The voltage at the BUR pin (V_BUR) sets a target peak current-sense threshold at the CS pin (V_CST(BUR)) which programs the onset of adaptive burst mode (ABM). V_BUR also determines the clamped peak current level of switching cycles in each burst packet. When V_BUR is set higher, ABM will start at heavier output load conditions with higher peak primary current, so the benefit is higher light-load efficiency but the side effect is larger burst-mode output voltage ripple. Therefore, 50% to 60% of output load at high line is the recommended highest load condition to enter ABM (I_o(BUR)) for both Si and GaN-based designs. The gain between V_BUR and V_CST(BUR) is a constant gain of K_BUR-CST, so setting V_CST(BUR) just requires properly selecting the resistor divider on the BUR pin formed by R_BUR1 and R_BUR2. V_BUR should be set between 0.7 V and 2.4 V. If V_BUR is less than 0.7 V, V_CST(BUR) holds at 0.7 V / K_BUR-CST. If V_BUR is higher than 2.4 V, V_CST(BUR) stays at 2.4 V / K_BUR-CST.

Equation 1.

In order to enhance the mode transition between ABM and LPM, a programmable offset voltage (ΔV_BUR(LPM)) is generated on top of the V_BUR setting in ABM through an internal 2.7-μA current source (I_BUR(LPM)), as shown in Figure 7-1. In ABM, V_BUR is set through the resistor voltage divider to fulfill the target average efficiency. On transition from ABM to LPM, I_BUR(LPM) is enabled in LPM and flows out of the BUR pin, so ΔV_BUR(LPM) can be programmed based on the Thevenin resistance on the BUR pin, which can be expressed as

Equation 2.

Figure 7-1 Hysteresis Voltage Generation on BUR Pin

When V_BUR steps higher on transition into LPM, the initial peak magnetizing current in LPM is increased with larger energy per switching cycle in each burst packet. This increases the output voltage which forces higher feedback current to restore regulation. Higher feedback current causes UCC28781 to stay in LPM, forming a hysteresis effect. If ΔV_BUR(LPM) is designed too small, it is possible that mode toggling between LPM and ABM can occur resulting in audible noise. For that situation, ΔV_BUR(LPM) greater than 100 mV is recommended.

To minimize the effects of external noise coupling on V_BUR, a filter capacitor on the BUR pin (C_BUR) may be needed. C_BUR needs to be properly designed to minimize the delay in generating ΔV_BUR during mode transitions. It is recommended that C_BUR should be sized small enough to ensure ΔV_BUR(LPM) settles within 40 μs, corresponding to the burst frequency of 25 kHz in LPM (f_LPM). Based on three RC time constants, representing 95% of a settled steady-state value from a step response, the design guide for C_BUR is expressed as

Equation 3.

In order to enhance the mode transition between ABM and AAM, a programmable offset voltage (ΔV_BUR(AAM)) is generated to lower the V_BUR with an internal 5-μA pull-down current (I_BUR(AAM)), as shown in Figure 7-1. After transition from ABM to AAM, I_BUR(AAM) is enabled in AAM and flows into the BUR pin, so ΔV_BUR(AAM) is also programmed based on the Thevenin resistance on the BUR pin, which can be expressed as

Equation 4.

When V_BUR reduces after transition to AAM, the initial peak magnetizing current in AAM is reduced with less energy per switching cycle, which forces controller to continu operating in AAM. If ΔV_BUR(AAM) is too small, it is possible that either mode toggling between ABM and AAM or low-frequency ABM burst packets less than 20 kHz can occur and result in audible noise concern. For that situation, ΔV_BUR(AAM) greater than 150 mV is recommended. In some power stage designs, LPM in hard switching condition may cover a wider output load current range, so the light-load efficiency in LPM may be lower than ABM with ZVS condition. Besides, the ABM-to-AAM mode transition may be affected potentially when the load current condition of LPM-to-ABM transition is too close to the load current condition of ABM-to-AAM transition.

In order to optimize the output load current range in LPM, lower V_BUR(ABM), smaller ΔV_BUR(LPM), larger R_OPP, and smaller C_CS help to reduce the peak magnetizing current in LPM. If the LPM energy needs to be further reduced but V_BUR in AAM is limited by the 0.7-V minimum programmable level, the optional application circuit in Figure 7-2 can be considered. When the output load current is reduced, duty cycle of each burst packet becomes smaller, so as the duty cycle of RUN-pin voltage. C_BUR is discharged by the RUN driver through the small-signal diode (D_BUR) and the current limit resistor (R_RUN). Proper selection of R_RUN value can further reduce V_BUR(ABM) when the load current is reduced close to the transition point from ABM to LPM. One example BUR-pin setting is R_BUR1 = 182 kΩ, R_BUR2 = 37.4 kΩ, C_BUR = 330 pF, and R_RUN = 20 kΩ.

Figure 7-2 Optional Application Circuit to Reduce V_BUR in LPM