7.3.9 Soft-Start Function and Powergood Status

The soft-start function of the SMPS is achieved by ramping up reference voltage and two-stage current clamp. At the starting point, the reference voltage is set to 650 mV (87% of its target value) and the overcurrent threshold is set half of the nominal value. When UVP comparator detects VDDQ become greater than 80% of the target, the reference voltage is raised toward 750 mV using internal 4-bit DAC. This takes approximately 85 μs. The overcurrent threshold is released to nominal value at the end of this period. The powergood signal waits another 45 μs after the reference voltage reaches 750 mV and the VDDQ voltage becomes good (above 95% of the target voltage), then turns off powergood open-drain MOSFET.

The soft-start function of the VTT LDO is achieved by current clamp. The current limit threshold is also changed in two stages using an internal powergood signal dedicated for LDO. During VTT is below the powergood threshold, the current limit level is cut into 60% (2.2 A).This allows the output capacitors to be charged with low and constant current that gives linear ramp up of the output. When the output comes up to the good state, the overcurrent limit level is released to normal value (3.8 A). The device has an independent counter for each output, but the PGOOD signal indicates the status of VDDQ only and does not indicate VTT powergood status externally. See Figure 35.

Figure 35. VDDQ Soft-Start and Powergood Timing

Soft-start duration, t_VDDQSS, t_VTTSS are functions of output capacitances.

Equation 2.

where

• I_VDDQOCP is the current limit value for VDDQ switcher calculated by Equation 5

Equation 3.

where

• I_VTTOCL = 2.2 A (typ)

In both Equation 2 and Equation 3 , no load current during start-up are assumed. Note that both switchers and the LDO do not start up with full load condition.