Application of TLC5615 serial digital-to-analog converter in switching power supply

The switching power supply has the advantages of small size, high efficiency, light weight, and low noise, and its application is becoming more and more extensive. When we design the battery charger, we use the switching power supply as the main circuit, in which the switching device uses the third-generation IGBT, whose main advantages are high voltage resistance, low driving power, high switching frequency, and low on-resistance. In the design of the main control board, a variable reference power supply is required. Changing the size of the reference voltage can change the size of the charging voltage, and the variable reference voltage needs to be generated with the help of a digital-to-analog converter. At present, digital-to-analog converters can be divided into two categories based on the interface: parallel interface digital-to-analog converters and serial interface digital-to-analog converters. The parallel interface digital-to-analog converter has many pins, a large size, and occupies more port lines of the single-chip microcomputer; while the serial digital-to-analog converter is small in size and occupies fewer port lines of the single-chip microcomputer. In order to reduce the area of ​​the circuit board and the port lines occupied by the single-chip microcomputer, the TCL5615 serial digital-to-analog converter can be used to generate a variable reference voltage.

1 Introduction to TLC5615 Serial Digital-to-Analog Converter

TLC5615 is a product launched by Texas Instruments in the United States in 1999. It is a digital-to-analog converter with a serial interface. Its output is voltage type, and the maximum output voltage is twice the reference voltage. It has a power-on reset function, which resets the DAC register to all zeros. TLC5615 has a high performance-price ratio and is currently very easy to purchase in the domestic market.

1?1 Features of TLC5615

(1) 10-bit CMOS voltage output;

(2) 5V single power supply;

(3) Three-wire serial interface with CPU;

(4) The maximum output voltage can reach twice the reference voltage;

(5) The output voltage has the same polarity as the reference voltage;

(6) Settling time 12 ± 5 μs;

(7) Internal power-on reset;

(8) Low power consumption, maximum only 1.75mW.

1?2TLC5615 pin description

The TLC5615 is available in a small plastic DIP package. The pinout of the TLC5615 chip in the DIP package is shown in Figure 1.

The pin functions are described as follows:

——Pin 1DIN: serial data input terminal;

——Pin 2SCLK: serial clock input;

——Pin 3CS: The chip uses the pass terminal, and low level is effective;

——Pin 4DOUT: used for serial data output when cascading;

——Pin 5AGND: analog ground;

——Pin 6REFIN: reference voltage input terminal;

Figure 1 TLC5615 pinout

Figure 2 Timing diagram of TLC5615

Figure 3 Interface circuit between TLC5615 and AT89C51 microcontroller

——Pin 7OUT: DAC analog voltage output terminal;

——Pin 8VDD: positive power supply terminal.

1?3TLC5615 Timing Analysis

The timing of TLC5615 is shown in Figure 2.

It can be seen from the timing diagram that when the chip select CS is low, the input data DIN is synchronously input or output by the clock SCLK, and the most significant bit is in front and the least significant bit is in the back. During input, the rising edge of SCLK shifts the serial input data DIN into the internal 16-bit shift register, the falling edge of SCLK outputs the serial data DOUT, and the rising edge of the chip select CS transfers the data to the DAC register.

When the chip select CS is high, the serial input data DIN cannot be sent to the shift register synchronously by the clock; the output data DOUT keeps the latest value unchanged and does not enter the high-impedance state. Therefore, in order to serialize the input data and output data, two conditions must be met: the first is the effective jump of the clock SCLK; the second is that the chip select CS is low. Here, in order to minimize the internal feedthrough of the clock, when the chip select CS is high, the input clock SCLK should be low.

There are two ways to use the serial digital-to-analog converter TLC5615 , namely the cascade method and the non-cascade method. If the cascade method is not used, DIN only needs to input 12 bits of data. Among the 12 bits of data input by DIN, the first 10 bits are the D/A conversion data input by TLC5615, and the high bit is in front and the low bit is in the back when inputting. The last two bits must be written to the bits below the LSB with a value of zero, because the DAC input latch of TLC5615 is 12 bits wide. If the cascade function of TL5615 is used, the data from DOUT needs to input the 16-bit clock falling edge, so it takes 16 clock cycles to complete a data input, and the input data should also be 16 bits. Among the input data, the first 4 bits are high virtual bits, the middle 10 bits are D/A conversion data, and the last 2 bits are bits below the LSB, that is, zero.

2 Application Circuit Examples

Figure 3 shows the interface circuit of TLC5615 and AT89C51 microcontroller in the switching power supply. In the circuit, P3.0-P3.2 of AT89C51 microcontroller respectively control the chip select CS, serial clock input SCLK and serial data input DIN of TLC5615. The circuit connection adopts non-cascade mode. According to the design requirements of the switching power supply, the variable reference voltage range is 0V~4V. Therefore, the reference voltage of TLC5615 is selected as 2.048V, and its maximum analog output voltage is 4.096V. It can meet the requirements of the switching power supply.

TLC5615 uses a non-cascaded method to store the 12-bit data to be input in registers R0 and R1. Its D/A conversion procedure is as follows:

CLRP3?0; chip select valid

MOVR2, #4; the first four data bits to be sent

MOVA, R0; the first four bits of data are sent to the lower four bits of the accumulator

SWAPA; the upper and lower four bits of A are swapped

LCALLWR?data; DIN inputs the first four bits of data

MOVR2, #8; the last eight bits of data to be sent

MOVA, R1; eight bits of data are sent to accumulator A

LCALLWR?data; DIN inputs the last eight bits of data

CLRP3?1; clock low level

SETBP3?0; chip select high level, input 12-bit data is valid

END: End

The sending subroutine is as follows:

WR?data: NOP; no operation

LOOP: CLRP3?1; clock low level

RLCA; data input flag CY

MOVP3?2, C; data input is valid

SETBP3?1; clock high level

DJNZR2, LOOP; loop sending

RET Return

3 Conclusion

By using the analog-to-digital converter TLC5615 with a simple interface, the hardware circuit of the switching power supply is greatly simplified, the circuit board area is reduced, and the cost is reduced.