Switch-type voltage regulator with reduction of occupied...

Details Switch-type voltage regulator with reduction of occupied... Switch-type voltage regulator with reduction of occupied...

2000-11-15

2003-04-22

Nguyen, Matthew (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S285000

Reexamination Certificate

active

06552517

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a switch-type voltage regulator with reduction of occupied space for internal soft-start functions.
BACKGROUND OF THE INVENTION
As is known, for switch-type voltage regulators, it is necessary to provide integrated circuit elements and external circuit elements which perform the so-called soft-start function, to prevent overloading of some components and causing dangerous output overvoltages during switch-on.
For greater clarity, reference is made to
FIG. 1
, which shows by way of an example a switch-type regulator
1
of a Buck-type circuit configuration.
In detail, the regulator
1
comprises an integrated circuit
100
, which has an input terminal
2
receiving an input voltage V
IN
, an output terminal
3
connected to a load
4
and supplying an output voltage V
OUT
smaller than the input voltage V
IN
, a compensation terminal
25
, and a soft-start terminal
26
, supplying a soft-start voltage V
SS
. A compensation resistor
27
and a compensation capacitor
28
, connected in series to each other, are coupled between the compensation terminal
25
and ground, whereas a soft-start capacitor
29
is connected between the soft-start terminal
26
and ground.
A DC-DC converter
5
is connected between the input terminal
2
and the output terminal
3
of the integrated circuit
100
, and comprises a switch
6
(typically formed by a power transistor, opening and closing whereof is controlled by a driving stage
7
), a diode
8
, an inductor
10
, and a filter capacitor
11
.
The regulator
1
also comprises a voltage divider
15
, connected between the output terminal
3
and ground, including a first resistor
16
and a second resistor
17
, connected to each other at a feedback node
18
supplying a feedback voltage V
FB
, proportional to output voltage V
OUT
.
An error amplifier
20
supplies to an intermediate node
21
an error voltage V
E
which is function of the difference between the feedback voltage V
FB
and a constant reference voltage V
REF
, supplied by a voltage source
22
.
The error voltage V
E
and a comparison voltage V
C
, supplied by an oscillator
32
and having a saw-tooth plot with pre-determined frequency, are supplied to the input of a differential-type comparator (PWM—Pulse Width Modulator comparator
30
), which in turn generates at the output a driving signal V
P
, which is supplied to the input of the driving stage
7
of switch
6
.
The intermediate node
21
is also directly connected to the compensation terminal
25
and to a soft-start circuit
35
, comprising a current source
36
, supplying a constant current I
SS
, and an enable transistor
37
, for example a bipolar transistor of the PNP type. In particular, an output terminal of the current source
36
is connected to the soft-start terminal
26
and to a base terminal of the enable transistor
37
, which also has an emitter terminal connected to the intermediate node
21
, and a collector terminal connected to ground.
When the regulator
1
is switched on, all capacitors
11
,
28
,
29
are discharged, and the feedback voltage VFB is zero. The error amplifier
20
is completely unbalanced, but since the enable transistor
37
is on (in fact it has a base-emitter drop greater than its switching-on voltage), the error voltage V
E
of the intermediate node
21
is equal to the soft-start voltage V
SS
at the soft-start terminal
26
, plus the base-emitter voltage of the enable transistor
37
. Since the current source
36
charges the soft-start capacitor
29
with the constant current I
SS
, the soft-start voltage V
SS
and the error voltage V
E
increase linearly, until a predetermined threshold is reached, beyond which the enable transistor
37
turns off, and the soft-start circuit
35
switches off.
This prevents overvoltages at the output; in fact, in the absence of the soft-start circuit
35
, the error voltage V
E
would immediately reach its maximum value, thus giving rise to the maximum possible duty-cycle, and inducing the power switch
6
to support the maximum current, with a risk of an output overvoltage. As an example,
FIG. 2
a
shows a comparison between the plot of the error voltage V
E
in a regulator provided with a soft-start circuit (broken line), and the plot of the error voltage V
E
in a regulator without a soft-start circuit (dot-and-dash line); on the other hand, the continuous line represents the comparison voltage V
C
.
FIGS. 2
b
and
2
c
show the plot of the driving voltage V
P
respectively in the case of absence of the soft-start circuit and in presence of the soft-start circuit.
However, the known switch-type regulators have a drawback caused by the need to use components external to the integrated circuit
100
(i.e., the soft-start capacitor
29
), specifically in order to provide the soft-start function. This is particularly disadvantageous, since the presence of the relevant pin (soft-start terminal
26
) and of the soft-start capacitor
29
involves a significant bulk and an increase in costs, which is particularly detrimental in portable electronic devices, and in particular in power supply for portable PCs.
SUMMARY OF THE INVENTION
The disclosed embodiment of the present invention provides a switch-type regulator with a soft-start function, free from the described disadvantages, and in particular allowing a reduction in the number of external components and pins.
According to the present invention, a switch-type voltage regulator is provided that includes an output terminal supplying an output voltage, a compensation network, and an error amplification circuit, the error amplification circuit having a first input receiving a reference voltage, a second input receiving a feedback voltage correlated to the output voltage, and an output connected to a compensation terminal of the compensation network, the error amplifier circuit supplying an error voltage correlated to the difference between the reference voltage and the feedback voltage, and further comprising a soft start circuit incorporated in the compensation network that is connected to the compensation terminal.
In accordance with another aspect of the invention, a switch-type regulator is provided that includes a converter circuit having a first input coupled to an input node, a second input, and an output coupled to an output node; a comparison voltage source having an output; a differential-type comparator having a first input coupled to the output of the comparison voltage source, a second input, and an output coupled to the second input of the converter circuit; and an error amplifier circuit having an input coupled to the output node to receive a feedback voltage, and an output coupled to the second input of the differential-type comparator, the error amplifier circuit comprising a comparator circuit having a first input coupled to the error amplification circuit input, a second input coupled to a first reference voltage, and an output coupled to the error amplification circuit output, and further including a differential amplifier having a first input coupled to the error amplifier circuit input, a second input coupled to a second reference voltage, and an output coupled to the error amplifier circuit output, the error amplifier circuit further coupled to a compensation circuit input terminal that in turn is coupled to the input node and to the output node, the error amplification circuit configured to supply an error voltage at its output that is correlated to the difference between the first reference voltage and the feedback voltage.


REFERENCES:
patent: 4806842 (1989-02-01), Bittner
patent: 4823070 (1989-04-01), Nelson
patent: 6185082 (2001-02-01), Yang

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