JPH04107888A - Led driver lsi - Google Patents

Abstract

PURPOSE:To always keep constant the quantity of light emitted from LED without relation to ambient temperature by storing a temperature compensating voltage value of the reference voltage for controlling a current and reading a temperature compensating voltage value corresponding to a measured value of ambient temperature sent from a temperature detecting means to control the reference voltage which determines an output current value of an LED driver. CONSTITUTION:A temperature information converted to a digital value is inputted to EEPROM 3 and content is read by designating an address. A value for compensating temperature characteristic of quantity of light emission of LED is previously written in EEPROM 3 using temperature information as an address. Digital temperature information read from EEPROM 3 is inputted to a D/A converter 4 and is then converted again to voltage value. An output voltage of the D/A converter 4 is inputted to a subtraction amplifier OP1 of a driver stage 5. An output voltage, namely temperature compensating reference voltage (VRO) of this D/A converter 4 corresponds to the conventional reference voltage (VR). Therefore, an operating current I of each LED of an LED array 6 is determined by reading VR as VRO with a resistor R.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an LED driver LSI, and particularly to an LED for an LED recording head used in electrophotographic recording such as a facsimile.
Regarding driver LSI.

[Conventional technology]

As an electrophotographic recording method for high-performance facsimile receivers, etc., due to its advantages such as high recording speed and high resolution, the LED array recording method, which uses an LED array in which light emitting diodes (hereinafter referred to as LEDs) are arranged to cover the full recording width, is used as a light source has recently been adopted. There are many cases where this is the case.

The LED recording head, which has an LED array as its main component, is an L head consisting of several dozen LEDs arranged in a line across the entire recording width.
ED array and LE arranged on both sides or one side of it
It consists of a D driver circuit.

The LED driver circuit is usually implemented as an LSI, and a constant current drive method is generally used in order to reduce variations in the amount of light emitted by the individual LEDs forming the LED array.

Conventional constant current drive type LED driver LSI of this type
An example is shown in FIG.

Note that the circuit in FIG. 2 shows only the LED driving circuit, and the data input/output circuit is omitted.

Referring to FIG. 2, the conventional LED driver LSI is
It was composed of a driving section 5, an LED array 6, a reference voltage source VR, and a resistor R.

The drive unit 5 includes an operational amplifier OP1, a transistor Ml,
Ql and driver circuits 51 to 5n.

The transistor Ql constitutes a current mirror circuit with the transistors of each driver circuit.

The driver circuits 51 to 5n each have a transistor Q5i2 (i=
1 to n) and a CMOS switch circuit 55i1.

The LED array 6 was composed of n LEDs D1 to Dn.

Next, the operation of the conventional LED driver LSI shown in FIG. 2 will be explained.

The current flowing through the transistor Q1 constituting the current mirror circuit of each of the driver circuits 51 to 5n is given by the following equation based on the resistance value R of the resistor R and the voltage value VR of the reference voltage source VR.

I=VR/R (1) At this time, for example, if the CMOS switch circuit 5511 of the driver circuit 51 is in the on state, the transistor Q5
Similarly, current I flows through 12 as well.

However, it is assumed that the transistor sizes of Ql and Q512 are the same. Note that the CMOS switch circuits 5511 to 55n1 of the driver circuits 51 to 5n are turned on and off by input data input from the terminals T1 to Tn. The operational amplifier OPI and the transistor M1, which is its buffer amplifier, are a stabilizing circuit for suppressing fluctuations in the current I caused by external factors such as the power supply VDD.

The current I of the transistor Q2 is the LED of the LED array 6.
It was supplied to the DI to drive it.

The driver circuits 52 to 5n perform similar operations when the CMOS switch circuits 8521 to 55nl are turned on.

[Problem to be solved by the invention]

The conventional LED driver LSI described above always supplies a constant current.

However, the amount of light emitted by LEDs changes depending on the temperature. - As an example, if the drive current is constant, when the temperature changes from 25°C to 70°C, the amount of light emission decreases by 20-30%;
There was a drawback that when changing from 5°C to 0°C, the amount of light emission increased by 20 to 30%.

Under the usage conditions of an LED recording head, it is normal for the temperature to rise to about 70°C. In this case, since the amount of light emitted from the LED is reduced by 20 to 30%, there is a drawback that the recorded image in the case of reversal development becomes thinner.

That is, there was a drawback that the density of the recorded image was affected by the environmental temperature.

[Means to solve the problem]

The LED driver LSI of the present invention drives an LED recording head for electrophotographic recording using an LED array in which LEDs are arranged in a row, using a constant current drive circuit that outputs a current set by the voltage value of a reference voltage source. Driver L
In the SI, a temperature measurement means for measuring the temperature of the LED, a storage means for storing in advance a temperature correction voltage value of the voltage value corrected corresponding to the temperature, and outputting the read temperature correction voltage value. It has an output means.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

Referring to FIG. 1, the LED driver LSI of the present invention includes a drive section 5, an LED array 6, and a resistor R similar to the conventional one, and also includes a temperature sensor 1 and a temperature sensor A instead of a reference voltage source VR. /D converter 2, EEPROM (non-volatile memory in which stored information can be electrically erased and rewritten) 3, and
It has a configuration in which an A converter 4 is added.

Next, the operation of this embodiment will be explained.

The temperature sensor 1 is provided within the LED array 6 and measures the temperature of the LEDs during operation and converts the temperature into a voltage value.

The operating principle of the temperature sensor 1 is to utilize the temperature coefficient of the pace emitter voltage VBH of a bipolar transistor.

The output of the temperature sensor 1 is temperature information expressed as a voltage value, which is input to the A/D converter 2 and converted into a digital value.

Temperature information converted to digital values is stored in EEFROM.
B, the address of EEFROMB is specified and its contents are read.

In EEFROMB, a value that compensates for the temperature characteristics of the amount of light emitted by the LED is written in advance using temperature information as an address.

The digital temperature information read from EEFROMB is
The signal is input to the D/A converter 4 and converted into a voltage value.

The output voltage VRO of the D/A converter 4 is input to the operational amplifier OPI of the drive section 5.

The output voltage of this D/A converter 4, that is, the temperature-corrected reference voltage VRO corresponds to the reference voltage VR in the conventional example described above, and therefore, by the resistor R,
By replacing VR with VRO in the relationship shown in equation (1), the operating current I of each LED of the LED array 6 is determined.

Here, the bit configuration of A/D converter 2 and EEFRO
The MB capacity can be set arbitrarily.

- As an example, the temperature correction range of the LED recording head is
℃ to 100℃, A/D converter 2 is 8 bits, EEF
It is assumed that the ROMB is 256 words x 6 bits, and the output voltage value can be set from 0.5V to 31.5V at intervals of 0.02V.

Here, the voltage value VR that satisfies the reference light amount at room temperature 25°C
If it is 1.0■, then 1.3V for a temperature of 70℃,
Further, data is written in the EEFROMB so that the output voltage is 0.7 cm for 0°C.

The temperature correction reference voltage VRO is set so as to obtain a current value that corrects the change in light intensity of each LED of the LED array due to temperature change.6 Although the present invention has been described in detail above, the present invention is Various modifications are possible without being limited to the above embodiments.

〔Effect of the invention〕

As explained above, the present invention stores the temperature correction voltage value of the reference voltage for controlling the current in the storage means so as to correct the temperature characteristics of the light emission amount of the LED, and the ambient temperature measurement from the temperature detection means is performed. By reading out the temperature-corrected voltage value corresponding to the temperature-corrected voltage value and controlling the reference voltage that determines the output current value of the LED driver circuit, the amount of light emitted by the LED can be kept constant regardless of the ambient temperature. .

PROM, 4...D/A converter, 5...Driver, 6
...LED array, 51-5n...driver circuit.

Claims (1)

[Claims] An LED driver LSI that drives an LED recording head for electrophotographic recording using an LED array in which LEDs are arranged in a row by a constant current drive circuit that outputs a current set by the voltage value of a reference voltage source. A temperature measuring means for measuring the temperature of the LED, a storage means for storing in advance a temperature-corrected voltage value of the voltage value corrected according to the temperature, and an output for outputting the read temperature-corrected voltage value. LED characterized in that it has a means
Driver LSI.