4.5ns single/dual power supply rail-to-rail output comparator LT1719 and its applications

    Abstract: LT1719 is a high-speed dual-power comparator produced by Linear Technology using optimized processes. It adopts rail-to-rail output mode and has a response time of only 4.5ns. It can be used in applications where the input signal changes slowly, and can interface directly with TTL and CMOS signals. The article introduces the main features, pin functions, working principle and typical applications of LT1719. Finally, an oscillation circuit composed of LT1719 is given.

    Keywords: comparator rail-to-rail voltage isolation LT1791

1 Overview

LT1719 is a 4.5ns high-speed dual-power comparator manufactured by Linear Technology using optimized production process technology. The comparator adopts rail-to-rail output mode and can operate normally under low voltage conditions. Its input voltage range is between VEE-100mV and Vcc-1.2V. Since the LT1719 comparator uses delay technology internally, it is particularly suitable for applications where the input signal changes slowly. Its rail-to-rail output ensures direct interface with TTL and CMOS signals, and its balanced output allows it to be conveniently used in analog or single-supply logic level conversion. In addition, the LT1719 also has a sleep function, which can greatly reduce power consumption and therefore can be widely used in portable battery-powered systems.

LT1719 has two package forms, one is SO-8-pin form and the other is SOT-23 package form. The LT1719 in an SO-8-pin package has power isolation capabilities, including the ability to isolate analog input and output logic levels. Therefore its operation is more flexible.

The main features of LT1719 are as follows:

●LT1719 in SO-8 package can isolate input and output power voltages;

●Optimized output with 3V and 5V power supply voltage;

●Has rail-to-rail output compatible with TTL/CMOS levels;

●With low power consumption power-down mode, the power supply current in power-down mode is 0.1μA;

●The power supply current at 3V is 4.2mA;

●Fast speed, the response time when driven by 20mV is only 4.5ns, and the response time when driven by 5mV is 7ns;

●LT1719 comparator can operate with 3V and 5V dual power supplies, and has fast response speed and flexible operation. It can be widely used in high-speed differential receiving systems, crystal oscillator circuits, level conversion, pre-detectors, zero-distortion detectors, high-speed sampling circuits and delay lines.

2 Pin functions and main parameters

2.1 Pin function

The pinout of the LT1719 comparator is shown in Figure 1. The function description of each pin is listed in Table 1.

Table 1 Pin function description of LT1719

2.2 Main parameters

The LT1719 in SO-8 package has power isolation function. The following are the main parameters of SO-8 package:

●Input power supply voltage (Vcc-VEE): 2.7~10.5V;

●Maximum input power voltage: 12V;

●Maximum voltage range of +VS to ground: 7V;

●VEE maximum voltage range to ground: -12V～0.3V;

●Output power supply voltage range +VS: 2.7V~6V;

●Maximum input voltage VCMR (MIN): VEE-0.1V;

●Maximum input voltage VCMR (MAX): VCC-1.2V;

●Maximum input current: ±10mA;

●Maximum output current: ±20mA.

The operating temperature range of LT1719 is divided into two levels: Class C and Class I. The operating temperature range of Class C is 0~70℃, and the working temperature range of Class I is -40~+85℃.

3 Working principle

Figure 2 shows the internal schematic block diagram of the LT1719 dual-supply comparator. As can be seen from the figure: LT1719 has a pair of differential input terminals, a bias control terminal and a complementary common emitter output terminal. Since all its internal signal channels use low-voltage feedback, the LT1719 comparator has high speed and extremely low power consumption.

Since the input terminals of the LT1719 require no power supply within its dynamic range, there is a certain dead zone at the two input terminals of the rail-to-rail input comparator. In this way, even for a 2.7V power supply signal, the LT1719 still has a common-mode input range of 1.6V. Additionally, the rail-to-rail format of the differential input voltage range does not require the use of larger input currents in the comparator. Likewise, phase reversal protection at the input can be used to prevent output errors when the input voltage drops to VEE-100mV.

The internal delay of LT1719 is completed by a nonlinear forward feedback circuit added to the second stage. Before the second stage output, the circuit completely uses differential signals. After the second stage output, the output signal is divided into two channels to drive two symmetrically matched transistors respectively. The two transistors are connected rail-to-rail in a common-emitter output. Two Schottky diodes limit the LT1719's output voltage to approximately 300mV, eliminating the need for 50mV or 15mV rail-to-rail linear amplifiers and other devices. But this does not change the digital form of the LT1719 comparator output. Its output status can still be used to directly drive TTL or CMOS levels. It can also drive ECL levels.

4 application circuit

4.1 Typical application circuit

Due to its fast response speed and flexible operation, the LT1719 comparator is widely used in application fields such as high-speed differential receiving systems, crystal oscillator circuits, level conversion, pre-detectors, zero-distortion detectors, high-speed sampling circuits and delay lines. application. Figure 3 is a typical application circuit of a crystal oscillator with TTL and CMOS level outputs composed of LT1719. This circuit operates from a voltage of 2.7 to 6V and uses a 2kΩ and a 620Ω voltage divider resistor to bias the forward input of the LT1719. On its reverse end, a 0.1μF capacitor and two resistors (resistance values ​​2kΩ and 1.8kΩ respectively) are used to set a suitable straight level output reference for its output end. In addition, using a 1MHz to 10MHz crystal oscillator can provide positive feedback for the circuit, and at the same time, it can also stabilize the oscillation frequency. It can be seen that this circuit uses very few components. It can work with only a few resistors, a capacitor and a crystal oscillator. The circuit composition is very simple.

4.2 Oscillator with 50% duty cycle

Although LT1719 can provide a relatively stable logic output. However, when the LT1719 is working, there is always a certain output delay. Therefore, the DC voltage bias on the LT1719's input stage should be set near the midpoint of its common-mode range. At the same time, when a change between 2.7V and 6V is added to the positive feedback loop, the crystal oscillator frequency is also adjusted between 1MHz and 10MHz. In fact, LT1719 can be activated at 2.0~2.2V. In other words: LT1719 can obtain a more ideal oscillation frequency at 2.0~2.2V.

Figure 4 is an oscillator circuit that can obtain a 50% duty cycle. This circuit is useful for applications that require only a 50% duty cycle. In practice, the circuit's duty cycle is also related to the error range of all resistors and the specific selection of the LT1719. In addition, it should be noted that the working cycle of this circuit will have a slight offset when the LT1719 comparator load is different. Therefore, in critical applications, the minimum capacitive reactance of its load should also be considered.