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# ADP7159 - Operate around minimum Vin

I'm designing a power network that get 15v input and deliver 2A 1.8V output.

I used ADP2384 switching regulator to convert 15v -> 2.3v and used ADP7159 LDO to bring it down to 1.8v. In order to minimum the power dissipation, I want to keep the LDO headroom voltage as low as possible.

The minimum Vin of ADP7159 is 2.3. Assuming that there is an error in output of switching regulator, for a proper operation of LDO how far should the input voltage be above the minimum Vin specified in datasheet?

• Hi,

The ADP7159 LDO will still be operational above the minimum VIN provided it will NOT hit the maximum junction temperature of 125°C that will trigger it to thermal shutdown. You may refer to pages 17-19 to guide you in the thermal considerations of ADP7159.

For example, you are operating at 2A and VOUT=1.8V, assuming your board temperature (TB) is 85°C and you are using LFCSP package, referring to Figure 61 of the datasheet, your maximum VIN allowable to avoid thermal shutdown is:

TOTAL POWER DISSIPATION = (VINmax-VOUT) x ILOAD

1.6W = (VINmax-1.8V) x 2A

VINmax=2.6V

Regards,

Errgy

• Hi Errgy,

I think you misunderstand the question. My design is a battery operated system and the power dissipation on LDO should kept low to prolong the battery life not to avoid hitting max junction temperature.

If my question is vague here I try to clearify it, what I'm asking is

1. How operating below (not above) minimum input voltage would affect the ADP7159 parameter? (How much it degrade PSRR and output voltage)

2. What is recomended minimum operating point for Vin, assuming there is ripple in Vin rail and noise, to avoid above problems?

• Hi,

ADP7159 can still regulate below minimum VIN provided the input voltage is greater than or equal to VREG (VINVREG). Please see table 10 of the datasheet. Operation below the minimum VIN is not recommended as it will degrade PSRR and noise performance. A slight degradation of PSRR will occur but we don't have the exact data below minimum VIN because we only characterize and guarantee optimum performance at the recommended VIN conditions.

In summary, you can still operate ADP7159 below minimum VIN as long as VIN is greater than or equal to VREG (VINVREG).

Regards,

Errgy