Reference-corrected ratiometric MOS current sensing circuit

Details Reference-corrected ratiometric MOS current sensing circuit Reference-corrected ratiometric MOS current sensing circuit

2000-07-14

2001-10-16

Kim, Jung Ho (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S543000

Reexamination Certificate

active

06304108

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to current sensing circuits and methods; and in particular, the present invention relates to a ratiometric current sensing circuit for accurately sensing the current flowing through a power-controlling pass device.
2. Background of the Invention
In circuits employing a power switch for power switching or power distribution functions, there is often a need to sense the current passing through the power switch. For example, current sensing is needed to monitor the load current passing through the power switch and the load coupled to the power switch. Current sensing is also needed to control and limit the load current in order to prevent damage to the load or to the power switch itself. Power switches are commonly implemented as n-channel or p-channel MOS devices. Although the current through the power switch can be sensed directly by placing a resistor in series with the power switch, this arrangement is undesirable because the resistor conducts the entire current through the power switch, resulting in a large power dissipation. Instead, a ratiometric current sensing technique is typically used for MOS power switches. In ratiometric current sensing, the current through the power switch is measured using a sense device which matches the power switch in electrical characteristics but is smaller by a known factor. The current through the sense device, which is a known ratio of the current through the power switch, is measured using a resistor connected in series with the sense device. The size of the sense device can be made small enough such that the current through the sense device is measured without undesirable power dissipation.
A conventional ratiometric current sensing circuit for use with a MOS power switch is illustrated in FIG.
1
. Current sensing circuit
10
for sensing the current through a power device M
Power
and a load
13
includes a sense device M
Sense
and a resistor R
Sense
connected in series. Power device M
Power
and sense device M
Sense
are matching n-channel MOS transistors. Sense device M
Sense
is chosen to be K times smaller than power device M
Power
. Typically, K is in the range of 1000 or more. The gate terminals of power device M
Power
and sense device M
Sense
are connected together and the source terminals of both devices are connected together to a ground terminal (node
15
). Therefore, power device M
Power
and sense device M
Sense
are driven with identical gate to source voltages. An input voltage V
in
from an input voltage source
12
is applied across load
13
and power device M
Power
. A load current flowing through load
13
is equivalent to the drain current I
DS,P
of power device M
Power
.
Resistor R
Sense
is connected between the drain terminal (node
14
) of power device M
Power
and the drain terminal (node
16
) of sense device M
Sense
and is used to measure the current flowing through the sense device M
Sense
. As long as the voltage across resistor R
Sense
is small compared to the drain-to-source voltage of M
Sense
, the drain-to-source voltages across power device M
Power
and sense device M
Sense
are essentially equal. Since the power device and the sense device have the same drain-to-source voltages and the same gate-to-source voltages, the drain current I
DS,S
of sense device M
Sense
is essentially I
DS,P
/K. A voltage drop develops across resistor R
Sense
which is equal to the product of the drain current I
DS,S
of sense device M
Sense
and the resistance of resistor R
Sense
.
The sensed current of sense device M
Sense
and the sensed voltage of sense resistor R
Sense
can be used to control circuit protection mechanisms for preventing excessive current flow in power device M
Power
and load
13
. To that end, current sense circuit
10
further includes an error amplifier
20
, a reference current source
19
, and a reference resistor R
Ref
. Reference current source
19
provides a fixed reference current I
Ref0
which flows through reference resistor R
Ref
and generates a reference voltage across the reference resistor. Reference resistor R
Ref
and sense resistor R
Sense
are either matching resistors having the same resistance values or resistors having ratioed resistance values. Error amplifier
20
compares the voltage across reference resistor R
Ref
(node
18
) and the voltage across sense resistor R
Sense
(node
16
) and provides a control signal on lead
17
to the gate terminals of sense device M
Sense
and power device M
Power
. In operation, the reference current I
Ref0
is selected so as to set the current limit of power device M
Power
. Error amplifier
20
operates to limit the power device's current whenever the sensed voltage at sense resistor R
Sense
is equal to or exceeds the reference voltage generated by reference resistor R
Ref
. When a current limit condition is detected, error amplifier
20
regulates the gate-to-source voltages of power device M
Power
and sense device M
Sense
to limit the current through the sense device to the maximum allowable current value of I
Ref0
.
As mentioned above, in current sense circuit
10
of
FIG. 1
, as long as the voltage drop across sense resistor R
Sense
is negligible as compared to the voltage drop across sense device M
Sense
, the drain-to-source voltages across the power device M
Power
and the sense device M
Sense
are essentially equal and the current through the sense device tracks the current through the power device. The drain current I
DS,P
through power device M
Power
and load
13
is given by:
I
DS,P
<=K*I
DS,S
*R
Ref
/R
Sense
,
 =K *I
Ref0
*R
Ref
/R
Sense
.
Through the use of a scaled-down sense device, current sensing circuit
10
operates at a low power dissipation level because the sensed current I
DS,S
is only a fraction of the power device's actual current. Furthermore, current sensing circuit
10
is applicable when the power device is biased either in the saturation region or in the linear (triode) region.
However, conventional current sensing circuit
10
has a significant drawback. In particular, conventional current sensing circuit
10
becomes grossly inaccurate when the power device is operated in the linear region where the drain-to-source voltage across the power device is small. In this case, the voltage drop across the sense resistor is no longer negligible and the drain voltage at the sense device does not track that of the power device. Thus, sense device M
Sense
grossly underestimates the power device's current.
For sense device M
Sense
to measure the power device current accurately, the terminal conditions of the two devices should be equal. That is, the gate-to-source voltages and the drain-to-source voltages should be the same for both devices. However, by virtue of the use of sense resistor R
Sense
, some voltage is dropped across the sense resistor. Consequently, the drain voltage at sense device M
Sense
is less than the drain voltage at power device M
Power
. In the case where the drain-to-source voltage across the power device is large, the voltage drop across the sense resistor is negligible and the drain-to-source voltages of the power and sense devices are essentially equal. However, when the drain-to-source voltage across power device M
Power
is small, the voltage drop across resistor R
Sense
is large compared with the drain-to-source voltage of power device M
Power
such that the drain voltage of the sense device is significantly less than the drain voltage of the power device. The disparity in the drain voltages results in a disparity in the drain current of the two devices such that the sense device grossly underestimates the current flow in the power device.
FIGS. 10
a-c
are graphs of the current and voltage characteristics obtained by simulation of the conventional current sensing circuit
20
in FIG.
13
. Current sensing circuit
20
is constructed in the same manner as conventional current sensing circuit
10
with the only exception that the load, including load

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