7.3.1 High Voltage Start-Up Regulator

The LM25574-Q1 contains a dual-mode internal high voltage startup regulator that provides the V_CC bias supply for the PWM controller and boot-strap MOSFET gate driver. The input pin (VIN) can be connected directly to the input voltage, as high as 42 Volts. For input voltages below 9 V, a low dropout switch connects V_CC directly to V_IN. In this supply range, V_CC is approximately equal to V_IN. For V_IN voltage greater than 9 V, the low dropout switch is disabled and the V_CC regulator is enabled to maintain V_CC at approximately 7 V. The wide operating range of 6 V to 42 V is achieved through the use of this dual mode regulator.

The output of the V_CC regulator is current limited to 25 mA. Upon power up, the regulator sources current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the V_CC UVLO threshold of 5.35 V and the SD pin is greater than 1.225 V, the output switch is enabled and a soft-start sequence begins. The output switch remains enabled until V_CC falls below 5 V or the SD pin falls below 1.125 V.

An auxiliary supply voltage can be applied to the V_CC pin to reduce the IC power dissipation. If the auxiliary voltage is greater than 7.3 V, the internal regulator will essentially shut off, reducing the IC power dissipation. The V_CC regulator series pass transistor includes a diode between V_CC and V_IN that should not be forward biased in normal operation. Therefore the auxiliary V_CC voltage should never exceed the V_IN voltage.

In high voltage applications extra care should be taken to ensure the VIN pin does not exceed the absolute maximum voltage rating of 45 V. During line or load transients, voltage ringing on the V_IN line that exceeds the Absolute Maximum Ratings can damage the IC. Both careful PC board layout and the use of quality bypass capacitors located close to the VIN and GND pins are essential.

Figure 8. V_IN and V_CC Sequencing