . A converter circuit comprising:
a high-side switch configured to conduct a rising current for an ON period to charge a coil when the high-side switch is configured in an ON condition;
a high-side sensor coupled to the high-side switch configured to sense the rising current; and
a high-side blanking circuit configured to block an averaging subcircuit from receiving the rising current for a first blanking period and a second blanking period during the ON period, the first blanking period starting after the high-side switch is configured in the ON condition and the second blanking period starting after the rising current reaches a peak threshold, the averaging subcircuit configured to output an average rising-current for the ON period based on the rising current between the first blanking period and the second blanking period, which is not blocked by the high-side blanking circuit.
2 . The converter circuit according to claim 1 , further comprising:
a low-side switch configured to conduct a falling current for an OFF period as the coil discharges when the low-side switch is configured in the ON condition;
a low-side sensor coupled to the low-side switch configured to sense the falling current; and
a low-side blanking circuit configured to block the averaging subcircuit from receiving the falling current for a third blanking period and a fourth blanking period during the OFF period, the third blanking period starting after the low-side switch is configured in the ON condition and the fourth blanking period starting after the falling current reaches a valley threshold, the averaging subcircuit configured to compute an average falling-current for the OFF period based on the falling current between the third blanking period and the fourth blanking period, which is not blocked by the low-side blanking circuit.
3 . The converter circuit according to claim 2 , further comprising a controller configured to:
compute an average current based on the average rising-current and the average falling-current; and
adjust the peak threshold and the valley threshold based on the average current.
4 . The converter circuit according to claim 3 , wherein the peak threshold and the valley threshold are separated by a range, the range shifted up or down based on the average current.
5 . The converter circuit according to claim 2 , further comprising a controller configured to:
compute a period based on a time corresponding to the rising current and the falling current; and
adjust the peak threshold and the valley threshold based on the period.
6 . The converter circuit according to claim 5 , wherein the peak threshold and the valley threshold are separated by a range, the range increased or decreased based on the period.
7 . The converter circuit according to claim 2 , further including:
a high-side comparator coupled to the high-side sensor and the peak threshold, the high-side comparator configured to output a first trigger signal when the rising current equals the peak threshold;
a high-side delay circuit coupled to the high-side comparator and activated by the first trigger signal to output a high-side switching signal after a first delay;
a high-side driver coupled between the high-side delay circuit and the high-side switch, the high-side driver configured to control the high-side switch in an OFF condition based on the high-side switching signal;
a low-side comparator coupled to the low-side sensor and the valley threshold, the low-side comparator configured to output a second trigger signal when the falling current equals the valley threshold;
a low-side delay circuit coupled to the low-side comparator and activated by the second trigger signal to output a low-side switching signal after a second delay; and
a low-side driver coupled between the low-side delay circuit and the low-side switch, the low-side driver configured to control the low-side switch in the OFF condition based on the low-side switching signal.
8 . The converter circuit according to claim 7 , wherein:
the first delay equals the first blanking period; and
the second delay equals the third blanking period.
9 . The converter circuit according to claim 2 , wherein:
the first blanking period and the second blanking period are equal; and
the third blanking period and the fourth blanking period are equal.
10 . The converter circuit according to claim 2 , wherein the high-side sensor and the low-side sensor are integrated within a semiconductor package containing the high-side switch and the low-side switch.
11 . The converter circuit according to claim 1 , wherein the averaging subcircuit is a low-pass filter.
12 . A method for controlling a converter circuit, the method comprising:
turning ON a high-side switch for an ON period to conduct a rising current to charge a coil;
sensing the rising current;
blanking the rising current for a first blanking period and a second blanking period during the ON period, the first blanking period starting after turning ON the high-side switch and the second blanking period starting after the rising current reaches a peak threshold;
computing an average rising-current for the ON period based on the rising current between the first blanking period and the second blanking period;
turning ON a low-side switch for an OFF period to conduct a falling current as the coil discharges;
sensing the falling current;
blanking the falling current for a third blanking period and a fourth blanking period during the OFF period, the third blanking period starting after turning ON the low-side switch and the fourth blanking period starting after the falling current reaches a valley threshold;
computing an average falling-current for the OFF period based on the falling current between the third blanking period and the fourth blanking period;
computing an average current for a pulse width modulation (PWM) cycle based on the average rising-current and the average falling-current; and
adjusting the peak threshold and the valley threshold based on the average current.
13 . The method according to claim 12 , wherein:
the first blanking period equals the second blanking period; and
the third blanking period equals the fourth blanking period.
14 . The method according to claim 12 , wherein adjusting the peak threshold and the valley threshold based on the average current includes:
increasing the peak threshold and the valley threshold when the average current for the PWM cycle is below a regulation value; and
decreasing the peak threshold and the valley threshold when the average current for the PWM cycle is above the regulation value.
15 . The method according to claim 14 , wherein:
the peak threshold and the valley threshold are increased or decreased equally to maintain a range of currents defined as a difference between the peak threshold and the valley threshold.
16 . The method according to claim 12 , further comprising:
computing a period of the PWM cycle based on the ON period and the OFF period; and
adjusting the peak threshold and the valley threshold based on the period of the PWM cycle.
17 . The method according to claim 16 , wherein adjusting the peak threshold and the valley threshold based on the period of the PWM cycle includes:
increasing the peak threshold and decreasing the valley threshold when the period of the PWM cycle is below a target value; and
decreasing the peak threshold and increasing the valley threshold when the period of the PWM cycle is above the target value.
18 . The method according to claim 12 , further comprising:
computing the average current for the PWM cycle based on only the average falling-current when the rising current reaches the peak threshold before an end of the first blanking period.
19 . An LED driver comprising:
a high-side switch configured to conduct a rising current when the high-side switch is turned ON for an ON period of a pulse width modulation (PWM) cycle;
a high-side sensor coupled to the high-side switch configured to sense the rising current;
a low-side switch configured to conduct a falling current when the low-side switch is turned ON for an OFF period of the PWM cycle;
an averaging subcircuit configured to output an average current based on:
an average rising-current for the ON period based on the rising current between a first blanking period and a second blanking period of the ON period, the first blanking period starting after the high-side switch is turned ON and the second blanking period starting after the rising current reaches a peak threshold; and
an average falling-current for the OFF period based on the falling current between a third blanking period and a fourth blanking period of the OFF period, the third blanking period starting after the low-side switch is turned ON and the fourth blanking period starting after the falling current reaches a valley threshold; and
a controller configured to control the peak threshold and the valley threshold based on the average current.
20 . The LED driver according to claim 19 , further comprising:
a high-side delay configured to delay turning the high-side switch OFF for the second blanking period after the rising current reaches the peak threshold; and
a low-side delay configured to delay turning the low-side switch OFF for the fourth blanking period after the falling current reaches the valley threshold.