Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
2000-07-28
2002-04-23
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S108000, C327S110000, C327S424000, C327S588000, C318S667000
Reexamination Certificate
active
06377109
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a driver circuit for controlling a high power load such as a actuator driver for CD or CD-ROM and a power amplifier for audio systems, and more particularly, to a current detector for use in such load driver circuits.
BACKGROUND OF THE INVENTION
Metal oxide semiconductor field effect transistors (MOSFETs) are widely used as output stage elements and control elements of driver circuits controlling high power loads such as a actuator driver for CD or CD-ROM and a power amplifier for audio systems. These MOSFETs are mostly controlled by controlling their gate voltages.
FIGS. 1 and 2
show typical load driver circuits having a single bridge structure (
FIG. 1
) and a three-phase bridge structure (FIG.
2
).
Load driving MOSFETs Q
1
-Q
4
of the load driver circuit shown in
FIG. 1
are controlled for conduction thereof by respective gate control signals
52
sent from a control circuit
51
. Normally, the load driving MOSFETs Q
1
and Q
4
are simultaneously turned ON in pairs to provide a load current Io in one direction or another in the bridge. So are the load driving MOSFETS Q
2
and Q
3
. The load current Io passing through a resistor for determining the current through a load L (referred to as current detection resistor Rs) generates a voltage drop across the resistor Rs, which is equal to the product Io*Rs. This voltage drop is amplified A times, say, by an amplifier
53
, which generates a detection voltage Vo.
The detection voltage Vo is fed back to the control circuit
51
along with a reference voltage Vi. The control circuits supplies gate control signals
52
to the load driving MOSFETs Q
1
-Q
4
. The gate control signals
52
can be voltage control signals or pulse control signals such as pulse width modulation (PWM) control signals. Through the feedback loop the load current Io through the load L is adjusted to a constant level associated with the reference voltage Vi until the detection voltage Vo matches the reference voltage Vi.
The load driver circuit of
FIG. 2
includes three loads LU, LV, and LW operable in three different phases and six load driving MOSFETs QU, QV, QW, QX, QY, and QZ forming an associated three-phase bridge for switching the three loads. Coupled to the three phase bridge is a control circuit
61
generating suitable three-phase gate control signals
62
for the three-phase bridge. An amplifier
63
is similar to the amplifier
53
of
FIG. 1
, and has a given amplification factor of A, for example. Since the load driver circuit of
FIG. 2
operates in the same manner as the one shown in
FIG. 1
, further details of the circuit will be omitted.
It is noted that in such conventional load driver circuits as described above, the current detection resistor Rs is connected to a power source or the ground in series with the load in order to detect the load current through the load L or one of LU, LV, and LW. The load current Io is normally in the range from the order of 100 mA to the order of 1 A, and the resistance of the current detection resistor Rs for generating a measurable detection voltage is in the range from the order of 0.1 ohms to the order of 1 ohm.
Therefore, the power consumed by the current detection resistor Rs itself is disadvantageously large. That is, it is not negligible as compared with the entire power consumption. In addition, since such nominally large load current Io passes the detection resistor Rs, the resistor must be fairly large in dimension, requiring an extra large space for connection thereof with the driver circuit on a semiconductor chip.
Another drawback pertinent to such current detection resistor is that since the current detection resistor Rs is inserted in series with the load, the output dynamic range of the load is limited by the current detection resistor Rs.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a load driver circuit including at least one FET such as MOSFET for driving a load, said MOSFET controlled by a control circuit providing gate control voltage to the load driving FET by means of a feedback loop connecting the FET and the control circuit, without utilizing any resistor (referred to as detection resistor) connected in series with the load to detect the load current. Because the invention utilizes a MOSFET instead of a resistor in detecting the load current, the control circuit:
(a) has a compact form, requiring only a minimum space for connection with external components;
(b) cuts down significant power loss that would be otherwise entailed by a current detection resistor;
(c) is capable of detecting the load current at high precision, thereby allowing precise control of the load current; and
(d) provides an improved output dynamic range.
In accordance with one embodiment of the invention, a load driver circuit comprises:
at least one load driving MOSFET;
means for detecting the level of a load current including:
a detection transistor;
a detection resistor connected in series with said detection transistor for detecting a voltage drop across said detection resistor; and
a difference amplifier for controlling said detection transistor, said difference amplifier fed at one input end thereof with a voltage drop across the drain and the source of said load driving MOSFET in conduction, and at the other input end thereof with said voltage drop across said detection resistor; and
control means for receiving a signal associated with said voltage drop across the drain and the source of said load driving MOSFET along with a reference value indicative of a target level for said signal, and generating gate control signal to said load driving MOSFET to thereby equilibrate said level of said signal with said reference value.
In actuality, the control means described above may receive a reference voltage to compare with the voltage drop across the drain and the source of the load driving MOSFET, or a reference current with which the load current Io as calculated from the detected voltage drop across the drain and the source of the load driving MOSFET is compared.
The inventive load driver circuit makes it unnecessary to use a current detection resistor in the driver circuit by detecting the voltage drop across the drain and the source of the load driving MOSFET. This is based on the fact that an excellent linear relationship exists between the current passing through the MOSFET and the voltage drop across the drain and the source of the load driving MOSFET over a certain range of voltage, so that the effective resistance is constant/invariable, which enables accurate detection of the voltage drop across the load.
It should be appreciated that no current detection resistor is required in the invention, so that power consumption due to such current detection resistor as is used in conventional load driver circuit is zero. In addition, no mounting space for a terminal for connection with such resistor is required on the semiconductor chip.
The elimination of a current detection resistor to be connected in series with the load and a load driving MOSFET as in conventional driver circuits widens the working range of voltage of the load, and hence the dynamic range of the driver circuit.
The means for detecting the level of a load current includes a detection transistor, a detection resistor, and a difference amplifier such that the current through the detection resistor (referred to as detection current) is far smaller than the load current, yet it is precisely proportional to the load current. Consequently, power consumption by the detection resistor can be made very small. Further, since the detection resistor can be fabricated on the same semiconductor chip as the load driving MOSFET, they can have the same temperature dependent characteristics.
In cases where the control circuit includes a multiplicity of load driving MOSFETs forming a multi-phase bridge configuration, they can be selectively turned on at different times or phases by a switching means to provide their voltage drops across the drain and the source to the diff
Cunningham Terry D.
Hogan & Hartson LLP
Nguyen Long
Rohm & Co., Ltd.
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