SNVSBJ0A December 2019 – June 2020 LM5170
Place a ceramic capacitor CSS between the SS pin and AGND to program the soft-start time. When the EN1 voltage is < 1 V, an internal pulldown switch holds the SS pin at AGND. When the EN1 pin voltage is > 2 V, the SS pulldown is released, and CSS is charged up slowly by the internal 25-µA current source, as shown in Figure 25. The slow ramping SS voltage clamps the COMP1 and COMP2 pins through two separate clamp circuits. Once the SS voltage exceeds the 1-V offset voltage, the PWM duty cycle starts to increase gradually from zero.
When EN1 is pulled below 1 V, CSS is discharged by the internal pulldown FET. Once this pulldown FET is turned on, it remains on until the SS voltage falls below 0.23 V, which is the threshold voltage indicating the completion of SS discharge.
Note that the EN2 pin does not affect the SS pin. When EN1 and EN2 are enabled together, the CH-2 output will follow CH-1 by going through the same soft-start process. If EN2 is enabled at a later time and CH-1 has already completed soft start, CH-2 will not be affected by the SS pin. This allows the CH-2 current to ramp up quickly to supply the increased load current. However, when SS is pulled low, both CH-1 and CH-2 are affected at the same time.