Accurate, fast, and user programmable hysteretic comparator

Details Accurate, fast, and user programmable hysteretic comparator Accurate, fast, and user programmable hysteretic comparator

1998-12-28

2001-05-08

Callahan, Timothy P. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S206000, C363S041000

Reexamination Certificate

active

06229350

ABSTRACT:

A. BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to integrated circuits and, more particularly, to integrated circuits having voltage regulator circuits generating an internal power supply voltage from an external power supply voltage.
2. Relevant Background
Integrated circuits (ICs) comprise thousands or millions of individual devices interconnected to provide desired functionality. Significant effort is expended to improve processing techniques so as to reduce the size of each individual device in order to provide greater functionality on a given IC chip at reduced cost. In general, smaller geometry devices operate faster while dissipating less power than do larger geometry devices. As device geometries are reduced the breakdown voltages of the devices and the isolation that separates the devices decreases also.
Electronic systems usually comprise ICs manufactured with a variety of technologies. This has created a need for multiple power supply voltages to be supplied to a single printed circuit board to support the various types of devices on that board. For example, devices are available that require a power supply voltages ranging from 5.0 volts to 3.3 volts, 2.8 volts or lower. A practical solution to this disparity is to provide voltage regulator circuitry that decreases the higher voltage (e.g., 5.0 V in the above example) to the lower voltage required by the small geometry device (e.g., 3.3 V or 2.8 V). Hence, it is necessary to regulate the available power supply voltage to provide voltages consistent with that required by each of the small geometry ICs.
A conventional voltage regulator is designed to generate a lower voltage than the available supply voltage. Typically, a transistor is coupled in series between the external voltage node and the internal voltage supply node. The conductivity of the transistor is modulated to drop the excess voltage across the transistor. Linear regulators have many desirable characteristics such as simplicity, low output ripple, high quality line and load regulation, and fast recovery time. However, linear regulators are inefficient resulting in wasted power and excess heat generation.
Switching regulators are becoming more common because of their characteristic high efficiency and high power density (i.e., power-to-volume ratio) resulting from smaller magnetic, capacitive, and heat sink components. Switching regulators convert one DC voltage into another DC voltage by selectively storing energy by switching energy on and off in an inductor. By comparing the output voltage to a reference voltage the inductor current is controlled to provide the desired output voltage.
Switching regulators exhibit longer hold-up times than linear regulators which is a characteristic that is important in computer applications. Switching regulators accept a wider range of input voltages with little effect on efficiency making them particularly useful in battery powered applications. However, peak-to-peak output voltage ripple of a switching regulator is typically greater than that of linear regulators. Hence, significant development effort is directed at reducing the voltage ripple of switching regulators.
To limit undesirable voltage ripple on the internal voltage supply node, the time constant of the regulator is desirably much longer than the internal cycle of the loading device. This prevents undesired voltage ripple within a cycle that can upset analog voltage levels. One way of controlling ripple is to heavily filter the regulator output by coupling a large capacitor between the internal voltage supply node and ground. In practice, however, filter capacitors consume a great deal of area without adding functionality. Cost and circuit size considerations dictate limiting the filter capacitor to more modest sizes. Hence, it is desirable to minimize voltage ripple in ways that do not require large filter capacitors.
Another technique to effectively increase the time constant of the regulator is to use hysteretic comparators to compare the output voltage to a reference voltage. The hysteretic comparator output drives a switching transistor that controls current in the inductor. However, it is difficult to generate accurate hysteresis as well as provide the ability to program the hysteresis using off-chip components. One prior solution is to use a Schmidtt trigger with an amplifier/comparator having an output and a non-inverting input brought out to pins of the IC. Although this allows the user to program the hysteresis by connecting the external feedback resistor, in many cases, the internal resistor that defines the hysteresis cannot be connected externally because the reference voltage used is not allocated a pin. As a result, the hysteresis is not accurate because the temperature coefficients of the internal and external resistors do not track. Furthermore, the internal and external resistors do not match because they are physically different. Although this limitation can be overcome by bringing the reference voltage out to a pin of the IC, this solution degrades the system's noise performance as well as raises the cost to manufacture the device. Moreover, the load capacitance created by the pins is significant making the design more complex in addition to degrading the overall performance device.
Another solution is to provide a regulator with fixed hysteresis (i.e., all hysteresis determining components are located on chip). Some control could be provided by using a digitally controlled hysteresis network to select the value of the hysteresis determining components. As the selection set increases the number of pins required increases. A user could than select the hysteresis value from among a finite number of choices dictated by the digital input of the hysteresis network. Although this solution yields high speed and accuracy, this comes at a cost of limited user programmability and increased pin allocation. For example, for a two-bit word, requiring two input pins, only four hysteretic settings can be programmed.
B. SUMMARY OF THE INVENTION
The present invention involves a hysteretic comparator for comparing a sample voltage to a reference voltage V
REF
. A hysteresis voltage generator providing a voltage V
HYST−
and a voltage V
HYST+
. A first differential input stage generates a signal coupled to a summing node determined from a difference between the sample voltage and V
REF
. A second differential input stage generates a signal coupled to the summing node determined from a positive difference between V
HYST−
and V
HYST+
. A third differential input stage generates a signal coupled to the summing node determined from a negative difference between V
HYST−
and V
HYST+
. A control device coupled to selectively enable the second and third differential input stages to select among the first mode and second mode.
In another aspect, the present invention involves a method of generating a pulse width modulated signal for driving an output stage of a DC regulator. The DC regulator includes an input stage receiving an input voltage V
IN
and an output stage providing an output voltage V
OUT
. A reference voltage V
REF
and a hysteresis voltage V
HYST
are generated. V
OUT
to V
REF
are compared to determine a difference signal. In a first mode, V
HYST
is added to the difference signal to generate a trigger signal. In a second mode V
HYST
is subtracted from the difference signal to generate the trigger signal. The pulse width modulated signal is generated by amplifying the trigger signal.


REFERENCES:
patent: 5528185 (1996-06-01), Lewicki et al.
patent: 5656957 (1997-08-01), Marlow et al.
patent: 5745352 (1998-04-01), Sandri et al.
patent: 5917313 (1999-06-01), Callahan, Jr.

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