JPH04257463A - Driver of led head - Google Patents

Abstract

PURPOSE:To offer a driver of an LED head which is used for an LED printer and is capable of substantially correcting the sensibility of a sensitizing element that increases with changes in temperature and of giving optical outputs corresponding to the sensibility of the sensitizing element in order to prevent deterioration of printing quality. CONSTITUTION:The electric potential of a terminal F' that changes depending on temperature is earthed as an electric current Ix through a resistor R5 and a transistor Q, and a rise in voltage of the terminal F' is restrained by a circuit Rx consisting of resistors R3-R5 and the transistor Q, so that optical outputs of an LED element that corresponds to a rise in the sensibility of a sensitizing element are given to the sensitizing element.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED head used in an LED printer, and more particularly to an LED head driving device.

0002

[Prior Art] An LED printer using an electrophotographic method has a charger, an LED head, a developer, a transfer device, etc. arranged around the photoreceptor, and the LED head illuminates the photoreceptor based on print data. This is a device that performs printing by writing, converting the electrostatic latent image formed on a photoreceptor into a toner image using a developing device, and transferring the toner image onto paper using a transfer device. In order to perform optical writing based on print data, the LED head used in such an LED printer has a drive circuit that controls the light emission of the LED element together with the LED element.

The circuit shown in FIG. 5 is an example of a driving circuit for an LED head. This drive circuit is composed of a plurality of LED array chips 1a to 1m, each of which has a plurality of LED array chips 1a to 1m.
n LED elements are arranged inside each 1 m. It should be noted that about 30 LED array chips, shown as m in the figure, are usually arranged in an actual LED head.

FIG. 6 shows the LED array chips 1a to 1m.
It is a figure showing the structure of one LED array chip in detail. In the figure, n LEDs 1 to LEDn are LED elements as light-emitting segments that actually perform optical writing on the above-mentioned photoreceptor, and the light emission is controlled by drive signals output from respective corresponding driver circuits A1 to An. Ru. Each of the driver circuits A1 to An serving as driving means for the light emitting segments is composed of switch circuits S1 to Sn and MOS transistors Q1 to Qn, and print data D1 output from a printer controller (not shown).
A drive signal is outputted to LED1 to LEDn based on -Dn. FIG. 7 shows a specific circuit configuration of the switch circuits S1 to Sn, in which two MOS transistors QS1
, QS2 and inverter 2 form the above switch circuits S1 to S
Indicates that n is configured. Print data D1 to Dn are directly input to the gate (G) of the MOS transistor QS1, and the gate (G) of the MOS transistor QS2 is directly inputted to the gate (G) of the MOS transistor QS1.
Print data D1 to Dn are input to the inverter 2 through the inverter 2. When the print data is a high signal, the MOS transistor QS1 is cut off, and the MOS transistor QS2 is turned on, so that the potential at point G, which is indicated by a circle in FIG. It is output to the gates (G) of transistors Q1 to Qn. Furthermore, when the print data is a low signal, the MOS transistor QS2 is cut off, and the MOS transistor QS1 is turned on, so that the power supply current is output from the power supply VDD to the gates (G) of the MOS transistors Q1 to Qn through the MOS transistor QS1. Ru. Therefore, when the print data is a high signal, the MOS transistor Q1
~Qn is turned on and drain current flows, and when the print data is a low signal, the power supply current supplied to the gate (G) of MOS transistors Q1 ~Qn is the same as the current supplied to the source (S), so it is cut. Turns off. From the above, for example, driver circuit A1 has print data D1
When is a high signal, MOS transistor Q1 is driven and a drive signal is output to LED1 to emit light, and when is a low signal, MOS transistor Q1 is turned off and LED1 does not emit light. Furthermore, when the print data D2 is a high signal, the drive circuit A2 drives the MOS transistor Q2 and outputs the LE signal.
D2 emits light, and when the signal is low, MOS transistor Q2
Turn off LED2 so that it does not emit light. Furthermore, the other drive circuits A3 to An operate in the same manner, and the corresponding LEDs
3 to control the light emission of LEDn.

On the other hand, the MOS transistor Qc shown in FIG.
, resistors R1, R2, and Rx are circuit configurations that determine the potential at point G shown in the figure. As mentioned above, the potential of this point G is MO when the print data D1 to Dn are high signals.
This is the voltage applied to the gates (G) of MOS transistors Q1 to Qn through the source (S) of S transistor QS2, and as a result, MOS transistors Q1 to Qn
This is the voltage that determines the drain current of . therefore,
Specifically, the current value of LED1 to LEDn is determined by the current value (drain current) supplied by the potential at point G, and the amount of light emitted by LED1 to LEDn is determined. Furthermore, the potential at point G depends on the resistance values of the resistors R1, R2, and Rx, and therefore LED1 to LEDn
The amount of light emitted depends on the resistance values of the resistors R1, R2, and Rx.

The resistors R1 and R2 are formed as part of an integrated circuit within the LED array chips 1a to 1n. Further, the resistor Rx is connected to the LED array chips 1a to 1m as a so-called external resistor.

Next, in the conventional LED head drive circuit, the gates (G) of the MOS transistors Q1 to Qn
The relationship between the voltage supplied to LED1 to LEDn and the amount of light emitted from LED1 to LEDn will be explained. First, generally the MOS transistor Q
The relationship between the gate voltage VGS (gate (G)-source (S) voltage) applied to c and the drain current ID is VDS
>VGS-Vth is expressed by the following formula.

ID=IDSS (VGS/Vth-1)2
...(1) However, Vth indicates the pinch-off voltage, and VDS indicates the drain-
IDSS indicates the source-to-source voltage, and IDSS indicates the saturated drain current.

Therefore, when the MOS transistor Qc is on, at the point G, the MOS transistor Qc and the resistor R
1, a current supplied from the power supply VDD flows into it. And this MOS transistor Qc and LED1
MOS transistor Q1 that emits light from ~LEDn ~Q
If the mutual conductances of n are equal, the drain current ID obtained by the above equation (1) flows to the MOS transistors Q1 to Qn as the drain current of the MOS transistors Q1 to Qn due to the so-called current mirror effect. Incidentally, the mutual conductance gm of the MOS transistor Qc and the MOS transistors Q1 to Qn is expressed by the following equation.

0010

[Math 1]

When comparing the overall configuration of the LED head drive circuit shown in FIG. 5 with the circuit configuration of the LED array chip shown in FIG. 6, it is found that the resistance corresponding to the resistor R2 shown in FIG. It is composed of R2a, R2b, and R2c, and can be selected by a switch 3. This configuration is such that since there is a difference in the amount of light emitted from the LED elements between the LED array chips, the difference is corrected by switching the switch 3.

0012

[Problems with the Prior Art] In the above-mentioned LED head driving device, in order to obtain a printed image of excellent quality, it is necessary above all that the amount of light emitted from each LED1 to LEDn be stable. However, a change in temperature within the device brings about a change in the resistance values of the resistors R1, R2, and Rx, and therefore the potential at point G is also affected by temperature and VGS changes, causing MOS transistors Qc and Q1 to change.
The mutual conductance between Q1 to Qn changes, and the drain current ID flowing through the MOS transistors Q1 to Qn also changes. This will be explained below.

First, as mentioned above, each LED1 to LED
The current flowing into n is based on the drain current IDC of transistor Qc, and therefore VGS shown in equation (1)
is required to be stable, that is, the potential at point G is required to be stable. Here, the temperature characteristics of the potential difference VGS across the resistor R1 are as follows in FIGS. 5 and 6, ignoring the drain current IDC, and assuming that the temperature coefficient of the resistors R1 and R2 is a, and the temperature coefficient of the resistor Rx is b.

[0014]

[Math 2]

It can be expressed as: Therefore, for example, the rate of change U of VGS with respect to the reference temperature T=0 is:

[0016]

[Math 3]

[0017] Here, suppose that the number of LED1 to LEDn arranged in each LED array chip is, for example, 64 (n=64), and a current of 6 mA is passed through one LED, and all of the LEDs in one LED array chip are LED1
~When the LED 64 emits light, the power consumption P per LED array chip is: P≒6×10-3×64×(5-1.5)×1/4
=0.336 [W]

...(5). However, the power supply voltage is 5V, and the Vf of LED1 to LED64 is
1. Calculations are made assuming that the voltage is 5V and the duty of the light emission time is 1/4.

Therefore, as LED1 to LEDn emit light, the temperature inside the LED array chip also rises, and the resistances R1 and R2 (R2a to R2 formed in the same integrated circuit) rise.
2c) also continues to rise in temperature as the LED head is used, reaching a high temperature that is balanced with the ambient conditions. For this reason, the resistors R1 and R2 when actually performing circuit operation
temperature is high.

On the other hand, since the resistor Rx is an external resistor, it is a resistor (
For example, temperature coefficient ±100PPM/resistance below °C)
is easily available. Incidentally, since the normal operating environment temperature of the LED head is 60° C. or lower, the resistance value change of the resistor Rx is within ±1%.

From the above, in equation (4), a≫b,
If a≪1, then

[0021]

[Math 4]

[0022] Therefore, in the above formula (6), a>
When 0, U > 1, and VGS increases as the temperature rises,
When a<0, U<1, and VGS decreases as the temperature rises. FIG. 8 is a diagram based on the above results, and when a>0, U
>1, and VGS exhibits a characteristic of increasing as the temperature T increases (changes). As a result, the drain current IDC flowing through the aforementioned MOS transistors Q1 to Qn, that is, the current flowing to the LED elements increases, and the drain current IDC flowing through the MOS transistors Q1 to Qn increases.
The optical power PDOT of n increases. Here, the light output of the LED element decreases due to a decrease in efficiency (0.3%/℃) as the temperature rises, but the light output PDOT in Figure 8 shows the increase in light output due to an increase in VGS and the decrease in light output due to a decrease in efficiency. It is shown synthesized.

On the other hand, the exposure amount E shown in the same figure is
Emission wavelength λm of LEDn (660-740nm
), and as the temperature rises, the sensitivity of the photoreceptor increases, so the required exposure amount decreases.

Therefore, in the conventional LED head driving device, although it is necessary to perform optimal exposure with a small amount of light as the temperature rises, the resistance R1 decreases as the temperature rises.
, R2, the amount of light emitted from LED1 to LEDn increases by about 30% due to a change in the resistance value of LED1 to LEDn, and the printed image becomes smudged at high temperatures and decreases in density at low temperatures, resulting in a problem of deterioration of print quality due to temperature changes. Ta.

[0025]

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, the present invention aims to reduce the light output of an LED head to compensate for the increase in sensitivity of the photoconductor due to temperature rise, and to prevent the print quality from deteriorating due to temperature changes. An object of the present invention is to provide a driving device for an LED head that enables the following.

[0026]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a drive device for an LED head that arranges a large number of light emitting segments in an array and performs optical writing on a photoreceptor. The present invention is characterized in that it has a compensating means connected to the means for reducing the amount of light emitted from the light emitting segment as the temperature rises in order to compensate for the increase in sensitivity of the photoreceptor as the temperature rises.

[0027]

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the circuit configuration of a part of the LED head driving device of this embodiment. The circuit shown in the figure is a circuit as a compensation means for compensating for temperature rise, and corresponds to the resistor RX shown in FIGS. 5 and 6 described above.

In the figure, terminal F' is connected to terminal F in FIGS. 5 and 6, and one end of resistor R3 is connected to power supply VDD. Resistors R3 and R4 are bias voltage setting resistors for transistor Q, and resistor R5 is the emitter (
E) It is resistance. Also, the collector of transistor Q (C
) and one end of resistor R4 is grounded. Note that the circuits other than the above-mentioned configuration have the same configuration as the circuits shown in FIGS. 5 and 6 described above, and therefore, description of the circuits having the same configuration will be omitted.

In the above configuration, the potential VB at point B is determined by the resistance values of resistors R3 and R4, and if the current amplification factor of transistor Q is sufficiently large, VB = {R3 /
(R3 +R4)}VDD. If the potential of the above point F (point F') is VCOM, the current IX flowing through the resistor R5 is:

[0030]

[Math 5]

[0031] However, VBE indicates the base-emitter voltage of the transistor Q. Here, as mentioned above, L
When the ED head is used, LED1 to LEDn emit light and the temperature of the LED array chips 1a to 1n rises, and with this temperature rise, the resistors R1 and R2 in the same integrated circuit increase.
The resistance value of (R2a to R2c) also increases, and the potential at point E decreases. Then, the drive circuit of this embodiment is expressed by the above equation (7).
Since the voltage value of VCOM decreases based on
(VCOM/IX) is increased to suppress the rise in VGS occurring across the resistor R1. Therefore, when the temperature rises, by suppressing the rise in the potential at point F and, as a result, suppressing the rise in the potential at point G, the MOS transistor Q
The drain current ID flowing through Q1 to Qn can be controlled to be constant or lower than that at the appropriate temperature.

FIG. 2 shows LED 1 when controlled in this way.
~It is a characteristic diagram showing the amount of light emitted from LEDn. The amount of emitted light PDOT shown in the figure decreases as the temperature rises, but this kind of control is due to the resistance R3 of the circuit shown in Figure 1.
This becomes possible by appropriately selecting the constant of ~R5. Therefore, by connecting the circuit shown in Fig. 1 in place of the resistor Rx shown in Figs. 5 and 6, even if the sensitivity of the photoreceptor increases in response to a rise in temperature, the rise in the potential at point G can be suppressed. By reducing the amount of light emitted from LED1 to LEDn, an electrostatic latent image with a constant potential is formed on the photoreceptor, and printing quality can be maintained constant.

FIG. 3 shows a configuration in which a resistor R6 is connected to the circuit in FIG.
The amount of light emitted by LEDn can be reduced to maintain a constant printing quality, and the current IX' flowing through the series circuit of resistor R5 and transistor Q can be reduced by a certain amount based on the resistance values of resistors R5 and R6, so the minimum current can be compensated. Thus, the load on the transistor Q can be reduced.

FIG. 4 shows a circuit in which a resistor R7 and a buffer transistor Q' are connected instead of the resistor R6, and by using two transistors Q and Q', the VBE of the transistors Q and Q' is This is a configuration that utilizes the effect more effectively.

It should be noted that each of the circuits shown in FIGS. 1, 3, and 4 shown above corresponds to the LED array chips 1a to 1m (driver circuits).
For example, it can be easily mounted as an external chip component on the drive circuit board of an LED head on which the LED head is mounted, and there is no need to increase the number of wire harnesses.

[0036]

As described in detail above, according to the present invention, even if the sensitivity of the photoreceptor changes due to temperature changes, the amount of light emitted by the LED element can be corrected and the light output to the photoreceptor can be easily adjusted. It performs printing with a well-balanced amount of light and can form images with excellent print quality.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a main part of an LED head driving device according to an embodiment.

[Figure 2] Light output P of LED1 to LEDn in one embodiment
It is a characteristic diagram of DOT, required exposure amount E, and VGS.

FIG. 3 is a circuit diagram of a main part of an LED head driving device according to an embodiment.

FIG. 4 is a circuit diagram of a main part of an LED head driving device according to an embodiment.

FIG. 5 is a circuit diagram of an LED array chip in an LED head.

FIG. 6 is a circuit diagram of a driver circuit within the LED head.

FIG. 7 is a circuit diagram of switch circuits S1 to Sn.

FIG. 8 is a characteristic diagram of optical output PDOT, required exposure amount E, and VGS of a conventional LED element.

[Explanation of symbols]

1a ~1m LED array chip 2 Inverter 3 Switch Q, Q', Q1 ~Qn, Qc Transistor A1
~An driver circuit

Claims (2)

[Claims] 1. A drive device for an LED head in which a large number of light-emitting segments are arranged in an array and optical writing is performed on a photoreceptor, which is connected to a drive means for driving the light-emitting segments, and is connected to a drive means for driving the light-emitting segments, and the light-emitting segment is An LED head driving device comprising a compensating means for reducing the amount of light emitted from the light emitting segment as the temperature rises in order to compensate for the increase in sensitivity of the body. 2. The compensation means has a characteristic that the resistance value decreases as the output voltage of the drive means decreases.
A driving device for the LED head described above.