7.3.6 Soft Start and Voltage Tracking (SS/TRK)

The LM43601 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 LM43601 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. The LM43601 employs 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 shut down by hiccup protection due to excessive load, temperature shutdown due to overheating or ENABLE = logic low. A large C_SS capacitor takes 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 follows the internal soft-start ramp. The output follows the ramp programmed by C_SS only when the soft-start voltage reaches the leftover voltage on CSS. This behavior appears as if there are two slopes at start-up. 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 LM43601 is capable of start-up into prebiased output conditions. When the inductor current reaches zero, the LS switches turned off to avoid negative current conduction. This operation mode is also called diode emulation mode. It is built-in by the DCM operation at light loads. With a prebiased output voltage, the LM43601 waits until the soft-start ramp allows regulation above the prebiased voltage. The device then follows the soft-start ramp to the regulation level.

When an external voltage ramp is applied to the SS/TRK pin, the LM43601 FB voltage follows the external 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 external ramp voltage applied at the SS/TRK pin must 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 Start-up Time Longer Than the Internal Ramp

Figure 39. Tracking With Start-up Time Shorter Than the Internal Ramp