TPS546D24: Clarification on UVLO and VDD5 operation

Ella KIM 

Ella KIM said:

(1) My customer is considering to utilize the internal VDD5 output to the external circuits - FET gate pull-up voltage. Even though the datasheet mentions not recommended, I think customer can use it because FET gate pull-up would not consume much of current. Pull-up resistor would be 100k, thereby ~50uA (=5V/100k). Please advise if you have concern here.

As noted in the datasheet, the VDD5 output is not recommended for powering external circuitry, but an external loading of a 100kΩ resistor should not negatively affect the LDO, though there will be significant switching frequency AC noise on the VDD5 output due to the high current pulses resulting from the turn-on of the low-side FET and the recharging of the BOOT to SW capacitor.  They may want to consider a 10μs or higher R-C filter between the VDD5 pin and the external load.

Ella KIM said:

(2) Is VDD5 always-on regardless of EN/UVLO pin level, once AVIN reaches its VULO level?

Yes, VDD5 is enabled and "ON" regardless of the EN/UVLO pin level.  VDD5 will start rising before AVIN reaches its UVLO level, typically between 1V and 2V on AVIN.

Ella KIM said:

(3) It's little confusing with UVLO.  AVIN has UVLO of 2.5V, then the internal 5V regulator will start to turn on once AVIN > 2.5V. But VDD5 outputs 4.7V with 130mV dropout. How AVIN 2.5V can turn on the internal 5V regulator?

When internally powered from AVIN, the VDD5 regulator tracks slightly below AVIN  as normal for a low-drop out linear regulator (LDO).  The 4.7V specification for VDD5 is at the "standard" test conditions of AVIN = 12V, but is generally good down to about AVIN = 5V.  The 130mV drop out specification is, as listed, for AVIN = 4.5V (VDD5 = 4.37V)  At lower AVIN voltages, VDD5 will track with AVIN.  At AVIN = 2.5V, VDD5 will be approximately 2.5V, since the VDD5 loading current will be significantly less than the 25mA the drop out is specified at.

Note:  When VDD5 is internally powered from AVIN, switching will not be enabled until AVIN is high enough to raise VDD5 above it's 3.9V UVLO level.

Ella KIM said:

(4) When it comes to UVLO level and VDD5 operation, TPS546D24 and TPS546C24A would show the same operation. Could you confirm?

Yes.  The TPS546D24 and TPS546x24A have the same PVIN, AVIN, and VDD5 UVLO functions.