“In the semiconductor world, there are many I/O standards that usually require different voltage levels for input voltage (VIH or VIL) and output voltage (VOH or VOL) according to the operating voltage of the device. These voltage levels define how the device communicates with other devices, and the voltage levels are expressed as bus standards. Some of these bus standards include 5 V CMOS, 5 V TTL, 3.3 V LVTTL, 2.5 V AGP graphics port and 1.5 V GTL + host bus.
Philips Semiconductors Gunning Transceiver Logic Converter Voltage Clamping (GTL-TVC) series of bidirectional low voltage converters are designed using BiCMOS technology to protect sensitive I/O on new advanced sub-micron components. The GTL-TVC device protects these new devices from the overvoltage and ESD applied by the old and old devices, and converts the VIH and VOH switching levels. The information provided in this application note describes the I/O protection application and voltage conversion of the GTL-TVC series, and enables design engineers to successfully interface devices with different I/O voltage levels.
Voltage level shift
GTL-TVC devices can be used to establish an interface between the I/O of devices operating at different voltage levels. Since both sides of the GTL-TVC device are open-drain, pull-up resistors may be required, depending on the I/O interface type (totem pole or open-drain) and conversion direction (from high to low, from low to high) Or both). Orientation). As long as the voltage difference between the gate and source voltages is maintained at approximately 1 V, the GTL-TVC device can switch between any voltage from 1.0 V to 5.0 V. The recommended circuit in Figure 1 connects the gate (GREF) and the reference drain (DREF) together to reach a voltage through a 200 kΩ resistor, which should be at least 1.5V higher than the reference source (SREF) level. This circuit biases the gate to a threshold higher than the reference source voltage and compensates for threshold changes between parts.
Figure 1 Typical two-bit two-way application
For bidirectional conversion, the drivers on both sides of the GTL-TVC device must have open-drain outputs, or at least they must be controlled in such a way that the contention between the high level and the low level on the output driver on one side prevents the other The voltage on the output driver on the side is too high. The simplest solution is to use an open-drain device (standard GTL and I2 C/SMBus outputs are open-drain outputs).
When using an open-drain device, a pull-up resistor must be used, and its size must ensure that the output driver will not be overloaded, nor will it exceed the recommended 15 mA current when using GTL-TVC devices in translation applications. If the device is used for I2C bus conversion, the size of the resistance must be determined to provide a current of less than 3 mA (the I2C device is specified to drive a maximum of 3 mA at 0.4 V).
When using a GTL-TVC device as shown in Figure 1, since the GTL-TVC device will rise to the reference power supply voltage, there is no need for a pull-up resistor on the lower voltage side. When a 200kΩ resistor is connected to the reference drain (DREF) and reference gate (GREF) and pulled up to a power supply voltage that is at least 1.5 V higher than the reference source (SREF) level, about 2.5 µA of current will flow to the reference voltage Source (SREF). The lower voltage side is the reference source voltage value. If leakage on the low-voltage side is known (greater than 2.5 µA above), a pull-up resistor must be included on both the low-voltage side and the high-voltage side to provide the required current value.
When performing down conversion, since there is no driver on the lower voltage side, the higher voltage driver can be a totem pole without any pull-up resistor, or it can be an open-drain with a pull-up resistor, and it is not required on low voltage Unless there is excessive leakage (greater than the 2.5 µA described in the previous paragraph) for the resistor pull-up, ensure that it is leak-free.