High-Voltage Serial-to-Parallel Converter: Designing with the Microchip HV57708PG-G
In the realm of industrial automation, medical imaging, and printing systems, the need to control numerous high-voltage actuators, piezoelectric elements, or LEDs efficiently is paramount. This is where high-voltage serial-to-parallel converters (HV SPCs) become indispensable. These specialized integrated circuits (ICs) allow a low-voltage microcontroller to command a large array of high-voltage outputs using a minimal number of GPIO pins. The Microchip HV57708PG-G stands out as a powerful and versatile solution in this category, enabling designers to simplify system architecture and enhance performance.
The HV57708PG-G is a 16-channel, high-voltage serial-to-parallel converter. Its core function is to receive serial data from a microcontroller (MCU) via a standard SPI (Serial Peripheral Interface) protocol and latch that data into 16 parallel output registers. Each output channel is capable of driving loads with voltages up to 200V, making it suitable for a wide range of demanding applications. A key advantage of this device is its ability to drastically reduce the number of control lines needed; controlling 16 outputs requires only three or four MCU pins (SPI CLK, Data In, Latch, and optionally Enable), a significant reduction from 16 individual lines.
Design Considerations and Key Features
Successful implementation of the HV57708PG-G hinges on understanding its critical features and their implications for the design:
1. Serial Data Daisy-Chaining: One of the most powerful features of this IC is its daisy-chain capability. The serial data output (SDO) pin of one HV57708 can be connected to the serial data input (SDI) of the next. This allows hundreds of output channels to be controlled from a single SPI bus, creating expansive arrays without a proportional increase in MCU pin count. This architecture is highly scalable for large systems.
2. Integrated Level-Shifting and Drivers: The device incorporates all necessary circuitry to translate low-voltage logic levels (e.g., 3.3V or 5V) into high-voltage output signals. Each channel features an open-drain NMOS transistor capable of sinking up to 150mA (per channel), with a maximum power dissipation of 1.5W. This eliminates the need for external level-shifters or discrete high-voltage transistors, saving board space and reducing component count.
3. Robust Protection and Diagnostics: Designing for high voltage necessitates robust protection. The HV57708PG-G includes thermal shutdown circuitry to protect the IC from overheating. Furthermore, its open-drain architecture inherently provides short-circuit protection for the outputs, a critical safety feature in high-power environments.
4. PCB Layout and Power Decoupling: High-voltage and high-current switching can introduce significant noise. A meticulous PCB layout is non-negotiable. This includes:
Using separate, low-inductance ground planes for the low-voltage logic (VDD) and the high-voltage power (VPP).
Placing bulk and bypass capacitors as close as possible to both the VDD and VPP pins. A combination of a large electrolytic or tantalum capacitor (e.g., 10µF) and a small ceramic capacitor (e.g., 100nF) is recommended for each supply rail to filter both low and high-frequency noise.
Keeping high-current output traces short and wide to minimize parasitic inductance and voltage drops.
5. Load Compatibility and Flyback Diodes: The open-drain outputs are designed to sink current. This means the load must be connected between the high-voltage supply (VPP) and the output pin. For inductive loads, such as solenoids or motors, external flyback diodes are absolutely mandatory across each load to suppress voltage spikes generated when the output transistor turns off, protecting the IC from damage.
Application Circuit Overview
A typical application circuit involves connecting the MCU's SPI pins to the SDI, CLK, and LATCH pins of the HV57708. The VDD pin is supplied with the MCU's logic voltage (e.g., 3.3V). The high-voltage rail, connected to VPP, is provided by an external DC-DC converter, the value of which depends on the load requirements (e.g., 100V for piezoelectric elements). Each output pin is connected to one side of its respective load, with the other side of the load tied to VPP. The daisy-chain feature is implemented by connecting the SDO pin to the SDI pin of the next converter in the array.
The Microchip HV57708PG-G is an exceptional integrated circuit that empowers engineers to tackle the challenge of controlling extensive arrays of high-voltage devices with remarkable efficiency and simplicity. Its daisy-chain architecture, integrated high-voltage drivers, and built-in protection features make it an ideal choice for applications ranging from industrial automation and medical equipment to advanced lighting systems. By adhering to sound design practices, particularly in power decoupling and layout, designers can leverage this IC to create robust, scalable, and high-performance solutions.
Keywords
1. High-Voltage Driver
2. Serial-to-Parallel Converter
3. SPI Interface
4. Daisy-Chaining
5. Open-Drain Output
