7.3 Feature Description

7.3.1 Enable and Disable

Forcing EN/FLT pin low disables the device, which greatly reduces the VINP power consumption. Pull the

EN/FLT pin high to enable normal device functionality. The EN/FLT pin has a weak internal pull-down resistor so

it is not recommended to leave this pin floating in noisy systems.

7.3.2 Output Voltage Soft-Start

The UCC33020 has soft-start mechanism that ensures a smooth and fast soft-start operation with minimum input

inrush current. The output voltage Soft-Start diagram is shown in Figure 7-1. After VINP > VVINP_UVLO_R and

EN/FLT is pulled high, the soft-start sequence starts with a primary duty cycle open loop control. The power

stage operates with a fixed burst frequency with an incremental increasing duty cycle starting at 6.5%. The

rate of change of the duty cycle is pre-programmed in the part to reduce the input inrush current while building

the output voltage VCC. The primary side limits the maximum duty cycle to 62.5% during this phase till the

secondary side VCC voltage passes VVCC_UVLO = 2.7V threshold before releasing this duty cycle limit. This limit

will ensure minimum input current in case the device starts on a short circuit and the VCC is not building up.

The soft-start time will vary depending on the output capacitors, input voltage and loading conditions. The

UCC33020 has a soft-start timeout feature by which the VCC output voltage state is monitored during soft-start.

In certain conditions, the VCC might not reach steady-state regulation threshold due to short circuit on the output

voltage as shown in Figure 7-2, heavy loading conditions above recommended operating conditions or higher

output capacitor values as shown in Figure 7-3. In these conditions, if the soft-start timeout duration of 16ms

expires without the VCC reaching steady-state regulation, the part will shutdown and EN/FLT pin will be pulled

low for 200μs to report the fault condition. An auto-restart timer will start afterwards, the part will attempt to

restart after that timer expires. More details regarding fault reporting and auto-restart can be found in Fault

Reporting and Auto-Restart. If the same conditions continue to exist the same cycle will repeat again as shown

in Figure 7-2 and Figure 7-3 below.

UCC33020

SLUSFK4A – FEBRUARY 2025 – REVISED MAY 2025

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