Field of the Invention
[0001] The present invention generally relates to electronic circuits, and, more particularly,
to regulators, and relates to a method for regulating.
Background of the Invention
[0002] A voltage regulator provides a substantially temperature independent output voltage
V
OUT to a consuming circuit ( "consumer circuit"). It is often required that the regulator
has a current limiting function which substantially reduces or cuts off output current
I
OUT when I
OUT reaches a maximum allowable current I
OUT MAX. The current limiting function should not interfere with the normal operation of
the regulator when (I
OUT ≤ I
OUT MAX). The time between reaching I
OUT MAX and reducing (and/or cutting off) I
OUT is the response time T
RESP and should be short enough to prevent damages in the current regulator and in the
consumer circuit.
[0003] FIG. 1 illustrates a simplified circuit diagram of prior art regulator 100. Regulator
100 supplies power (output voltage V
OUT', output current I
OUT) to consumer circuit 199 (illustrated dashed). Regulator 100 comprises output transistor
110, feedback resistors 115 and 116, current limiting portion 101 (dashed frame, with
transistor 120, resistor 130 and transistor 140), and driver portion 111 (dashed frame,
with transistors 150, 160, 170 and 180, current source 155, and resistors 165 and
166). Preferably, transistor 120 in current limiting portion 101 has an effective
emitter area which is the 1 / N fraction of the emitter area of output transistor
110.
[0004] The components of regulator 100 are coupled as follows: Output transistor 110 has
an emitter coupled to reference line 191 and a collector coupled to output node 105.
Output node 105 is coupled to reference line 192 via resistors 115 and 116.
[0005] In driver portion 111, transistor 180 is coupled in a current mirror arrangement
to transistor 110. Transistor 180 has an emitter coupled to reference line 191 and
a base and a collector coupled together to the base of transistor 110. The collector
of transistor 180 is coupled to a collector of transistor 170. An emitter of transistor
170 is coupled to reference line 192. Transistor 160 has a collector coupled to reference
line 191 via resistor 165 and has an emitter coupled to reference line 192 via resistor
166. The emitter of transistor 160 is coupled to a base of transistor 170. Current
source 155 is coupled between reference line 191 and an emitter of transistor 150.
The emitter of transistor 150 is also coupled to the base of transistor 160. A collector
of transistor 150 is coupled to reference line 192. Transistor 150 receives control
signal V
IN' at a base.
[0006] In current limiting portion 101, transistor 120 has a base coupled to the base of
transistor 110, an emitter coupled to reference line 191 and a collector coupled via
resistor 130 to reference line 192 (current I
CL'). A base-emitter path of transistor 140 is coupled in parallel to resistor 130 (between
the collector of transistor 120 and reference line 191). A collector of transistor
140 is coupled to the base of transistor 160 of driver portion 111.
[0007] Output transistor 110 provides output current I
OUT' on node 105 depending on control signal V
IN which is propagated by driver portion 111. The feedback signal V
FB at node 106 between resistors 115 and 116 can be feed back to control signal V
IN'. For simplicity, the feedback circuit is not illustrated.
[0008] In current limiting portion 101, transistor 120 provides auxiliary current I
CL' which is scaled to output current I
OUT' transistor 110 (preferably,
). Resistor 130 (value R
CL') provides voltage V
CL' proportional to auxiliary current I
CL'.
is applied as the base-emitter voltage V
BE' of transistor 140. When current I
OUT' reaches I
OUT MAX', I
CL' reaches a maximum value I
CL MAX'. Also, voltage V
CL' reaches V
BE START' (e.g., 0.6 volts) at which transistor 140 becomes conductive. This keeps the driving
capacity of driver portion 111 at a predetermined minimum. With driver portion 111
in such a state, transistor 110 substantially limits output current I
OUT'.
[0009] Current limiting portion 101 of regulator 100 exhibits an unwanted temperature dependency.
Base-emitter voltage V
BE' has a negative temperature coefficient (e.g., about -2mV / K) and V
CL' has a positive temperature coefficient. Also, limiting portion 101 acts as a feedback
path from output transistor 110 to driver portion 111 so that regulator 100 can oscillate
when I
OUT' is near I
OUT MAX'. To prevent oscillating, frequency compensation with additional components (e.g.,
capacitors on extra die area) should be used when necessary. But this is also not
wanted. The response time T
RESP' of regulator 100 is often not short enough. It is a further disadvantage of regulator
100 that current I
CL' in current limiting portion 101 contributes to the total power consumption of regulator
100 without contributing to output current I
OUT'. Regulator 100 can be modified as follows.
[0010] FIG. 2 illustrates a simplified circuit diagram of further prior art regulator 200.
Regulator 200 comprises output transistor 210, output resistors 215 and 216, current
limiting portion 201 (dashed frame, with resistor 230 and transistor 240), and driver
portion 211 (dashed frame, with transistors 250, 260 and 270, current source 255,
and resistors 265 and 266). The components of regulator 200 are coupled similarly
as in regulator 100 of FIG. 1. In FIGS. 1-2, reference numbers 105/205, 106/206, 110/210,
115/215, 116/216, 101/201, 111/211, 130/230, 140/240, 150/250, 155/255, 160/260, 165/265,
166/266, 170/270, 180/280, 191/291, and 192/292 refer to analogous components. However,
their function can be different as described in the following.
[0011] Output transistor 210 and transistor 280 form a current mirror. Auxiliary current
I
CL'' is substantially equal to the base current of output transistor 110 (or to a scaled
base current). Current I
CL'' is therefore related to I
OUT'' by the collector current to base current ratio ("β-value") of transistor 110 (preferably,
). Similar as in regulator 100, resistor 230 (value R
CL'') provides voltage V
CL'' proportional to auxiliary current I
CL''. Voltage V
CL'' (
) is applied as the base-emitter voltage V
BE'' of transistor 140. When current I
OUT'' reaches I
OUT MAX'', I
CL'' also reaches I
CL MAX'', voltage V
CL'' reaches V
BE START''. Transistor 140 keeps driver portion 211 at a predetermined driving capacity and
transistor 210 substantially limits output current I
OUT''. Regulator 200 consumes a smaller total current than regulator 100 (see transistor
120). But the other mentioned disadvantages remain the same.
[0012] The present invention seeks to provide regulators with current limiting function
which mitigate or avoid these and other disadvantages and limitations of the prior
art.
Brief Description of the Drawings
[0013]
- FIG. 1
- illustrates a simplified circuit diagram of a prior art regulator;
- FIG. 2
- illustrates a simplified circuit diagram of a further prior art regulator;
- FIG. 3
- illustrates a simplified circuit diagram of a regulator according to a first embodiment
of the present invention; and
- FIG. 4
- illustrates a simplified circuit diagram of a regulator according to a second embodiment
of the present invention.
Detailed Description of a Preferred Embodiment
[0014] Embodiments of a regulator with current limiting function of the present invention
are explained in connection with FIGS. 3-4. For convenience, the figures and the description
refer to bipolar transistors. Persons of skill in the art are able, based on the description
herein, to implement the present invention with other technologies, such as CMOS or
others.
[0015] The term "transistor" is intended to include any device or arrangement having at
least two main electrodes (e.g., emitters and collectors) and a control electrode
(e.g., a base). The impedance between the main electrodes is controlled by a signal
applied to the control electrode. The transistors are classified into npn-transistors
and pnp-transistors. This is convenient for the embodiments of the present invention,
but not essential. Persons of skill in the art are able, based on the description
herein, to use transistor types in other combinations without departing from the scope
of the invention.
[0016] Signals are conveniently introduced as voltages (e.g., V
IN, V
REF) or currents (I
B, I
CL) as they appear in the embodiments. Persons of skill in the art are able, based on
the description herein, to change voltage signals to current signals or vice versa
without departing from the scope of the present invention.
[0017] The present invention has a number of advantages over the prior art, such as:
(a) less temperature dependency,
(b) shorter response time T
RESP, and
(c) higher stability for high output currents (I
OUT ≈ I
OUT MAX) due to lower feedback gain.
[0018] FIG. 3 illustrates a simplified circuit diagram of regulator 300 according to a first
embodiment of the present invention. Regulator 300 has reference line 391 (e.g., voltage
V
CC), reference line 392 (e.g., ground "GND" at around zero volts), control terminal
394 (control signal V
IN), reference terminal 393 (reference signal V
REF), and feedback node 306 (feedback signal V
FB). Regulator 300 comprises output transistor 310, input transistor 350, clamp transistor
352, chain transistors 360 and 370, mirror transistor 380, resistors 315, 316, 330,
365, 366, and current source 355. In the example of FIG. 3, transistors 310, 350,
352 and 380 are pnp-transistors, and transistors 360 and 370 are npn-transistors.
As mentioned above, the transistor types are not important. Having chain transistors
360 and 370 of the same type (e.g., npn) is convenient. Transistors 350, 360, 370,
and 380, current source 355, and resistors 365 and 366 form driver portion 311 (dashed
frame). Transistors 360 and 370 are preferably arranged as emitter followers and form
chain 335 ("arrangement", dashed frame). Resistor 330 and clamp transistor 352 in
connection with other components in driver portion 311 provide the current limiting
function. Unlike as in the prior art examples of FIGS. 1-2, components of driver portion
311 also participate in limiting the output current. Regulator 300 receives input
current I
CC at reference line 391. Regulator provides power (output voltage V
OUT, output current I
OUT) at output node 305 to a consumer circuit (not illustrated in FIG. 3, cf. circuit
199 in FIG. 1). For controlling, an optional operational amplifier (not illustrated)
can have inputs at feedback node 306 and at reference terminal 393 and an output at
control terminal 394.
[0019] The components of regulator 300 are coupled as follows: Output transistor 310 has
an emitter coupled to reference line 391 and a collector coupled to output node 305.
Resistors 315 and 316 are serially coupled between output node 305 and reference line
392 via feedback node 306 (voltage sensor arrangement). Output transistor 310 and
mirror transistor 380 have their bases coupled together to a collector of transistor
380 and have emitters coupled to reference line 391 to form a current mirror. A collector
of transistor 380 is coupled to a collector of chain transistor 370. An emitter of
transistor 370 is coupled reference line 392 via resistor 330. Transistor 360 has
a collector coupled to reference line 391 via resistor 365, and has an emitter coupled
to reference line 392 via resistor 366. The emitter of transistor 360 is also coupled
to a base of transistor 370. Current source 355 is coupled between reference line
391 and intermediate node 351. Intermediate node 351 is coupled to a base of transistor
360, to an emitter of clamp transistor 352 and to an emitter of input transistor 350.
A collector of input transistor 350 is coupled to reference line 392. A base of input
transistor 350 is coupled to control terminal 394. A collector of clamp transistor
352 is coupled to reference terminal 392. A base of clamp transistor 352 is coupled
to reference terminal 393. In other words, input transistor 350 and clamp transistor
352 have their main electrodes couple in parallel between intermediate node 351 and
reference line 392.
[0020] Further current, voltages and resistor magnitudes are conveniently defined as follows:
Input current I
CC is the current going into regulator 300 at reference terminal 391. Output current
I
OUT is the collector current of output transistor 310 wherein the current through resistors
315 and 316 is neglected. Control current I
B is the base current of output transistor 310 and flow from the base of transistor
310 to reference terminal 392. For bipolar transistors operating such as transistor
310, the base current is substantially proportional to the collector current (β-relation
known in the art).
[0021] Preferably, resistor current I
CL through resistor 370 is substantially proportional to current I
B, that is:
wherein "k" is a proportional factor. This is convenient for explanation, but not
essential for the present invention. When the emitter current of mirror transistor
380 is neglected, then factor k is about k = 1.
[0022] Preferably, voltage drop V
CL is related to I
CL proportionally, that is:
wherein R
CL is the Ohmic resistance value of resistor 330. Such a linear relation is convenient,
but not essential for the present invention. V
BE3 is the base-emitter voltage of chain transistor 370, V
BE2 is the base-emitter voltage of chain transistor 360, and V
BE1 is the base emitter voltage of input transistor 350. Control signal V
IN is defined as the voltage between control terminal 394 and reference line 392 across
the base-collector path of input transistor 350, and reference voltage V
REF is defined as the voltage between reference terminal 393 and reference line 392 across
the base-collector path of clamp transistor 352. Feedback signal V
FB is the voltage between feedback node 306 and reference line 392 across resistor 316.
Current I
Q is the current of current source 355 and is, preferably, a constant current. Node
voltage V
X is the voltage between intermediate node 351 and reference terminal 392.
[0023] The current limiting function in regulator 300 is achieved by the operation of regulator
300 as follows: Output transistor 310 receives input voltage V
CC at the emitter and provides output voltage V
OUT at the collector (also current I
CC and I
OUT, respectively). Driver portion 311 controls transistor 310 through the base current
I
B. Resistor 330 carries current I
CL (substantially proportional to I
OUT,I
B, explained above) and provides voltage drop V
CL. Chain 335 forwards voltage drop V
CL to node 351. When I
OUT increases, then voltage V
X at node 351 also increases. Optionally, input transistor 350 receives V
IN and controls the base current I
B through chain 335. Transistor 352 clamps the voltage V
X at node 351 to V
XMAX (V
X ≤ V
X MAX). This affects also the maximum magnitude of V
CL and therefore limits I
OUT to I
OUT MAX.
[0024] Unlike in the prior art, voltage drop V
CL is not applied to base-emitter path of a transistor which could limit capacity of
the driver portion. In other words, changes of V
CL act directly on driver portion 311. There is no time required to wait until a base-emitter
voltage V
BE reaches V
BE START (as in the prior art, see FIGS. 1-2).
[0025] Voltage changes due to temperature variations are partly compensated. Voltage V
CL across resistor 330 has a positive temperature coefficient ("temp co"). The chain
voltage V
CL- V
IN from the emitter of transistor 370 to the base of input transistor 350 can be calculated
as follows:
Voltage V
CL - V
IN has a temperature coefficient (e.g., negative) which compensates the opposite temp
co of V
CL across resistor 330 (e.g., positive). In other words, chain 335 provides a pn-junction
voltage (V
CL - V
IN) with a negative temp co which compensates the positive temp co of voltage drop V
CL. Persons of skill in the art are able, based on the description herein, to replace
transistors 360 and 370 by other semiconductor devices whose pn-junctions provide
similar voltages.
[0026] Preferably, reference voltage V
REF is substantially independent from the temperature. This is convenient, but not necessary.
It is possible to apply a V
REF which substantially does depend on the temperature. In such a case, the temperature
dependency of the current limiting portion can be arbitrarily adjusted as required
by, for example, the consumer circuit. In regulator 300 of the present invention,
the response time T
RESP is shorter than in prior art solutions. This is an important advantage of the present
invention. As mentioned above, regulator 300 has a better stability due to lower feedback
gain and higher stability for high output currents (I
OUT ≈ I
OUT MAX).
[0027] FIG. 4 illustrates a simplified circuit diagram of regulator 400 according to a second
embodiment of the present invention. In FIGS. 3-4, reference numbers 300/400, 305/405,
306/406, 310/410, 315/415, 316/416, 330/430, 352/452, 350/450, 335/435, 370/470, 391/491,
392/492, 393/493 and 394/494 refer to analogous components. However, their function
can be different as described in the following. The analogous components are: output
transistor 410 (e.g., pnp-type), input transistor 450 (e.g., npn-type), clamp transistor
452 (e.g., npn-type), chain transistor 470 (forming chain 435, e.g., pnp-type, dashed),
resistors 415, 416 and 430, and current source 455. Regulator 400 further comprises
resistor 431.
[0028] The components are coupled as follows: Output transistor 410 has an emitter coupled
to reference line 491 and a collector coupled to output node 405. A base of transistor
410 is coupled to reference line 491 via resistor 431 and is coupled to an emitter
of transistor 470 via resistor 430. Output node 405 is coupled to reference line 492
via resistor 415, feedback node 406, and resistor 416 (voltage sensor). Chain transistor
470 has a collector coupled to reference line 492 and a base coupled to intermediate
node 451 (base-emitter voltage V
BE3). Input transistor 450 has a collector coupled to reference line 491, an emitter
coupled to intermediate node 451, and a base coupled to input terminal 494. Clamp
transistor 452 has a collector coupled to reference line 491, an emitter coupled to
intermediate node 451, and a base coupled to reference terminal 493. Current source
is coupled between intermediate node 451 and reference line 492.
[0029] Resistor current I
CL through resistor 430 is, preferably, proportionally related to the control current
I
B (base current) of output transistor 410. Voltage V
X is referred to reference line 491 (different from FIG. 3).
[0030] Having described details in connection with FIGS. 3-4, the present invention can
also be described as regulator 300/400 with a current limiting function which comprises:
(a) output transistor 310/410 having an emitter to receive input voltage V
CC (on line 391/491), a collector to provide output current I
OUT, and a base to receive control current I
B (base current);
(b) resistor 330/430 which carries resistor current I
CL proportional to control current I
B at the base of transistor 310/410 and which provides voltage drop V
CL;
(c) chain 335/435 to forward voltage drop V
CL to intermediate node 351/451, such that an increase of I
OUT leads to an increase of node voltage V
X between intermediate node 351/451 and reference line 392/491;
(d) input transistor 350/450 having an emitter coupled to intermediate node 351/451,
a collector coupled reference line 392/491, and a base to receive control signal V
IN, input transistor 350/450 controlling output transistor 310/410 (e.g., via transistors
360, 370, 380 in FIG. 3, or via transistor 470 in FIG. 4);
(e) clamp transistor 352/452 having an emitter coupled to intermediate node 351/451,
a collector coupled to reference line 392/491, and a base to receive reference signal
V
REF, such that node voltage V
X is limited ("clamped") when output current I
OUT increases to predetermined magnitude I
OUT MAX, and that input transistor 350/450 prevents I
OUT from increasing beyond I
OUT MAX.
[0031] Further, using the reference to FIG. 3 only for convenience of explanation, and with
no intention to limit the scope, the present invention can be described as circuit
300 having output transistor 310 to transfer an input 391 to an output 305 wherein
output current I
OUT is limited. Circuit 300 is characterized by features
(a) and
(b):
(a) Driver portion 311 for controlling output transistor 310 has (i) mirror transistor 380 coupled to output transistor 310; (ii) resistor 330 coupled to mirror transistor 380 to carry a scaled replica (e.g., ICL) of output current IOUT, the potential VCL at resistor 330 corresponding to output current IOUT; (iii) transistor chain 335 to transfer the potential VCL at resistor 330 to input node 351 and to transfer an external control signal (e.g.,
VIN from terminal 394) to output transistor 310; and (iv) clamping transistor 352 to limit an increase of the potential VX at input node 351.
(b) A rise of output current IOUT is communicated to input node 351 by transistor chain 335 so that clamping transistor
352 prevents driver portion 311 from allowing output transistor 310 to drain output
current IOUT above a predetermined maximum.
[0032] Further, a method of the present invention can be described as follows. Regulator
300/400 has output transistor 310/410 that receives a first current (e.g., I
CC) at a first main electrode (e.g., emitter) and a second current (e.g., I
B) at a control electrode (e.g., base). Output transistor 310/410 provides a third
current (e.g., output current I
OUT) at a second main electrode (e.g., collector). The third current is limited to predetermined
maximum current (e.g., I
OUT MAX) by the following steps:
(a) propagating a replica (e.g., emitter-collector voltage of transistor 352/452) of
a reference voltage (e.g., VREF) to a node (e.g., node 351/451) to limit a magnitude of a voltage at the node (e.g.,
VX limited to VX MAX);
(b) providing a voltage drop (e.g., VCL) across a resistor (e.g., resistor 330/430) which carries a resistor current (ICL) substantially proportional to the second and third currents (cf. equation (1) and
β-relation); and
(c) keeping the voltage drop in parallel to the magnitude of the voltage (VX) at the node, so that a magnitude of the voltage drop (VCL) is limited in reference to the maximum magnitude of the voltage at the node (e.g.,
VX MAX and VBE2, VBE3), thereby limiting a magnitude of the second current and limiting a magnitude of
the third current to the predetermined maximum current.
[0033] Further, the method of the present invention for limiting output current I
OUT to a maximum predetermined output current I
OUT MAX can described by the following steps:
(i) relating a magnitude of control signal VIN to a magnitude of control current IB (base current of output transistor 310/410) via intermediate signal VX at intermediate node 351/451;
(ii) receiving reference voltage VREF and propagating reference voltage VREF to intermediate node 351/451, VREF limiting a magnitude of VX at node 351/451 to maximum intermediate magnitude VX MAX;
(iii) providing voltage drop VCL across resistor 330/430 which carries resistor current ICL depending on control current IB (e.g., see current mirror 380/310 in FIG. 3 or resistor arrangement 430/431 in FIG.
4) and thereby represents IOUT; and
(iv) keeping in parallel VCL to the magnitude of VX, so that a magnitude of VCL is limited in reference to VX MAX, thereby limiting the magnitude of IB and limiting IOUT to IOUT MAX.
[0034] While the invention has been described in terms of particular structures, devices
and methods, those of skill in the art will understand based on the description herein
that it is not limited merely to such examples and that the full scope of the invention
is properly determined by the claims that follow.
1. A regulator (300) with a current limiting function, characterized by:
a first transistor (310) having a first main electrode to receive an input voltage,
a second main electrode to provide an output current, and a control electrode to receive
a control current for controlling an impedance between said first and second main
electrodes;
an arrangement (335) for forwarding a voltage drop which corresponds to said control
current to an intermediate node (351), such that an increase of said output current
leads to an increase of a node voltage between said intermediate node (351) and a
reference line (392);
a second transistor (350) having a first main electrode coupled to said intermediate
node (351), a second main electrode coupled to said reference line (391), and a control
electrode to receive a control signal (394), said second transistor controlling said
first transistor (310);
a third transistor (352) having a first main electrode coupled to said intermediate
node (351), a second main electrode coupled to said reference line (392), and a control
electrode to receive a reference signal (393), such that said node voltage is limited
when said output current increases to a predetermined magnitude, and that said second
transistor (350) prevents said output current from increasing beyond said predetermined
magnitude.
2. The regulator (300) of claim 1 further comprising a resistor (330) for carrying a
resistor current proportional to said control current on said control electrode of
said first transistor (310) and to provide said voltage drop.
3. The regulator (300) of claim 1 wherein said arrangement (335) comprises further transistors
(360, 370) in an emitter follower arrangement.
4. The regulator (300) of claim 1 wherein said arrangement (335) provides a chain voltage
having a first temperature coefficient so that a second, opposite temperature coefficient
of said voltage drop is substantially compensated.
5. The regulator (300) of claim 1 wherein said arrangement (335) provides a pn-junction
voltage with a negative temperature coefficient compensating a positive temperature
coefficient of said voltage drop.
6. A method for limiting the output current of a regulator (300) to a predetermined maximum
current, wherein the regulator (300) has an output transistor (310) receiving a first
current at a first main electrode and a second current at a control electrode and
providing said output current at a second main electrode, said method characterized
by the following steps:
propagating a replica of a reference voltage to a node (351) to limit a magnitude
of a voltage at said node;
providing a voltage drop across a resistor (330) which carries a resistor current
substantially proportional to said second current and to said output current; and
keeping said voltage drop in parallel to said magnitude of the voltage at said node
(351), so that a magnitude of said voltage drop is limited in reference to the maximum
magnitude of the voltage at said node (351), thereby limiting a magnitude of said
second current and limiting a magnitude of said output current to said predetermined
maximum current.
7. A method for limiting the output current I
OUT of a regulator (300) to a maximum predetermined output current I
OUT MAX, said regulator (300) having an output transistor (310) receiving an input current
I
CC at a first main electrode, receiving a control current I
B at a control electrode, and providing the output current I
OUT at a second main electrode, said method characterized by the following steps:
relating a magnitude of a control signal VIN to a magnitude of said control current IB via an intermediate signal VX at an intermediate node (351);
receiving a reference voltage VREF and propagating said reference voltage to said intermediate node (351), said reference
voltage VREF limiting a magnitude of VX at said intermediate node (351) to maximum intermediate magnitude VX MAX;
providing a voltage drop VCL across a resistor (330) which carries a resistor current ICL which depends on said control current IB and thereby represents IOUT; and
keeping in parallel VCL to the magnitude of VX, so that a magnitude of VCL is limited in reference to VX MAX, thereby limiting a magnitude of IB and limiting IOUT to IOUT MAX.
8. The method of claim 7 wherein in said step relating, VIN is substantially related to IB proportionally.
9. The method of claim 7 wherein in said step providing, said related resistor current
is substantially proportional to control current IB.
10. A circuit (300) having an output transistor (310) to transfer an input to an output
wherein an output current is limited, the circuit (300) characterized in that
(a) a driver portion (311) for controlling the output transistor (310) has
(i) a mirror transistor ((380) coupled to the output transistor (310);
(ii) a resistor (330) coupled to the mirror transistor (380) to carry a scaled replica
of the output current, the potential at the resistor (330) corresponding to the output
current;
(iii) a transistor chain (335) to transfer the potential at the resistor (330) to
an input node (351) and to transfer an external control signal to the output transistor
(310); and
(iv) a clamping transistor (352) to limit an increase of the potential at the input
node (351), and
(b) a rise of the output current is communicated to the input node (351) by the transistor
chain (335) so that the clamping transistor (352) prevents the driver portion (311)
from allowing the output transistor (310) to drain output current above a predetermined
maximum.