I am looking to apply the principles of TIDU421 Verified Design (for the the TPS74401) to the TPS7A85 as I need higher current.
I am a little confused as to the sizing of the resistor between the op amp output and the feedback node. How does the 15K resistor selected equate to allowing for a maximum of 4% mismatch between the master/slave LDOs? A little clarification in this area would be appreciated.
Hi Rich,
In order to have a maximum of 4% mismatch, we need to make sure that the current driven by the op-amp through the resistor is greater than the current resulting in a 4% deviation in output voltage:
Current from op-amp > current through top resistor
(Vfb - 0.3 V) / R > [(Vout x 0.96 - Vfb) / R1] - (Vfb / R2)
*The 0.3 V is the headroom voltage of the op-amp to GND in order to ensure linearity.
(0.8 V - 0.3 V) / R > [(3.3 V x 0.96 - 0.8 V) / 3.57 k] - (0.8 V / 1.15 k)
(0.5 V / R) > (2.368 V / 3.57 k) - (0.8 V / 1.15 k)
(0.5 V / R) > 663.3 uA - 695.7 uA
(0.5 V / R) > -32.4 uA
R < 0.5 V / 32.4 uA
R < 15.4 k
Since the positive rail of the op-amp allows for a higher voltage drop across the resistor (and therefore more current), we do not need to repeat the calculation for a 4% higher output voltage.
We have an additional current sharing TI Design that you may be interested in as it does not require as many external components:
Very Respectfully,
Ryan
