SNVSA46B June 2014 – January 2018 LM46001
The LM46001 has a flexible and easy-to-use start up rate control pin: SS/TRK. The soft-start feature is there to prevent inrush current impacting the LM46001 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 LM46001 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 VOUT, or other special requirements, the soft-start time can be extended by connecting an external capacitor CSS 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 (ISSC = 2.2 µA) charges CSS and generates a ramp from 0 V to VFB to control the ramp-up rate of the output voltage. VFB value is typically 1V and therefore it is not mentioned in the below equation. For a desired soft start time tSS, the capacitance for CSS can be found by:
The soft start capacitor CSS is discharged by an internal FET when VOUT is shutdown by hiccup protection due to excessive load, temperature shutdown due to overheating or ENABLE = logic low. A large CSS capacitor takes a long time to discharge when ENABLE is toggled low. If ENABLE is toggled high again before the CSS is completely discharged, then the next resulting soft-start ramp follows the internal soft-start ramp. Only when the soft-start voltage reaches the leftover voltage on CSS does the output follow the ramp programmed by CSS. This behavior looks 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 CSS.
The LM46001 is capable of start up into prebiased 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 at light loads. With a prebiased output voltage, the LM46001 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 LM46001 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 be above 1.2 V to avoid abnormal operation.
VOUT tracked to an external voltage ramp has the option of ramping up slower or faster than the internal voltage ramp. VFB always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 38 shows the case when VOUT ramps slower than the internal ramp, while Figure 39 shows when VOUT 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.