Miscellaneous active electrical nonlinear devices – circuits – and – External effect – Temperature
Reexamination Certificate
2001-09-10
2002-12-17
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
External effect
Temperature
C327S512000
Reexamination Certificate
active
06496052
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to switching regulators and driver circuits. In particular, the present invention relates to a method and apparatus that provides for an improved temperature coefficient for the current limit in a switching regulator circuit. The improved temperature coefficient in the current limit may also be employed for use in a driver circuit such as an RS-232 driver.
BACKGROUND OF THE INVENTION
Current limit is an important parameter for switching regulator and driver circuits. A precisely controlled value for the current limit in a wide temperature range is always desired. An example of a current sensor circuit (
100
) that may be used in a switching regulator circuit is shown in FIG.
1
. Switching regulator circuit
100
includes a comparator (
110
), a differential gain amplifier (
120
), a reference voltage (V
REF
), a bi-polar junction transistor (BJT) (Q
1
), a diode (D
1
), an internal voltage reference (V
REF(I)
), a sense resistor (R
S
), and two resistors (R
1
and R
2
).
Comparator
110
includes a sensor input (SNS) that is coupled to node N
10
, a reference input (REF) that is coupled to node N
11
, and an output that is coupled to node N
16
. Differential gain amplifier
120
includes differential inputs that are coupled to node N
12
and power supply node N
PS10
, and an output that is coupled to node N
10
. Internal voltage reference V
REF(I)
is coupled between node N
11
and node N
13
. Diode D
1
is coupled between node N
13
and node N
14
. Transistor Q
1
includes a base that is coupled to node N
15
, an emitter that is coupled to node N
14
, and a collector that is coupled to power supply node N
PS11
. Resistor R
1
is coupled between node N
15
and power supply node N
PS12
. Resistor R
2
is coupled between node N
15
and power supply node N
PS11
.
The resistors (R
1
, R
2
) operate as a voltage divider. In this example, sense resistor R
S
is located “off-chip” as can be resistors R
1
and R
2
. A circuit ground potential (GND) is coupled to power supply node N
PS11
.
In operation, comparator
110
produces an output signal when the voltage signal level at node N
10
exceeds the signal level at node N
11
. Differential gain amplifier
120
produces the output signal based on the voltage drop across the sense resistor (R
S
). The voltage drop across the sense resistor (R
S
) is equal to the product of the value of the sense current (I
S
) and the value of the sense resistor (R
S
). The differential gain amplifier (
120
) then takes the resulting voltage value (I
S
·R
S
) and scales it (e.g., ×3). This scaled value is outputted by differential gain amplifier
120
at node N
10
. The sensor input (SNS) receives a signal from the output of the differential gain amplifier (
120
).
Similarly, the reference input (REF) receives a signal from the output of transistor Q
1
. Transistor Q
1
produces the signal based on the voltage present across resistor (R
2
) of the voltage divider (R
1
, R
2
) due to the reference voltage (V
REF
). Diode D
1
provides an offset to compensate for the voltage drop (V
BE
) across the base-emitter junction. The internal reference (V
REF(2)
) voltage is provided as a design adjustment. Therefore, the signal received at the reference input (REF) of the comparator (
110
) is proportional to the reference voltage (V
REF(1)
). The equation for the switching regulator circuit (
100
) of
FIG. 1
, when the sensor current (I
S
) is at its peak value, can be expressed as follows:
3·
I
S
·R
S
=[(
R
2
·V
REF(1)
)/(
R
1
+R
2
)]−
V
REF(2)
I
S
={[(
R
2
·V
REF(1)
)/(
R
1
+R
2
)]−
V
REF(2)
}/(3·
R
S
)
The value of I
S
depends on the ratio of R
2
/(R
1
+R
2
), the value of V
REF(1)
and V
REF(2)
, and R
S
. Assuming V
REF(1)
and V
REF(2)
have no appreciable temperature coefficient, the temperature coefficient of I
S
is only dependent upon the temperature coefficient of R
S
. R
S
may be an equivalent resistance such as the on resistance (R
DS(ON)
) of a MOSFET transistor in a switching regulator. In this instance, the I
S
·R
S
has a large temperature coefficient that is on the order of 4000 ppm/° C., which is intolerable in some applications.
SUMMARY OF THE INVENTION
The present invention is directed to a method and an apparatus that improves the temperature coefficient for the current limit in a switching regulator, and also in driver circuits. An improved switching regulator/driver circuit includes “on-chip” resistance circuits that allow for a reduced temperature coefficient associated with the current limit. The improved temperature coefficient of the current limit may be arranged to provide for a constant current limit in the switching regulator or driver circuit. High output currents are limited over a wide range of temperature changes, providing for improved protection to the switching regulator or driver circuit.
Briefly stated, a method and apparatus is provided that is directed to generating an improved temperature coefficient for the current limit in a switching regulator circuit. A current limit sense circuit is employed that includes a comparator that compares two signals to determine when the current limit has been exceeded. One signal is produced from an input voltage source that has no temperature coefficient, a trans-conductance cell, and a sensor resistor circuit. An active output circuit produces another signal that corresponds to the current associated with the switching regulator circuit. The current sensed by the regulator is temperature dependent due to the resistances in the active output circuit, the sensor resistor circuit, and the trans-conductance cell. Each of these resistances has a temperature coefficient. The sum of the temperature coefficients determines the amount of temperature dependence in the sensed switching regulator circuit current. The resistance materials are chosen such that the temperature dependence of the sensed current is minimized. Also, all resistors are integrated into a single chip, reducing the costs associated with external pins and external resistor components. A similar arrangement may be applied to a driver circuit with a limited current output.
REFERENCES:
patent: 4484089 (1984-11-01), Viswanathan
patent: 5825234 (1998-10-01), Sung et al.
patent: 6366068 (2002-04-01), Morishita
patent: 6441674 (2002-08-01), Lin
“A New Integrated Circuit for Current Mode Control,” Unitrode Application Note U-93 (1997), pp. 3-1 to 3-8.
Callahan Timothy P.
Hertzberg Brett A.
Merchant & Gould
National Semiconductor Corporation
Nguyen Linh
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