SLVSIL5 Data sheet

SLVSIL5A May 2025 – September 2025 UCC25661-Q1

PRODUCTION DATA

8.2.2.16 BLK Pin Voltage Divider

BLK pin senses the LLC DC input voltage and determines when to turn on and off the LLC converter. Also, BLK pin voltage is used for feedforward compensation. Size the resistor divider so that the current in the divider is larger than the leakage into the BLK pin (Iblksink). The maximum desired power consumption of the BLK pin resistor divider is P_BLKsns = 15mW. The BLK sense resistor total value is given by:

Equation 51.

R_{BLKsns} = R_{BLKupper} + R_{BLKlower} = \frac{{V_{IN (nom)}}^{2}}{P_{BLKsns}} = \frac{390^{2}}{0.015} = 10 MΩ

Select 365V as the LLC start-up voltage. Relate V_BLKStart to V_BLKStop, V_BLKStartHys, I_BLKSink as below:

Equation 52.

V_{BLKStart} = 365 (\frac{R_{BLKlower}}{R_{BLKupper} + R_{BLKlower}}) = V_{BLKStop} + V_{BLKStartHys} + I_{BLKsink} (\frac{R_{BLKupper} R_{BLKlower}}{R_{BLKupper} + R_{BLKlower}})

For V_BLKStop = 1V, V_BLKStartHys = 0.1V, I_BLKSink = 5μA, R_BLKupper, and R_BLKlower are obtained as 10MΩ and 35.4kΩ respectively.

A standard value of 35.4kΩ is selected for R_BLKlower and a standard value of ×3 3.3MΩ in series is selected for R_BLKupper.

The actual start-up voltage is calculated using:

Equation 53.

V_{BLKStart} (\frac{R_{BLKupper} + R_{BLKlower}}{R_{BLKlower}}) = (\begin{array}{l} V_{BLKStop} + V_{BLKStartHys} + \\ I_{BLKSink} [\frac{R_{BLKupper} R_{BLKlower}}{R_{BLKupper} + R_{BLKlower}}] \end{array}) \times (\frac{R_{BLKupper} + R_{BLKlower}}{R_{BLKlower}}) = 358 V

The power consumption in BLK resistors is calculated using:

Equation 54.

P_{BLKsns} = \frac{{V_{IN (nom)}}^{2}}{(R_{BLKupper} + R_{BLKlower})} = \frac{390^{2}}{(10 MΩ + 35.4 kΩ)} = 15.3 mW

The LLC turn off voltage is calculated using:

Equation 55.

V_{BLKStop} (\frac{R_{BLKupper} + R_{BLKlower}}{R_{BLKlower}}) = 280.6 V