7.3.2 High Voltage Start-Up Regulator

The LM5008 contains an internal high voltage start-up regulator. The input pin (V_IN) can be connected directly to the line voltages up to 95 Volts, with transient capability to 100 V. The regulator is internally current limited to 9.5 mA at V_CC. Upon power up, the regulator sources current into the external capacitor at V_CC (C3). When the voltage on the V_CC pin reaches the undervoltage lockout threshold of 6.3 V, the buck switch is enabled.

In applications involving a high value for V_IN, where power dissipation in the V_CC regulator is a concern, an auxiliary voltage can be diode connected to the V_CC pin. Setting the auxiliary voltage to 8 V to 14 V shuts off the internal regulator, reducing internal power dissipation. See Figure 8. The current required into the V_CC pin is shown in Figure 1.

Figure 8. Self-Biased Configuration