When there is no overlapping between PWML on time and BSW on time and the survival mode is not triggered, there are two operating modes for the bias regulator when V_BIN is between the UVLO(ON) threshold and the disable threshold. The first is the constant peak current (CPC) control mode for a heavier VDD load condition, and the second is the burst mode (BM) control for a light VDD load condition. The 18.5-V regulation in CPC is maintained by varying the boost switching frequency, and the peak current of the boost inductor is fixed at approx. 0.33 A. The 18.5-V regulation in BM is maintained by changing the burst frequency with a constant peak current for each switching cycle of a burst packet. There are at least 3 switching cycles in a burst packet for BM. E.g. when a 22-µH boost inductor (L_B) is used, the mode transition point is at approx. 3-mA VDD load.

In CPC mode, the V_VDD regulation is achieved by changing the boost switching frequency (f_BSW) with a fixed 0.33-A peak current. When f_BSW is reduced to the minimum controllable frequency, the control loop will automatically transition into BM. Figure 8-34 illustrates the switching pattern in CPC mode operating in the transition mode or discontinuous conduction mode (DCM). The internal ZCD detection on BSW pin only allows the turn on instant of the next boost switching cycle to happen after the BSW-pin voltage falls below the BIN-pin voltage, so that the boost inductor current drops to zero first.

Figure 8-34 Switching Pattern of Boost Converter in CPC and COT Modes

If survival mode is enabled, the regulator will start switching regardless of V_BIN level and enter into constant off time (COT) control mode. The COT control mode disables the ZCD detection on the BSW pin and creates a 250-ns off time in order to force the regulator into the continuous conduction mode (CCM). More energy per cycle can transfer from the BIN-pin capacitor to the VDD capacitor in CCM, so V_VDD can be ramped up above the 13-V survival mode threshold faster than DCM, and minimize the survival mode energy transfer to the output capacitor on the ACF secondary side, as a result of the drop in V_BIN. When V_VDD is higher than 13 V, the regulator will automatically change the operation mode back to CPC or BM.

Besides survival mode, when the voltage difference between V_VDD and V_BIN is less than 1 V, the regulator will also disable ZCD detection and allows CCM operation. If ZCD is not disabled, the low voltage difference makes the demagnetization time of the boost inductor current very long, so V_VDD would not be able to build up to the regulation level. The CCM operation can transfer the energy to VDD capacitor quickly, so V_VDD can recover back to the regulation level.