Band-gap reference (BGR) circuits provide stable reference voltages that vary little with process, supply-voltage, and temperature (PVT). Many circuits—including dynamic random-access memories, flash memories, and analog devices—employ BGR circuits.
The band-gap voltage Vbg of conventional BGR circuits is typically about 1.25 volts. Modern integrated circuits, however, are using ever-lower supply voltages, putting downward pressure on the output voltage of BGR circuits. Some modern devices, for example, employ 1.2-volt power supplies (e.g., Vdd is 1.2 volts), making it impractical to derive a 1.25-volt BGR voltage. Researchers have therefore directed their attention to the creation of BGR circuits with reduced band-gap voltage levels. For a more detailed discussion of BGR circuits, see U.S. Pat. No. 6,489,835 to Yu et al. and U.S. Pat. No. 6,323,630 to Banba, both of which are incorporated herein by reference.
The input terminals of differential amplifier Da1 connect to the drains of respective transistors P1 and P2 via respective voltage dividers R4 (resistors R4a and R4b) and R2 (resistors R2a and R2b). Assuming R2a=R4a and R2b=R4b gives:
I1a=I2a
I1b=I2b
Va=Vf1[R4b/(R4a+R4b)]
Vb=Vf2+dvf[R2b/(R2a+R2b)]
dVf=Vf1−Vf2
Because the voltage across R1 is dVf, this gives:
I2a=dVf/R1
I2b=Vf1/(R2a+R2b)
Thus, I2=I2a+I2b=[Vf1/(R2a+R2b)]+dvf/R1
The resistance ratio (R2a+R2b)/R1 can be set so that vref is not temperature dependent, and the resistance ratio R3/(R2a+R2b) can be used to adjust the Vref level within the range of the power supply. Voltage dividers R4 and R2 reduce voltages Va and Vb below Vf1, which may be advantageous for very low Vdd levels. The ratio between R2a and R2b (and similarly between R4a and R4b) is optimized for a given application. BGR circuit 100 is discussed in more detail in the above-referenced Banba patent.
Diagram 150 includes two response curves: a first curve 160 depicts the response of BGR signal Vbg to the application of supply voltage Vdd at a first temperature, and a second curve 165 depicts the response of BGR signal Vbg to the application of the same supply voltage Vdd at a second, lower temperature. The slower response of curve 165 indicates that the response of BGR circuit 100 shifts later in time with reduced temperatures. This shift occurs because the forward voltages vf of diodes D1 and D2 increase with reduced temperature, so Vdd must rise higher before diodes D1 and D2 conduct.
Typical integrated circuits (ICs) function over a range of power-supply voltages, and can be expected to fail if operating with supply voltage outside this range. ICs therefore commonly include a so-called “Power-On-Reset” (POR) circuit that resets the IC to a known state upon application of power and holds the known state until the power supply voltages settle at or near some predetermined level. Typically, the POR circuit is powered by the same source as the rest of the IC.
Reference voltage Va is merely a divided version of supply voltage Vdd, and thus increases linearly in proportion to Vdd as Vdd ramps up from zero to 1.8 volts. In contrast, BGR signal Vbg ramps up in a non-linear fashion due to the non-linear components (diodes and transistors) employed to generate BGR signal Vbg. The level of BGR signal Vbg can therefore cross the level of reference voltage Va a number of times, producing one or more undesirable “windows” W in the POR signal. In this example, the low-temperature Vbg curve 165 produces a troublesome window, but the higher temperature Vbg curve 160 does not. The relationship between reference voltage va and curves 160 and 165 illustrate that POR circuits are especially susceptible to producing windows when operating at extreme values of process, temperature, and voltage. If such windows are unavoidable, it is preferred that they disappear early and at a relatively low vdd value.
The present invention is directed to power-on reset (POR) methods and circuits for resetting and subsequently enabling integrated circuits in response to applied power. A POR circuit in accordance with one embodiment is capable of operating at exceptionally low temperatures and supply voltages, and is relatively tolerant to process variations. To achieve these benefits, the POR circuit compares a conventional BGR signal with a temperature-compensated reference signal that varies in inverse proportion to temperature.
The claims, and not this summary, define the scope of the invention.
POR circuit 300 includes many components in common with POR circuit 200 of
When Vdd rises high enough to forward bias PMOS transistors P1, P2, and P3, transistor P1 provides current I1b through voltage divider R5, causing control signal Vtc to rise abruptly (time T1). Later, when Vdd reaches Vtc plus Vtp (vdd=vtc+vtp) to forward bias transistor 315, reference signal Vr rises abruptly (time T2). With both of transistors P1 and 315 forward biased, reference signal Vr rises linearly with supply voltage vdd.
Reference signal Vr, in comparison to reference signal Va of conventional POR circuit 200 of
While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, while a BGR circuit is modified in the foregoing examples to provide the temperature-compensated reference signal Vtc, alternative methods of providing the requisite temperature compensation might also be used. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance the method of interconnection establishes some desired electrical communication between two or more circuit nodes, or terminals. Such communication may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.
Number | Name | Date | Kind |
---|---|---|---|
5477176 | Chang et al. | Dec 1995 | A |
5497112 | Hoang | Mar 1996 | A |
5867047 | Kraus | Feb 1999 | A |
6239630 | Bowers et al. | May 2001 | B1 |
6323630 | Banba | Nov 2001 | B1 |
6377090 | Bruno | Apr 2002 | B1 |
6472912 | Chiu et al. | Oct 2002 | B1 |
6489835 | Yu et al. | Dec 2002 | B1 |
6509768 | Polizzi et al. | Jan 2003 | B2 |