DC-to-DC converter circuit having a power saving mode

Details DC-to-DC converter circuit having a power saving mode DC-to-DC converter circuit having a power saving mode

1999-12-21

2002-07-16

Nguyen, Matthew (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

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

Reexamination Certificate

active

06420858

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority of Japanese Patent Application No. 10-369006 filed Dec. 25, 1998, the contents being incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC-to-DC converter circuit including a main switching element to perform DC-to-DC conversion by using the main switching element to turn an input voltage on and off. More specifically, the present invention relates to a DC-to-DC converter circuit that achieves a high conversion efficiency without using a sense resistance.
2. Description of the Related Art
Battery-driven devices, such as notebook personal computers (PCs), typically include DC-to-DC converter circuits to convert the voltage of AC adapters, dry-cell batteries and the like into a voltage required by a load. In order to increase the utility of such battery-driven devices, the conversion efficiency of the DC-to-DC converter circuit must be increased.
The conversion efficiency of conventional DC-to-DC converter circuits provided in battery-driven devices, such as notebook PCs, is made as high as possible using a switching regulator to perform pulse width modulation (PWM) control. DC-to-DC converter circuits using the type of switching element that performs PWM control can be either a voltage mode control DC-to-DC converter circuit or a current mode control DC-to-DC converter circuit, depending on the method of control.
FIG. 31
illustrates a conventional voltage mode control DC-to-DC converter circuit. As shown in
FIG. 31
, the voltage mode control DC-to-DC converter circuit generates a PWM control signal (Vpwm) and comprises a triangular wave generation circuit
2
to generate triangular wave signals, an error amp (AMP) to output a voltage Ver in response to an output voltage Vout, and a comparator (COMP) to compare the triangular wave signals output by the triangular wave generation circuit
2
and an output voltage Ver of the error amp (AMP). The DC-to-DC conversion circuit shown in
FIG. 31
performs DC-to-DC conversion by turning a main switching element Q
1
on and off via a driver circuit
4
.
The conventional voltage mode control DC-to-DC converter circuit shown in
FIG. 31
includes a synchronous commutating switching element Q
2
in place of a flywheel diode, and a main switching element Q
1
. The ON/OFF operation of the synchronous commutating switching element Q
2
is performed as a reverse operation of the ON/OFF operation of the main switching element Q
1
. When the main switching element Q
1
is off, the synchronous commutating switching element Q
2
supplies current to an output capacitor C
1
from an inductor L
1
with a smaller drop in voltage than with a flywheel diode.
An example of a conventional current mode control DC-to-DC converter circuit is the MAX 786 PWM controller made by the Maxim Co. of the U.S. As shown in
FIG. 32
, the MAX 786 PWM current mode control DC-to-DC converter circuit comprises a sense resistance R to detect a load voltage; an error amp (AMP
1
) to output a voltage in response to an output voltage Vout; a current amp (AMP
2
) to receive the voltage sensed by the sense resistance R and to output a voltage that becomes larger as the input voltage generated by sense resistance R becomes larger; a current comparator (COMP
1
) to compare the output of the error amp AMP
1
with the output of the current amp AMP
2
, and to output a high level when the current amp AMP
2
output voltage reaches an output voltage Ver of the error amp AMP
1
; and a flip-flop FF
1
to latch a high level in response to a predetermined frequency pulse and to reset the latch output at a low level when the current comparator COMP
1
outputs a high level. A control logic circuit
6
turns the main switching element Q
1
on and the synchronous commutating switching element Q
2
off when the flip-flop FF
1
outputs a high level, and turns the main switching element Q
1
off and the synchronous commutating switching element Q
2
on when the flip-flop FF
1
outputs a low level.
As shown in
FIG. 32
, a reverse current comparator (COMP
2
) receives the voltage output by the sense resistance R, detects the reverse current generated when the load current becomes small (that is, the current flowing from capacitor C
1
to inductor L
1
) and outputs a high level. A flip-flop FF
2
latches the high level when the reverse current comparator (COMP
2
) outputs a high level, and resets the latch output at a low level in response to the pulse input by the flip-flop FF
1
. When the flip-flop FF
2
indicates the generation of a reverse current, the control logic
6
then cuts off the reverse current by turning synchronous commutating switching element Q
2
off in order to prevent wasted consumption of power by the sense resistance R.
Further, a mini-current comparator (COMP
3
) receives the output voltage of the current amp AMP
2
as an input, detects when the load current is even smaller than the level generated by the reverse current, and outputs a high level. When the mini-current comparator COMP
3
detects a decrease in the load current, the control logic
6
enters a power saving mode (pulse skip mode). The drive current of the main switching element Q
1
(synchronous commutating switching element Q
2
), which is problematic when the charge current is low, can be reduced by selecting the drive instruction signals of the main switching element Q
1
(synchronous commutating switching element Q
2
) input from the flip-flop FF
1
.
In operation of the device shown in
FIG. 32
, when entering the power saving mode, after extra energy is injected into an output LC filter by turning the main switching element Q
1
on at the maximum duty on width, the power saving mode is entered by causing the main switching element signal Q
1
and the synchronous commutating switching element Q
2
to rest.
In the above-described manner, the current mode control DC-to-DC converter circuit shown in
FIG. 32
functions to stop the reverse current by turning the synchronous commutating switching element Q
2
off when the reverse current is generated in response to the load current becoming smaller, thereby preventing the waste of electric power caused by the reverse current flowing through the sense resistance R.
The current mode control DC-to-DC converter circuit also has the function of reducing the drive current of the main switching element Q
1
(synchronous commutating switching element Q
2
) when the load current becomes small, which becomes problematic when the load current becomes small, by selecting the drive instruction signals of the main switching element Q
1
(synchronous commutating switching element Q
2
).
On the other hand, unlike the current mode control DC-to-DC converter circuit, the voltage mode control DC-to-DC converter circuit shown in
FIG. 31
is unable to improve the conversion efficiency because it does not have the ability to measure the load current. Therefore, when a high conversion efficiency is required, the current mode control DC-to-DC converter circuit has been employed.
However, the current mode control DC-to-DC converter circuit shown in
FIG. 32
has the problem of wasteful use of energy by the sense resistance since the sense resistance is used to measure the load current.
Recently, the load current in notebook Pcs having DC-to-DC converter circuits has been constantly increasing as functions become more and more advanced, and it is now impossible to ignore the loss of power caused by the sense resistance. For example, when a sense resistance of 22 m&OHgr; is used, if the load current is 4 A, the power loss caused by the sense resistance is 22 m&OHgr;×4 A
2
=0.352 W. With an output of 3.3 V this becomes a power loss of 2.67%.
Moreover, the sense resistance has the problem of high price because it is a special article with low resistance in the tens of m&OHgr; and precision below 1%.
In order to solve the above-described problems, technology has been disclosed that uses the “on ” resistan

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