Hi Team,
I got an inquiry from one of my assigned accounts if we have an equivalent to Linear Techs LT1761. The critical feature they need is the soft start function. Below has more details on the situation from customer. Do we possibly have a pin to pin part that is similar to LT1761? Pin to pin is needed since this part is already going to mass production.
The requirement is to have soft-start inbuilt to the IC capable of delaying the output around 10ms after valid input is available to prevent loading the input during the initial turn on. If the IC give output immediately, a dip is observed in the output. Mechanism of the dip is explained below. Please also note that the input to LDO is loosely regulated because it is a slave winding and during startup, and when unloaded, the input to the LDO will exceed 5.5V max input limitation as stated by Alfred below.
Mechanism of the dip:
During startup the main loop of auxiliary which is PVCC overshoots momentarily due to very light load. This causes the pwm to stop pulsing momentarily. The slave winding 6VCC which is the input to the LDO dips because it is already loaded and the dip is reflected to the 5V output. In order to prevent this, we need to delay the 5V output. Unfortunately we do not have the space to add external circuit to delay the output due to space restriction and PCB already finalized (CBYP is already in the circuit). Hence we need chip with inbuilt soft-start.
Response of IC without softstart
Comparison of IC with and without soft-start.
Thanks!
Regards,
Alfred
