The LM73605-Q1/6-Q1 controlled output voltage ramp during start-up. The soft-start feature reduces inrush current during start-up and improves system performance and reliability.

If the SS/TRK pin is floating, the LM73605-Q1/6-Q1 up following the fixed internal soft-start ramp.

If longer soft-start time is desired, an external capacitor can be added from SS/TRK pin to ground. There is a 2-µA (typical) internal current source, I_SSC, to charge the external capacitor. For a desired soft-start time t_SS, capacitance of C_SS can be found by Equation 12.

where

• C_SS = soft-start capacitor value (F)
• I_SSC = soft-start charging current (A)
• t_SS = desired soft-start time or times

The FB voltage always follows the lower potential of the internal voltage ramp or the voltage on the SS/TRK pin. Thus, the soft-start time can only be extended longer than the internal soft-start time by connecting C_SS. Use C_SS to extend soft-start time when there are a large amount of output capacitors, the output voltage is high, or the output is heavily loaded during start-up.

LM73605-Q1/6-Q1 operating in diode emulation mode during start-up regardless of mode setting. The device is capable of starting up into pre-biased output conditions. During start-up, the device sets the minimum inductor current to zero to avoid back charging the input capacitors.

LM73605-Q1/6-Q1 can track an external voltage ramp applied to the SS/TRK pin, if the ramp is slower than the internal soft-start ramp. The external ramp final voltage after start-up must be greater than 1.5 V to avoid noise interfering with the reference voltage. Figure 8-9 shows how to use resistor divider to set V_OUT to follow an external ramp.

Figure 8-9 Soft-start Tracking External Ramp

V_OUT tracking also provides the option of ramping up faster than the internal start-up ramp. The FB voltage always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 8-10 shows the case when V_OUT ramps slower than the internal ramp, while Figure 8-11 shows when V_OUT ramps faster than the internal ramp. If the tracking ramp is delayed after the internal ramp is completed, V_FB follows the tracking ramp even if it is faster than the internal ramp. Faster start-up time may result in large inductor current during start-up. Use with special care.

Figure 8-10 Tracking With Longer Start-up Time Than the Internal Ramp

Figure 8-11 Tracking With Shorter Start-up Time Than the Internal Ramp

The SS/TRK pin is discharged to ground by an internal pulldown resistor R_SSD when the output voltage is shutting down, such as in the event of UVLO, thermal shutdown, hiccup, or V_EN = 0. If a large C_SS is used, and the time when V_EN = 0 V is very short, the C_SS may not be fully discharged before the next soft start. Under this condition, the FB voltage follows the internal ramp slew rate until the voltage on C_SS is reached, then follow the slew rate defined by C_SS.