7.3.2 Start-Up Regulator (V_CC)

The high voltage bias regulator is integrated within the LM5008A. The input pin (VIN) can be connected directly to line voltages between 6 V and 95 V, with transient capability to 100 V. Referring to the Functional Block Diagram and Figure 2, when V_IN is between 6 V and the bypass threshold (nominally 8.5 V), the bypass switch (Q2) is on, and V_CC tracks V_IN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 100 Ω, with inherent current limiting at approximately 100 mA. When V_IN is above the bypass threshold Q2 is turned off, and V_CC is regulated at 7 V. The V_CC regulator output current is limited at approximately 9.2 mA. When the LM5008A is shut down using the RT/SD pin, the V_CC bypass switch is shut off regardless of the voltage at V_IN.

When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 µs to 3 µs. The capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at V_CC from rising above its absolute maximum rating in response to a step input applied at V_IN. C3 must be placed as close as possible to the VCC and RTN pins. In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An auxiliary voltage of between 7.5 V and 14 V can be diode connected to the VCC pin to shut off the V_CC regulator, thereby reducing internal power dissipation. The current required into the VCC pin is shown in Figure 6. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than V_IN.

The turnon sequence is shown in Figure 8. During the initial delay (t1), VCC ramps up at a rate determined by its current limit and C3 while internal circuitry stabilizes. When V_CC reaches the upper threshold of its undervoltage lockout (UVLO, typically 5.3 V), the buck switch is enabled. The inductor current increases to the current limit threshold (I_LIM), and during t2 the V_OUT increases as the output capacitor charges up. When V_OUT reaches the intended voltage the average inductor current decreases (t3) to the nominal load current (I_O).

Figure 8. Start-Up Sequence