JP3236071B2 - Triangular wave signal generation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to pulse width modulation (PW).
The present invention relates to a triangular wave generation circuit that generates a triangular wave signal synchronized with an input clock signal used in M).

[0002]

2. Description of the Related Art LBP (Laser Beam Printer)
When printing a high-definition video (high-gradation) image, the PWM pixel modulation for controlling the beam irradiation time on a pixel-by-pixel basis is performed on the amount of laser light having a correlation with the print density (the amount of adhered toner).

FIG. 2 shows an example of this pixel modulation. The input terminal 1 has a BD indicating a horizontal reference position on the paper surface.
(Beam Detect) A video clock (FIG. 3A) representing a pixel unit synchronized with a pulse is input, and a triangular wave generating circuit 18 generates a triangular wave synchronized with the video clock as shown in FIG. 3D. Is input to the comparator 20.

On the other hand, the input terminal 17 receives pixel data (FIG. 3B) for setting the print density of each pixel, for example, 8 bits.
The input pixel data is latched by a video clock at the D / A converter 19 to convert the input pixel data into pixel data (FIG. 3C), and then analog data corresponding to each pixel data shown in FIG. Converted to voltage and comparator 2
Input to 0.

As shown in FIG. 3D, the comparator 20 outputs a laser drive pulse pulse-width-modulated according to the pixel data by the input triangular wave signal and the pixel analog voltage (FIG. 3E). ).

When the laser drive pulse is at the H level, the laser beam is emitted. Therefore, when the pixel data is DN + 2, it corresponds to a "dark pixel", and when it is DN + 1, it corresponds to a "light pixel". Since the print density is very sensitive to the pulse width (irradiation time), the peak level value of the triangular wave and the DC
The condition is that not only can the offset value be set, but also the stability is above all.

FIG. 4 shows a conventional triangular wave generating circuit 18.
The video clock input to the input terminal 1 is shaped in the buffer 21 to remove noise such as ringing, and has a time constant T = R1 · C that is appropriately larger than the clock cycle.
A triangular wave is generated by the time constant circuit of FIG.

The level of this triangular wave can be set by the resistance value of R1. Further, a DC offset can be set by VR1 via a sufficiently large capacity C4. In order to ensure the linearity of the slope of the triangular wave, the time constant T needs to be about three times the video clock cycle.

[0009]

However, the conventional triangular wave generating circuit shown in FIG. 4 has the following disadvantages. 1) To adjust the generation level of the triangular wave signal to the output characteristics of a general high-speed D / A converter, 0.6 to 0.7 vp
It is necessary to secure about p. Therefore, the buffer 21 must be driven at a large level of about 12 V,
It is disadvantageous in C configuration. 2) Since the time constant T for determining the triangular wave level is affected by environmental changes, when used for pixel modulation in the LBP,
The fact is that the temperature is controlled and used. This is L
This is a heavy load in configuring a BP PWM pixel modulation system. 3) Independent PWM adjustment of triangular wave level and offset value
Since the characteristics cannot be determined, the adjustment is very difficult.

[0010]

The triangular wave signal generating circuit according to the present invention comprises a capacitor, a first current source for charging the capacitor, and a second current source for discharging the charge stored in the capacitor. A current source;
Or a switch circuit for controlling a current output to a second current source in accordance with an input clock signal; and a third current source for increasing or decreasing the charge stored in the capacitor. A triangular wave generating circuit that generates a triangular wave signal having a period equal to the clock signal,
The triangular wave signal output from the terminal of the capacitor and the first
A first comparison circuit that compares the triangular wave signal output from the terminal of the capacitor with a second reference voltage that is different from the first reference voltage; In order to control the level value of the triangular wave signal, the first and second comparison circuits are controlled according to the output signals of the first and second comparison circuits.
And a circuit for controlling the second current source and controlling the third current source in accordance with an output signal of the first or second comparison circuit to control an offset value of the triangular wave signal. It is characterized by.

[0011]

【Example】

[First Embodiment] FIG. 1 shows a first embodiment of a triangular wave generating circuit according to the present invention. A clock signal such as a video clock is input to the terminal 1 to control the switch 2. When the clock signal is at H level, switch 2
The voltage rises monotonously due to the charging current of I1 + (I3-I4). On the other hand, when the clock signal is at the L level, the switch 2 is turned on and the capacitor C0 is supplied with I2-I1.
It falls monotonically due to the discharge current of-(I3-I4). Because when the clock duty is balanced,
This is because (I3−I4) = 0, and I2 = 2 · I1. The reason why the current source I2 is twice as large as I1 to generate the charge / discharge current for the capacitor CO is that the current source I1
This is because, in order to perform the switch operation, the circuit generally needs to be configured with a P-type transistor that is disadvantageous for high-speed operation.
Therefore, a triangular wave can be generated in one cycle of the clock. However, if the current sources I1 to I4 are fixed without being controlled, the level and offset value of the generated triangular wave signal cannot be defined due to variations in the capacitor CO, the current sources I1 to I4, and the clock duty.

Then, the generated triangular wave signal is supplied to the buffer 7.
Are connected to the subtraction input and the addition input of the comparators 8 and 9 respectively. On the other hand, as shown in FIG. 5A, reference voltages V90 and V10, which output a 90% -width pulse when a desired triangular wave signal is generated, are applied to the addition input and the subtraction input of the comparators 8 and 9, respectively. Is entered. FIG.
(B) and (c) show output pulse waveforms of the comparators 8 and 9 when a desired triangular wave signal is generated. These pulses are input to the charge pump circuits 12, 15. The charge pump circuit 12 is configured as shown in FIG. The output pulse of the comparator 8 is input to the input terminal 23, and the switch SW1 is turned on when it is at the H level. SW1 has a constant current source I5, a constant current source 0.9I5, and a capacitor C1.
Is connected. The output of the charge pump 12 is a loop
The data is input to the filter 11 and subjected to appropriate filtering. The output of the loop filter 11 is the error signal generation circuit 1
0 generates error signals 6,5. Error signal 6, 5
Control current sources I3 and I4, respectively. Comparator 8
When the output pulse width is larger than 90%, the value of (I3-I4) is increased, and conversely, when the output pulse width is smaller than 90% (I3-I4).
The absolute value and polarity of (I3-I4) are controlled by decreasing the value of 4). The pulses converge so that a pulse having a 90% width is output from the comparator 8.

On the other hand, the charge pump 15 is shown in FIG.
The configuration is as follows. Outputs of comparators 8 and 9 are connected to terminals 24 and 25, respectively.
3 is turned on when it is at the H level. SW2 has a constant current source I
6, SW3 is connected to a constant current source I7 equal to I6, and the other end is connected to a constant current source 1.8I6. Only when the sum of the output pulse widths of the comparators 8 and 9 is 180%, the charging / discharging current for the capacitor C2 is balanced and the output voltage is stabilized. The output of the charge pump 15 is the same as the output of the charge pump 12,
The error signals 4 and 3 are generated via the error signal generating circuit 13. Error signals 3 and 4 control current sources I1 and I2, respectively,
The current values of both current sources are controlled in proportion. When the sum of the output pulse widths of the comparators 8 and 9 is larger than 180%, the current values of the current sources I1 and I2 are increased, and when the output pulse width is smaller than 180%, the current values are decreased. The sum of the output pulse widths of the comparators 8 and 9 is 180%
Converge so that Therefore, the triangular wave signal generating circuit shown in FIG. 1 generates a desired triangular wave signal shown in FIG. The charge pump 15 defines the triangular wave level, and the charge pump 12 defines the offset value of the triangular wave signal. Here, the reason why the offset value of the triangular wave level is specified at the position where the output pulse width is narrow (10%) in the PWM pixel modulator having the configuration of FIG. This is because the fact that the influence on the image quality is large in (light pixels) is considered. However, it goes without saying that the reference voltages V10 and V90 that define the triangular wave signal are not limited to being set as in this embodiment.

[Second Embodiment] The triangular wave generating circuit shown in FIG. 7 is a second embodiment of the present invention in which the offset value of the triangular wave signal is specified by the center level of the triangular wave signal. The parts performing the same operations as those in the embodiment of FIG. 1 are given the same numbers. The difference from the triangular wave generation circuit of FIG. 1 will be described. In the triangular wave generation circuit of FIG. 7, the generated triangular wave signal is newly input to the addition input of the comparator 26,
When the desired triangular wave signal is generated at the subtraction input, FIG.
As shown in (a) and (d), a reference voltage V50 for generating a pulse having a 50% width is input to the output of the comparator 26. The output of the comparator 26 is input to the charge pump 27. The charge pump 27 has the configuration shown in FIG. SW4 is turned on when the output pulse of the comparator 26 is at H level. A constant current source I8 is connected to SW4, and a constant current source 0.5I8 and a capacitor C are connected to the other end.
3 are connected. Therefore, the output voltage of the charge pump 27 is stabilized only when the output pulse width of the comparator 26 is 50%.

According to the output of the charge pump 27, (I3
-Control the value of I4). When the pulse width of the comparator 26 is greater than 50%, the value of (I3-I4) is reduced,
When the pulse width is smaller than 50%, the pulse width is increased to generate and converge the desired triangular wave signal.

In the embodiment of FIG. 7, by appropriately configuring the reference voltage V50 input to the comparator 26 and the charge pump 27, the level value of the triangular wave signal can be set independently and the offset value can be set arbitrarily.

[Third Embodiment] FIG. 9 shows a triangular wave generating circuit according to a third embodiment of the present invention. The difference from the triangular wave generating circuit of FIG. 1 lies in that a starting circuit 28 is added. In the case of PWM pixel modulation of the LBP system, FIG.
For a video clock signal which is an intermittent clock signal as shown in FIG. 11A and whose clock phase changes before and after the missing period, a desired signal is obtained immediately after the missing period as shown by a solid line in FIG. It is required to generate a triangular wave signal. In the triangular wave generating circuit of FIG.
As shown by the dotted line in (b), the potential of the triangular wave signal output drops during the missing period, the desired triangular wave signal cannot be obtained immediately after the missing period, and it takes time for the feedback loop defining the offset value to converge. I will. The start-up circuit 28 is for bringing the potential of the triangular wave signal closer to the triangular wave signal shown by the solid line in FIG. FIG. 10 shows a configuration example of the activation circuit 28. The constant current source I9 has a current value larger than I2-I1- (I3-I4), and Q1 and Q1 are supplied through the current switches of Q2 and Q3.
A charging current is supplied to the terminal 29 by a current mirror circuit including Q4, R1, and R2 (Q1 = Q2, R1 = R2). As shown in FIG. 5A, the potential V100 at the lower vertex of the triangular wave signal or a slightly lower potential VL is input to Q2 / B, and the triangular wave signal is input to the terminal 29. When the triangular wave signal becomes equal to or lower than the VL potential, Q2 is turned on, a charging current is supplied to the terminal 29, the potential drop of the triangular wave signal is suppressed, and the potential is fixed near the potential VL during the clock missing period.

[0018]

As described above, the following effects can be obtained by the triangular wave generating circuit of the present invention. 1) The level value and offset value of the triangular wave signal are actually generated.
Since the triangular wave signal is defined based on the generated triangular wave signal, a stable triangular wave signal can be generated with respect to power supply voltage, environmental temperature, and element variation. 2) Unlike the conventional triangular wave generating circuit shown in FIG. 4, a large power supply voltage is not required, and a triangular wave signal having a good slope linearity can be generated. 3) By adding a driving circuit having a very simple configuration, the start-up characteristics are excellent even for a synchronous clock signal such as the video clock in which a missing period exists and the clock phase changes before and after the missing period. Can be configured.

[Brief description of the drawings]

FIG. 1 shows a triangular wave signal generation circuit according to a first embodiment.

FIG. 2 is a conceptual diagram of a PWM pixel modulation circuit.

FIG. 3 is a waveform diagram illustrating an operation of PWM pixel modulation.

FIG. 4 is a conventional example of a triangular wave signal generation circuit.

FIG. 5 is a waveform diagram illustrating the operation of the triangular wave signal generation circuit according to the first embodiment.

FIG. 6 is a charge pump circuit used in the first embodiment.

FIG. 7 shows a triangular wave signal generation circuit according to a second embodiment.

FIG. 8 shows a charge pump circuit used in the second embodiment.

FIG. 9 shows a triangular wave signal generation circuit according to a third embodiment.

FIG. 10 shows a starting circuit used in the third embodiment.

FIG. 11 is a waveform chart for explaining the operation of the third embodiment.

[Explanation of symbols]

 Reference Signs List 1 clock input terminal 2 current switch 3 first discharge current value control line 4 first charge current value control line 5 second discharge current value control line 6 second charge current value control line 7 triangular wave buffer 8 first Comparator 9 Second comparator 10 First error signal generation circuit 11 First loop filter circuit 12 First charge pump circuit 13 Second error signal generation circuit 14 Second loop filter circuit 15 Second charge pump circuit Reference Signs List 16 Triangular wave signal output terminal 17 Pixel modulation data input terminal 18 Triangular wave generating circuit 19 D / A converter 20 PWM comparator 21 Triangular wave generating clock buffer 22 Triangular wave buffer 26 Third comparator 27 Third charge pump circuit 28 Starter circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B41J 2/44 H04N 1/23 103

Claims (2)

(57) [Claims] And a capacitor charged in said capacitor.
And a first current source for charging the capacitor.
A second current source for discharging the charged electric charge,
Or a current output to the second current source as an input clock signal.
And a switch circuit that controls according to
And a third current source for increasing or decreasing the obtained charge.
A triangular wave generating circuit for generating a triangular wave signal having a period equal to the input clock signal from a terminal of a capacitor, wherein the triangular wave signal output from a terminal of the capacitor is connected to a first triangular wave signal .
A first comparison circuit that compares the reference voltage with a reference voltage, and a triangular wave signal output from a terminal of the capacitor and the first comparison circuit.
A second comparison circuit for comparing a second reference voltage different from the first reference voltage, and an output signal of the first and second comparison circuits for controlling a level value of the triangular wave signal. The first and second
To control the offset value of the triangular wave signal.
And a circuit for controlling the third current source in accordance with an output signal of the first or second comparison circuit for controlling the triangular wave signal. 2. The triangular-wave signal generating circuit according to claim 1, further comprising a drive circuit that regulates an output signal of a terminal of the capacitor to a predetermined potential during a period during which the input clock signal is lost.