8.3.6 Soft Start and Voltage Tracking (SS/TRK)

The LM43600 has a flexible and easy-to-use start up rate control pin: SS/TRK. Soft-start feature is to prevent inrush current impacting the LM43600 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 LM43600 employw the internal soft-start control ramp and start up to the regulated output voltage in 4.1 ms typically.

In applications with a large amount of output capacitors, or 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 needed 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 V_FB 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 2.

The soft start capacitor C_SS is discharged by an internal FET when V_OUT is shutdown by hiccup protection due to excessive load, temperature shutdown due to overheating or ENABLE = logic low. A large C_SS cap will take a long time to discharge when ENABLE is toggled low. If ENABLE is toggled high again before the C_SS is completely discharged, then the next resulting soft-start ramp will follow the internal soft-start ramp. Only when the soft-start voltage reaches the left-over voltage on C_SS, will the output follow the ramp programmed by C_SS. This behavior will look as if there are two slopes at startup. If this is not acceptable by a certain application, a R-C low-pass filter can be added to ENABLE to slow down the shutting down of VCC, which allows more time to discharge C_SS.

The LM43600 is capable of start up into prebiased output conditions. When the inductor current reaches zero, the LS switch will be 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 prebiased output voltage, the LM43600 will wait until the soft-start ramp allows regulation above the prebiased voltage and then follow the soft-start ramp to the regulation level.

When an external voltage ramp is applied to the SS/TRK pin, the LM43600 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 seen by the SS/TRK pin should not fall below 1.2 V to avoid abnormal operation.

Figure 37. Soft-Start Tracking External Ramp

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 38 shows the case when V_OUT ramps slower than the internal ramp, while Figure 39 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 38. Tracking with Longer Start-up Time Than The Internal Ramp

Figure 39. Tracking with Shorter Start-up Time Than The Internal Ramp