When the CS voltage is below ILIM, the driver output will follow the PWM input. The CLF digital output flag

can be monitored by the host controller to determine when a current limit event occurs and to then apply the

appropriate algorithm to obtain the desired current limit profile.

One of the main benefits of this local protection feature is that the UCD7K devices can protect the power stage

if the software code in the digital controller becomes corrupted and hangs up. If the controller’s PWM output

stays high, the local current sense circuit will turn off the driver output when an over-current condition occurs.

The system would likely go into a retry mode because; most DSP and microcontrollers have on-board watchdog,

brown-out, and other supervisory peripherals to restart the device in the event that it is not operating properly.

But these peripherals typically do not react fast enough to save the power stage. The UCD7K’s local current limit

comparator provides the required fast protection for the power stage.

The CS threshold is 25 mV below the ILIM voltage. This way, if the user attempts to command zero current

pin receives a pulse. At start-up it is necessary to ensure that the ILIM pin always greater than the CS pin for

the handshaking to work as described below. If for any reason the CS pin comes to within 25 mV of the ILIM

pin during start-up, then the CLF flag is latched high and the digital controller must poll the UCD7K device,

by sending it a narrow IN pulse. If the fault condition is not present the IN pulse resets the CLF signal to low

indicating that the UCD7K device is ready to process power pulses.

9.3.3 Handshaking

The UCD7K family of devices have a built-in handshaking feature to facilitate efficient start-up of the digitally

controlled power supply. At start-up the CLF flag is held high until all the internal and external supply voltages

of the UCD7K device are within their operating range. Once the supply voltages are within acceptable limits, the

CLF goes low and the device will process input drive signals. The micro-controller should monitor the CLF flag at

start-up and wait for the CLF flag to go LOW before sending power pulses to the UCD7K device.

9.3.4 Driver Output

The high-current output stage of the UCD7K device family is capable of supplying ±4-A peak current pulses and

swings to both VDD and GND. The driver outputs follows the state of the IN pin provided that the VDD and 3V3

voltages are above their respective under-voltage lockout threshold.

The drive output utilizes Texas Instruments’ TrueDrive™ architecture, which delivers rated current into the gate

of a MOSFET when it is most needed during the Miller plateau region of the switching transition providing

efficiency gains.

TrueDrive™ consists of pullup/ pulldown circuits using bipolar and MOSFET transistors in parallel. The peak

output current rating is the combined current from the bipolar and MOSFET transistors. The output resistance is

the bipolar transistor. This hybrid output stage also allows efficient current sourcing at low supply voltages.

Each output stage also provides a very low impedance to overshoot and undershoot due to the body diode of

the external MOSFET. This means that in many cases, external-schottky-clamp diodes are not required.

9.3.5 Source/Sink Capabilities During Miller Plateau

Large power MOSFETs present a large load to the control circuitry. Proper drive is required for efficient, reliable

operation. The UCD7K drivers have been optimized to provide maximum drive to a power MOSFET during

the Miller plateau region of the switching transition. This interval occurs while the drain voltage is swinging

between the voltage levels dictated by the power topology, requiring the charging/discharging of the drain-gate

capacitance with current supplied or removed by the driver device. See Reference [1]

9.3.6 Drive Current and Power Requirements

The UCD7K family of drivers can deliver high current into a MOSFET gate for a period of several hundred

nanoseconds. High peak current is required to turn the device ON quickly. Then, to turn the device OFF, the

driver is required to sink a similar amount of current to ground. This repeats at the operating frequency of the

power device. A MOSFET is used in this discussion because it is the most common type of switching device

used in high frequency power conversion equipment.

UCD7100

SLUS651E – MARCH 2005 – REVISED NOVEMBER 2021

www.ti.com

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