The recommended headroom voltage (headroom = VIN – VOUT) will depend on the
requirements in each individual application. These LDOs are typically operated
at 500mV to 1V of headroom. The higher the headroom, the higher the
rejection of noise/ripple from the input supply. The tradeoff is
efficiency, as running at higher headroom burns more power and also increases
The dropout voltage of the ADM7154/ADM7155 is very low, (120 mV typical at 600
mA load) allowing the ADM7154/ADM7155 to operate at lower headroom voltages.
The ADM7154/ADM7155 (or any LDO) should NEVER be operated in or near dropout if
the LDO is required to provide a clean supply rail. The headroom voltage should
be greater than the max dropout spec, and large enough to achieve the required
performance. The TPCs in the datasheet show the achievable PSRR at different
headroom voltages. Once the headroom voltage is selected, thermal calculations
should be carried out to ensure that the die temperature doesn’t exceed 125C
(or the max allowed die temperature in the application if lower than 125C).