ZHCSNM5C november 2020 – august 2023 UCC25800-Q1
PRODUCTION DATA
The UCC25800-Q1 transformer driver drives an LLC converter with constant switching frequency to make the LLC converter operate near its resonant frequency. When LLC converter operates at its resonant frequency, the impedance of the resonant tank is equal to zero. The input and output voltages are virtually connected together, through the transformer turns-ratio. Given the LLC converter is a half-bridge converter, the transformer primary side only sees half of the input voltage. If the secondary side uses voltage double rectifier, it also only sees half of the output voltage. The relationship between the input and output voltages is simply the transformer turns-ratio.
Given that, to achieve a fixed output voltage, the input voltage needs to be fixed. Even though the transformer driver is recommended to operate with an input voltage source between 9 V and 34 V, it is meant to be one fixed voltage within this voltage range. Because the relationship between input and output voltages is simply the transformer turns-ratio, the accuracy of the input would impact the accuracy of the output. There is no requirement from the transformer driver, while the input voltage accuracy is demanded by the output voltage requirements.
When the input voltage is very close to 9 V (because it is very close to the UVLO threshold UVLOF) sufficient input bypass capacitor is recommended to ensure the load transient does not cause the VCC voltage drops below UVLO threshold UVLOF.