The TPS65400 implements a soft-start function that minimizes discharge of the output when starting up in a prebiased condition. Soft-start time, t_SS, is set by t_{ON_TRANSITION_RATE} (digital soft-start) or by a capacitor connected to the corresponding SSx pin (analog soft-start). In this setup, the SSx pin sources a 5-µA current charging the capacitor, and the voltage at the SSx pin limits the reference voltage at the input of the error amplifier.

At the beginning of the soft-start, the soft-start input to the error amplifier is set to 0. The SSx input is raised gradually and reaches its target value during the time t_ss. If V_FB > V_SS, then no switching occurs. After the Soft-Start signal crosses VFB, the switching begins. The first switching pulse is on the low-side FET, which charges the high-side bootstrap capacitor. The unit runs in discontinuous conduction mode (DCM) with the zero-cross detector enabled on the low side (diode emulation). The high-side FET is pulsed according to the error amplifier output on the COMP pin. If the IC is configured for continuous conduction mode (CCM) operation (default), the low-side FET pulses gradually transition to normal CCM operation; at each successive switching cycle, the low-side gate pulse is gradually ramped until full synchronous switching occurs. At this point, the switcher enters normal CCM operation.

Figure 8-5 Soft-Start Under Prebiased Condition and CCM Mode Programmed

Figure 8-6 Soft-Start Under Prebiased Condition and DCM Mode Programmed