Digital power control technology new guarantee does not rely on digital control signal MOSFET driver

New digital power controllers such as the UCD9110 or UCD9501 need to be supported by new intelligent integrated MOSFET drivers. Power designers are still skeptical about digital power control technology. They often blame the blue screen phenomenon of the PC on software conflicts. Of course, this controversy hinders the promotion of digital control power supplies and power stage protection strategies during controller fault finding. This promotes the development of MOSFET drivers with internal protection functions of the power stage that do not rely on digital power controller signals.

Figure 1 is a typical implementation of a digitally controlled power supply. The digital power controller on the left side of the figure usually operates at 3.3V. Due to the digital low-voltage processing method used in the controller design, the digital controller cannot be used directly to drive the MOSFET for stability and noise considerations. The interface between the controller and the power stage is provided by the MOSFET driver. The MOSFET driver usually receives the output signal of the PWM or digital controller and converts it into a high current signal suitable for efficiently turning the MOSFET on and off. If the controller signal is interfered or erroneous, the ordinary MOSFET driver will not provide any protection function. The UCD7K series MOSFET driver introduced by TI will be able to protect the power stage from major faults caused by interference with the drive signal. The ultra-high-speed current sensing comparator built into the MOSFET driver provides power stage protection. Figure 2 is a related block diagram.

Integrated ultra-fast current limiting function

The UCD7K MOSFET driver receives logic level input signals from the digital controller and converts them into ±4A high current MOSFET gate drive signals that are connected to the power stage. The driver provides a periodic current limit function with programmable thresholds and digital output current limit flags. By monitoring the current flags, the host controller can select the appropriate algorithm and derive the required current limit configuration parameters (profile). In the rare case that the digital system cannot respond to the fault in time, this fast (25ns) periodic current limit protection function will shut down the power stage. The main advantage of the local overcurrent protection function is that the UCD7K device can protect the power stage when the software code in the digital controller is corrupted or terminated. If the controller PWM output maintains a high current, the local current detection circuit will shut down the driver output when an overcurrent condition occurs. The system is likely to enter a retry mode because most DSPs and microcontrollers are equipped with on-board watchdogs, power-down resets and other monitoring peripherals that can restart the device when it is not operating properly. However, these peripherals are usually slow to react and cannot protect the power stage from damage. The UCD7K's current limit comparator provides the required fast protection function for the power stage.

The current limit threshold can be set arbitrarily from 0.25V to 1.0V by applying the desired threshold voltage at the current limit (ILIM) pin. This voltage can be applied using a resistor divider or a digital controller plus a digital-to-analog converter. In any case, the maximum threshold voltage is internally limited to 1.0V, and external voltage settings above 1.0V have no effect, providing another protection function in the event of D/A converter damage.

TrueDrive output architecture

For fast switching speeds, the output of the UCD7K driver uses the TrueDrive output architecture, which inputs a rated current of ±4A to the gate of the MOSFET during the "Miller" plateau of the switching transition. TrueDrive consists of a pull-up/pull-down circuit consisting of a bipolar transistor and a MOSFET in parallel.

High voltage start-up JFET + precision reference

The UCD7K series devices with the second digit of the part number equal to or greater than 5 (such as UCD7500, UCD7601) have a built-in 110V startup JFET that can be directly connected to the 48V communication bus voltage without external resistors. The JFET provides current during startup and is disabled when the bias winding is connected to the VDD pin to obtain sufficient operating current.

The UCD7K series devices also include a 1% accurate, 3.3V, 10mA linear regulator that serves as both a reference voltage and a power supply for the digital controller.

Digital Power Applications

The UCD7500 driver in Figure 3 connects the digital controller on the left to the power stage on the right. Pin 1 of the UCD7500 is directly connected to the communication input voltage bus, and the internal JFET provides current during startup. The microcontroller is powered by the 3.3V voltage regulator of the MOSFET driver. The CLF flag remains high during startup until the internal and external supply voltages of the UCD7500 enter the operating range. At this time, the CLF flag will go low and the UCD7500 begins to process the input drive signal. During startup, the microprocessor monitors the CFL flag, and when the CLF flag goes low, the microprocessor sends a power pulse to the MOSFET driver. The MOSFET driver receives the input pulse from pin 3. At the same time, it receives the current limit setting from pin 6 of the microcontroller. The current sense resistor connected to pin 8 monitors the current through the power stage. Once the current through the current sense resistor exceeds the current limit setting on ILIM, the MOSFET driver immediately turns off the MOSFET gate drive and sends a current limit flag to the microcontroller. The current limit flag clears when the microcontroller sends a new gate drive pulse to the UCD7500. This technique enables the microcontroller to decide how to respond to a current limit event, such as providing more current to the load for a certain period of time (during the start-up of a motor drive). The microcontroller will increase the current limit threshold and may also count the number of current limit flag pulses to tolerate a certain number of current limit events before issuing a shutdown command.

Figure 3: Typical application of UCD7500 MOSFET driver in digital control power supply

Analog Power Applications

In Figure 4, the UCD7600 MOSFET driver is connected to the UCC28221 with built-in PWM controller. The UCD7600 provides two independent MOSFET drivers, each with independent current limit comparators and current limit flags. In the application of Figure 4, the fixed current limit thresholds of the two comparators are provided by resistor dividers with an internal 3.3V voltage applied. These thresholds are used as secondary current sensing limits for hiccup mode.