JPH05315913A - Output current driving circuit - Google Patents

Abstract

PURPOSE:To suppress a voltage change speed at the time of switching and to reduce spurious radiation. CONSTITUTION:The output of a transfer gate constituted of transistors(TRs) 3, 4 is connected to the gate of a TR 1 and proper voltage less than power supply voltage is impressed to the input Vi of the transfer gate, so that the drain current of the TR 1 can be restricted and the change speed of an output Vout of this output current driving circuit can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output current drive circuit in a semiconductor device.

[0002]

2. Description of the Related Art A conventional current drive circuit using a MOS transistor has a simple structure in which an input terminal is connected to the gate of the transistor and an output terminal is connected to the drain.
FIG. 2 shows a circuit diagram of an open drain type current drive circuit, and description will be made with reference to this.

In the conventional circuit, when Vi changes from low level to high level and the transistor is turned on, the following drain current I _DS flows through the drain of the transistor.

[0004] Vout <time of the Vi-V _{T is, I DS = β / 2 /} (Vi-V T) 2 .................. (1) Vout> time of the Vi-V _T is, I _DS = β _{{(Vi-V T) Vout} -Vout 2/2} ...... (2) provided that β = με (W / L) / t OX where, V _T is the threshold voltage of the transistor, mu is the electron mobility, ε is the dielectric constant of the gate insulating film, t
_OX is the thickness of the gate oxide film, W is the channel width, and L is the channel length.

[0005]

However, in order to drive the current outside the device according to the TTL standard, it must be designed so that a large current can be driven when Vout is low. Then, as can be seen from the above equations (1) and (2), a larger current flows when Vout is still at the high level immediately after the circuit starts current driving. Especially when multiple circuits change simultaneously like a bus driver,
This causes a problem of unnecessary radiation because a large current flows instantaneously when the driving transistor is switched. The present invention is intended to solve the above problems, and an object of the present invention is to provide an output current drive device capable of reducing unnecessary radiation while ensuring current drive capability.

[0006]

In order to solve the above problems, the present invention suppresses the voltage change speed of a transistor by setting the gate voltage of the transistor to be equal to or lower than the power supply voltage.

[0007]

According to the above circuit, unnecessary radiation can be reduced by setting the gate voltage of the transistor to be equal to or lower than the power supply voltage and suppressing the voltage change speed during switching.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the current drive circuit of the present invention will be described with reference to the drawings.

FIG. 1 shows the configuration of the open drain type current drive circuit of this embodiment. The input signal IN is a transistor T
It is connected to r3, Tr4, and the inverter I1. Tr
3, Tr4 and I1 form a transfer gate, and V
A voltage Vi satisfying i <VDD is input to the transfer gate, the output is input to the gate of the current drive transistor Tr1, and the drain of the transistor Tr2 having the output of I1 as the gate is connected to the gate of Tr1. Where V
DD is the power supply voltage of the circuit. If the voltage input to the gate of Tr1 has the above value, T
r1 drain of - source current I _DS is reduced, the voltage variation of the output Vout resulting slows. In this embodiment, when the input signal IN changes from low level to high level, Tr3 and Tr4 are turned on, Tr2 is turned off, and Vi is applied to the gate of Tr1 to output Vo.
At ut, draw current. When IN becomes low level, Tr3 and Tr4 are turned off, Tr2 is turned on, and the gate of Tr1 is fixed at low level.
There is no change in ut. By appropriately selecting the size of Tr1 and the voltage Vi, it is possible to obtain a current drive circuit that satisfies the standard in current drive capability and further reduces the voltage change speed in Tr1.

[0010]

As described above, according to the current drive circuit of the present invention, by turning on the gate of the transistor for current drive at a voltage lower than the power supply voltage, the voltage change at the switching time becomes slower than that of the conventional circuit. Therefore, unnecessary radiation can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a current drive circuit of the present invention.

FIG. 2 is a circuit diagram of a conventional current drive circuit.

[Explanation of symbols]

 Tr1 N-type MOS transistor Tr2 N-type MOS transistor Tr3 N-type MOS transistor Tr4 P-type MOS transistor I1 Inverter

Claims (1)

[Claims] 1. An output current drive circuit, wherein an output current is controlled by applying a voltage lower than a power supply voltage to a gate of a MOS type transistor.