During power up, the device will automatically determine if two-wire or three-wire mode is to be entered. Once two-wire or three-wire mode is determined, the mode is maintained until another power cycle is performed. Therefore, it is important to understand different scenarios that may affect the device operation.

In two-wire mode, the device is supplied power from a single external voltage source via EN, which charges the C_VDDP capacitance on VDDP. The voltage supply is required to meet the power supply needs at the selected PXFR setting, as well as, meet the recommended minimum ramp time, |ΔV_EN/Δt|. To ensure two-wire mode is entered properly, V_EN must reach V_{IH_EN} prior to V_VDDP reaching V_{VDDP_UV_R}. This is summarized in Figure 8-6. Similarly, it is recommend that V_EN meet the minimum recommended ramp down time to V_{IL_EN}. Too slow a ramp down time may cause insufficient power to be transferred while slowly transitioning between V_{IH_EN} and V_{IL_EN} leading to intermittent de-assertions and assertions of VDRV. This may continue until the power transfer reduces sufficiently to maintain VDRV low.

Figure 8-6 Two-wire Mode Entry

In most three-wire mode applications, EN and VDDP are supplied by the same voltage rail and source. It is recommended that V_EN remain below V_{IL_EN} until V_VDDP reaches V_{VDDP_UV_R}. It is also possible in some applications to connect EN directly to the VDDP supply. These two scenarios are shown in Figure 8-7.

Figure 8-7 Three-wire Mode Power Sequences

In three-wire mode applications with separate voltage sources supplying EN and VDDP, it is recommended that V_EN remain below V_{IL_EN} until V_VDDP reaches V_{VDDP_UV_R}. If V_EN reaches V_{IH_EN} prior to V_VDDP reaching V_{VDDP_UV_R}, current from the supply that sources EN will attempt to power VDDP. Depending on the other supply's impedance residing on VDDP and the amount of power available from the EN pin, V_VDDP may begin to rise and eventually exceed V_{VDDP_UV_R}. At that point, the device will begin to transfer power to the secondary and start charging the VDDM and VDDH rails. If VDDP remains above V_{VDDP_UV_R}, the device will continue to transfer power to the secondary eventually charging the VDDM and VDDH rails and VDRV may assert high.