A high-impedance resistor is integrated inside the BIN pin to sense the bias regulator input voltage and determine the regulator operating mode. A 30-V rated MOSFET (Q_BSW) with 1.4-Ω R_DS(on) is integrated in the controller, whose drain is connected to the BSW pin and the source is to the BGND pin. When V_BIN is less than the 2.2-V UVLO(ON) threshold (V_BIN(ON)) and V_VDD is still higher than the 13-V survival mode threshold (V_VDD(PCT) + V_VDD(OFF)), the regulator remains in the disabled condition. If the survival mode is triggered by V_VDD < 13 V, Q_BSW is forced to switch regardless of V_BIN < V_BIN(ON) or not, and the regulator operates in continuous conduction mode (CCM) to charge C_VDD quickly. If V_VDD > 13 V, Q_BSW switching is enabled when V_BIN > V_BIN(ON) , and the regulator operates in transition mode or discontinuous conduction mode (DCM) to boost V_VDD to the 18.5-V regulation level (V_VDD(BOOST)). When the regulator starts switching, the 190-ns leading edge blanking time of Q_BSW is used to sample V_BIN for under-voltage. If V_BIN drops below the 1-V UVLO(OFF) threshold (V_BIN(ON) - V_BIN(OFF)), the regulator switching will be terminated immediately.

When the ACF output voltage increases and V_BIN reaches to the 15-V boost disable threshold (V_BIN(EN) + V_BIN(DIS)), so the regulator will stop switching and V_VDD is directly supplied from the rectified auxiliary winding voltage through the boost inductor and boost diode. When V_BIN drops below the 14.8-V boost enable threshold (V_BIN(EN)), the switching regulator will take over boosting of the VDD supply.

Two separate capacitors are recommended for the regulator input capacitor bank of the BIN pin. One is placed closer to the auxiliary winding and its rectification diode, so the switching loop of the primary auxiliary winding output can be minimized. A 33-µF chip ceramic capacitor is recommended for energy storage. The other capacitor is placed closer to the boost inductor, BSW, and BGND pins, so the regulator input switching loop can be reduced as well. A 10-µF chip ceramic capacitor is recommended for high-frequency decoupling. Ground return of the regulator output capacitor (C_VDD) should be connected back to the BGND pin as close as possible in order to minimize the regulator output switching loop area. Rather than with the BGND pin, the low-noise ground terminal of C_VDD should be used to connect the BGND net to the AGND pin with a low impedance copper trace or copper pour.

When the boost inductor current flowing through Q_BSW reaches to the 0.33-A peak current threshold, Q_BSW turns off in every boost switching cycle. A 30-V rated Schottky diode with higher than 0.4-A rated peak current capability is needed between the BSW and VDD pins in order to handle the 0.33-A switching current. The boost inductor between the BIN and BSW pins should support higher than 0.4-A saturation current capability. Higher current peaks may ring through the inductor whenever C_BIN is charged higher than C_VDD.

As the following equation shows, the inductance (L_B) is determined based on the largest total supply current to the loading on the VDD pin and the highest boost switching frequency selection (f_BSW), which is limited by maximum boost switching frequency (f_BSW(MAX)) of the control loop. The minimum inductance is 22 µH (±10%) regardless of calculation result. Magnetic shielding is recommended to help avoid inducing noise into nearby networks.

Equation 12.

The voltage drop on the DC winding resistance of the boost inductor (R_LB) and R_DS(ON) of Q_BSW (R_BSW) reduces the actual voltage across the boost inductance from V_BIN. The boost inductor voltage needs to be high enough to build up the inductor current quickly. Therefore, it is recommended to choose the R_LB low enough to make the total resistive voltage drop at 0.33 A lower than the 1-V boost UVLO(OFF) threshold.

Equation 13.

When the ACF operates in the LPM, SBP1, and SBP2 modes, the high side switch is disabled, so the leakage inductance energy will charge the clamping capacitor and C_BIN in every ACF switching cycle. If the leakage inductance energy is big enough to build up V_BIN higher than 30 V under those operating modes, a unidirectional TVS between the BIN pin and AGND pin will be needed to protect the 30-V rated BIN and BSW pins. A high-voltage Schottky auxiliary winding rectifier diode maximizes the C_BIN voltage in the survival mode, so the regulator in CCM mode can transfer the stored C_BIN charge to C_VDD. The more survival mode energy is absorbed by the auxiliary power supply, the less residual energy is delivered to the output capacitor. This will ensure that the output voltage can stay within regulation range during survival mode under no-load condition.