I am planning on using the LP2951 LDO for an automotive application. I will be powering a device from the OBD-II port of a vehicle and I am expecting a load of ~60-80mA max.
The unregulated voltage of the OBD-II port is 12V, and I need 3.3V output for my device.
2 things I am concerned about: (1) Heat generated from power dissipation, and (2) Ignition voltage spikes from the battery.
1) I found an equation for worst case power dissipation for LDO's calculated as: P = (Vin - Vout)*I + Vin*Ignd
So I calculated using: Vin - Vout = 12V - 3.3V = 8.7V; I = 100mA (max load) , Vin = 12V, Ignd = 14mA (from Datasheet)
and got: P = 1038mW
I'm not sure if this calculation was useful, but would this kind of power being dissipated be too much for an 8-pin SOIC?
2) So, for voltage spikes, I think it makes sense to have a large capacitor (47uF, high max voltage) close to the 12V source on the PCB and then a 1uF capacitor at the input to the regulator. I've also seen lots of designs that use a diode in series with the 12V source before the input to the regulator (I assume this prevents negative voltages across the regulator when the vehicle is turned off). Does this seem like an adequate solution to protect the LP2951 in this device?
Thank You,
Seth Berggren
