7.3.6 Soft Start and Voltage Tracking (SS/TRK)

The LM76002/LM76003 has a flexible and easy-to-use start-up rate control pin: SS/TRK. The soft-start feature is to prevent inrush current impacting the LM76002/LM76003, and its supply when power is first applied. Soft start is achieved by slowly ramping up the target regulation voltage when the device is first enabled or powered up. The simplest way to use the device is to leave the SS/TRK pin open circuit or floating. The LM76002/LM76003 employs the internal soft-start control ramp and starts up to the regulated output voltage in 6.3 ms typically. In applications with a large amount of output capacitors, higher V_OUT, or other special requirements, the soft-start time can be extended by connecting an external capacitor C_SS from SS/TRK pin to AGND. Extended soft-start time further reduces the supply current required to charge up output capacitors and supply any output loading. An internal current source (I_SSC = 2.2 μA) charges C_SS and generates a ramp from 0 V to VFB to control the ramp-up rate of the output voltage. For a desired soft-start time t_SS, the capacitance for C_SS can be found by Equation 5:

The soft-start capacitor C_SS is discharged by an internal FET when V_OUT is shut down by hiccup protection or ENABLE = logic low. When a large C_SS is applied, and EN is toggled low only for a short period of time, C_SS may not be fully discharged. The next soft-start ramp follows internal soft-start ramp before reaching the leftover voltage on CSS and then follows the ramp programmed by C_SS. If this is not acceptable for a certain application, an R-C low-pass filter can be added to EN to slow down the shutting down of VCC, allowing more time to discharge C_SS.

The LM76002/LM76003 is capable of start-up into pre-biased output conditions. When the inductor current reaches zero, the LS switch is turned off to avoid negative current conduction. This operation mode is also called diode emulation mode. It is built in by the DCM operation in light loads. With a pre-biased output voltage, the LM76002/LM76003 waits until the soft-start ramp allows regulation above the pre-biased voltage and then follows the soft-start ramp to the regulation level. When an external voltage ramp is applied to the SS/TRK pin, the LM76002/LM76003 FB voltage follows the ramp if the ramp magnitude is lower than the internal soft-start ramp. A resistor divider pair can be used on the external control ramp to the SS/TRK pin to program the tracking rate of the output voltage. The final voltage detected by the SS/TRK pin must not fall below 1.2 V to avoid abnormal operation

V_OUT tracked to an external voltage ramp has the option of ramping up slower or faster than the internal voltage ramp. V_FB always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 14 shows resistive divider connection if external ramp tracking is desired.

Figure 14. Soft-Start Tracking External Ramp

Figure 15 shows the case when V_OUT ramps more slowly than the internal ramp, while Figure 16 shows when V_OUT ramps faster than the internal ramp. Faster start-up time may result in inductor current tripping current protection during start-up. Use with special care.

Figure 15. Tracking With Longer Start-up Time Than The Internal Ramp

Figure 16. Tracking With Shorter Start-up Time Than The Internal Ramp

The LM76002/LM76003 is capable of start-up into pre-biased output conditions. During start-up the device sets the minimum inductor current to zero to avoid discharging a pre-biased load.