Electricity: power supply or regulation systems – Input level responsive
Reexamination Certificate
2000-08-11
2001-10-09
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
Input level responsive
C323S222000, C323S285000
Reexamination Certificate
active
06300751
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a switching regulator control circuit. More specifically, the present invention is directed to a regulator control circuit capable of avoiding such a fact that when an impedance of an input power supply of a switching regulator is increased, a switch element is continuously turned ON and thus, a large current flows through a power supply and the switch element to thereby break down this switch element.
2. Description of the Related Art
FIG. 6
is a circuit diagram for representing one of the conventional switching regulator (SW regulator) control circuits. That is, there is provided an error amplifying circuit
13
which amplifies a difference voltage between a reference voltage “Vref” of a reference voltage circuit
10
and a voltage “Va” appeared at a junction point between a bleeder resistor
11
and a bleeder resistor
12
. The bleeder resistors
11
/
12
sub-divide an output voltage “Vout” of an output terminal
2
of an SW regulator. Assuming now that the output voltage of the error amplifier circuit
13
is “Verr”, the output voltage of the reference voltage circuit
10
is “Vref”, and the voltage appeared at the junction point between the bleeder resistor
11
and the bleeder resistor
12
is “Va”, if Vref>Va, then the output voltage “Verr” of the error amplifier circuit
13
is increased, whereas if Vref<Va, then this output voltage “Verr” is decreased. A pulse width control circuit
14
which enters the output voltage “Verr” of the error amplifying circuit
13
as an input signal controls the ON time of the switch element (SW element) and the OFF time thereof in response to a value of this output voltage “Verr”. The SW element is connected to an SW element drive circuit
16
so as to be turned ON/OFF.
The SW element drive circuit
16
is operated while the output voltage “Vout” is used as the power supply. The reference voltage circuit
10
, the error amplifying circuit
13
, and the pulse width control circuit
14
are operated while the voltage “Vin” of the input terminal
1
is used as the power supply. When a power MOS transistor is employed, for instance, as the SW element of the SW element drive circuit, if a high drive voltage (namely, gate-to-source voltage) is used, then the ON resistance of this power MOS transistor may be lowered. As a result, when the SW element drive circuit is driven by employing the boosted output voltage “Vout”, the efficiency of the SW regulator may be increased. A level shifter (will be referred to as an “L/S” hereinafter)
15
is employed between the pulse width control circuit
14
and the SW element drive circuit
16
, and converts a signal derived from the pulse width control circuit
14
of the Vin-power supply system into a signal level of the SW element drive circuit
16
of the Vout-power supply system having the different power supply voltage from that of the Vin-power supply system.
FIG. 7
shows an example of a step-up type SW regulator. In this SW regulator, both a coil
21
and an SW regulator control circuit
30
are connected to an input power supply
20
. A rectifying element
23
is connected between the coil
21
and an output capacitor
24
. A load
25
is connected parallel to the output capacitor
24
. In general, an output impedance
26
of the input power
20
is low, and therefore is negligible. However, in the case that an extraordinary condition happens to occur in the input power supply
20
, this output impedance
26
will have a certain impedance value. Also, when a cell and the like are employed as the input power supply
20
, the input power supply
20
will have an impedance value of approximately several &OHgr; to ten &OHgr;.
FIG. 8
indicates a waveform produced when the power supply is turned ON in such a case that the impedance
26
of the input power supply
20
of
FIG. 7
is negligibly small. FIG.
8
(
a
) shows both a voltage “V
20
” of the power supply
20
of
FIG. 7 and a
power supply voltage “Vin” of the SW regulator control circuit
30
, and FIG.
8
(
b
) represents an output voltage “Vout” of the SW regulator. In these drawings, abscissas denote time. Since the impedance
26
of the input power supply
20
is negligibly small, the waveform of “V
20
” is overlapped with the waveform of “Vin” in FIG.
8
(
a
). The reason why the output voltage “Vout” of FIG.
8
(
b
) is gradually increased is caused by a soft starting function of the SW regulator control circuit. This soft starting function is such a function that the output voltage is gradually increased in order that an overshoot phenomenon is not produced in the output voltage “Vout” when the power supply is turned ON. This soft starting function is not described in this specification.
FIG. 9
shows a waveform produced when the power supply is turned ON in the case that the impedance
26
of the input power supply
20
of
FIG. 7
is on the order of several &OHgr;. FIG.
9
(
a
) shows a voltage “V
20
” of the input power supply
20
and a power supply voltage “Vin” of the SW regulator control circuit
30
in
FIG. 7
, FIG.
9
(
b
) represents an output voltage “Vout” of the SW regulator, and FIG.
9
(
c
) denotes a current “I
20
” of the input power supply
20
. In FIG.
9
(
a
) to FIG.
9
(
c
), abscissas show time. When a current flows through the input power supply
20
by the impedance
26
of the input power supply
20
, the power supply voltage “Vin” of the SW regulator control circuit is decreased. In
FIG. 9
, while the SW regulator is operated in the step-up operation, a current flows through the input power supply
20
. As a result, the input voltage “Vin” of the SW regulator control circuit
30
is decreased lower than, or equal to the operation voltage of the SW regulator control circuit
30
, so that the SW regulator control circuit
30
cannot be operated under normal condition. Thus,
FIG. 9
represents such a condition that the output of the SW element drive circuit continuously turns ON the SW element. For example, in such a case that the value of the output voltage “V
20
” of the input power supply
20
is 2 V, the value of the output impedance
26
is 1.5 &OHgr;, and a current of 1 A flows through the input power supply
20
when the power supply is turned ON, the input voltage “Vin” of the SW regulator control circuit
30
is decreased up to 0.5 V. Assuming now that the minimum operation voltage of the SW regulator control circuit
30
is selected to be 1 V, the SW regulator control circuit
30
cannot be operated in the normal mode under this low-voltage condition, and also the output of the L/S
15
of
FIG. 6
becomes uncertain. As a result, when the voltage of the EXT terminal of the output of the SW element drive circuit
16
is stopped under such a condition that the SW element
22
of
FIG. 7
is turned ON, a large current continuously flows through the input power supply
20
, the coil
21
, and the SW element
22
. Thus, there is such a risk that these circuit elements are deteriorated, and will be broken down in the worst case.
However, in the conventional SW regulator, when the output impedance of the input power supply is increased, the following problem will occur. That is, while the SW regulator is operated under step-up operation, the power supply voltage of the SW regulator control circuit is lowered, the SW regulator control circuit cannot be operated under normal condition, and the SW element is continuously turned ON, so that the large current flows through the power supply circuit and the SW element, which may give damages to these circuit elements.
SUMMARY OF THE INVENTION
The present invention has been made to solve such a problem of the conventional SW regulator control circuit, and therefore, has an object to provide an SW regulator control circuit in which a voltage detecting circuit operable by an output voltage is provided, a voltage of an input power supply is detected by this voltage detecting circuit, and when the voltage of the input power supply is decreased lower than the operation
Adams & Wilks
Seiko Instruments Inc.
Sterrett Jeffrey
LandOfFree
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