Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-07-30
2003-01-21
Barhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S314000
Reexamination Certificate
active
06509726
ABSTRACT:
BACKGROUND
The invention generally relates to powering up a bandgap reference circuit.
Bandgap reference circuits are typically chosen due to their ability to produce reference voltages that vary little with temperature. For example,
FIG. 1
depicts a typical bandgap reference circuit
10
. The circuit
10
includes a high gain operational amplifier
12
, three resistors
14
,
16
and
17
and two PEP bipolar junction transistors (BATS)
18
and
20
.
Regarding the specific structure of the bandgap reference circuit
10
, the output terminal of the amplifier
12
provides a bandgap reference voltage (called “Vbg”). Each BJT
18
and
20
has its base terminal coupled to its collector terminal, and the collector terminal of each BJT
18
,
20
is coupled to ground. The emitter terminal of the BJT
18
is coupled to the output terminal of the amplifier
12
through the resistors
14
and
17
. The emitter terminal of the BJT
20
is coupled to the output terminal of the amplifier
12
through the resistor
16
. The inverting input terminal of the amplifier
12
is coupled to a node between the resistors
14
and
17
, and the non-inverting input terminal of the amplifier
12
is coupled to the emitter terminal of the BJT
20
. As depicted in
FIG. 1
, a current called I
1
flows through the emitter-collector path of the BJT
18
, and a current called I
2
flows through the emitter-collector path of the BJT
20
.
Due to the high gain of the amplifier
12
, the non-inverting and inverting input terminals of the amplifier
12
are approximately equal to establish the following relationship:
Vbe
1
+I
1
*
R
3
=Vbe
2
, Equation 1
where “Vbe
1
” and “Vbe
2
” are the base-emitter voltages of the BATS
18
and
20
, respectively, and “R
3
” represents the resistance of the resistor
17
. From this relationship, the I
1
current may be calculated as described below:
I
1
=(
Vbe
2
−Vbe
1
)/
R
3
Equation 2
If it is assumed that the resistors
14
and
16
have the same resistances, then the I
2
current equals the I
1
current, and from Equations 1 and 2, the Vbg bandgap reference voltage may be calculated as described below:
Vbg=Vbe
1
+(1
+R
1
/R
3
)*(
Vt
*ln(
n
)), Equation 3
where “Vt” is the thermal voltage that is equal to approximately 25.875 mV at room temperature, “n” is the ratio of the areas of the BATS
18
and
20
and “R
1
” is the resistance of the resistor
14
,
16
.
In Equation 3, the Vbel voltage has a negative proportional-to-absolute-temperature (PTAT) coefficient, and the second term on the right-hand side of the equation has a positive PTAT. Therefore, by controlling the ratio of the resistances
14
and
17
and the ratio n, the Vbg bandgap reference voltage may have very little dependency on temperature.
However, a potential difficulty with the bandgap reference circuit
10
is that there are two possible solutions for Vbg in Equation 3. Thus, the Vbg bandgap reference voltage may be either a well-controlled voltage (1.25 volts, for example) as desired, but the Vbg voltage may also be zero volts. For example, a scenario in which the Vbg bandgap reference voltage is zero volts may occur due to the circuit
10
being powered down, a state of the circuit
10
in which the Vbg bandgap reference voltage is zero volts. When the bandgap reference circuit
10
powers up and transitions into its normal mode of operation, however, the Vbg bandgap reference voltage may not change from zero volts.
Referring to
FIG. 2
, to prevent the above-described scenario from occurring, a start-up circuit, such as a start-up circuit
30
that is depicted in
FIG. 2
, typically accompanies the bandgap reference circuit
10
and is used for the purpose of ensuring that the Vbg bandgap reference voltage indicates the desired solution to Equation 3. The start-up circuit
30
may include several resistors, such as an explicit resistor
32
and n-channel metal-oxide-semiconductor field-effect-transistors (NMOSFETs)
34
,
36
and
38
that are configured as resistors. These resistors form a resistor divider to scale down a supply voltage (called Vcc) to provide a voltage and a current to the emitter terminal of the BJT
20
. Due to this arrangement, when the bandgap reference circuit
10
powers up, current flows through the emitter-collector path of the BJT
20
to produce a nonzero voltage at the non-inverting input terminal of the amplifier
12
. This voltage, in turn, produces a nonzero voltage at the inverting input terminal of the amplifier
12
if the input voltage swing of the amplifier
12
is sufficient. Thus, non-zero voltages and currents that are produced by the start-up circuit
30
should ideally prevent the Vbg bandgap reference voltage from being zero volts after power up.
There are potential drawbacks to the start-up circuit
30
. For example, the amplifier
12
may not operate correctly if the Vbe
2
voltage is too low, thereby causing the Vbg bandgap reference voltage to still come up at zero volts. Furthermore, the start-up circuit
30
consumes current during the normal mode of operation of the bandgap reference circuit
10
, after the power-up has been completed. This may be disadvantageous if the bandgap reference circuit
10
is used in, for example, a wireless or portable product that requires low power operation.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
REFERENCES:
patent: 4857823 (1989-08-01), Bitting
patent: 5610506 (1997-03-01), McIntyre
patent: 6150872 (2000-11-01), McNeill et al.
patent: 6232829 (2001-05-01), Dow
patent: 6313615 (2001-11-01), Fayneh et al.
Barhane Adolf Deneke
Intel Corporation
Trop Pruner & Hu P.C.
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