Question:
The MAX4820 device has eight main output transistors. The datasheet has a spec for the transistors called "OUT_Drive Current" and is specified at 70mA minimum. I anticipated a maximum spec for current instead of a minimum spec. Does this mean the transistors can sink more than 70mA? How much can they sink?
Answer:
Yes, the MAX4820 outputs can sink more than 70mA. You do want a minimum guaranteed specification to ensure that the relay armature current can be met. The OUT output transistors are simply open-drain n-channel mosfets, and the electrical characteristics table on page 2 provides specifications that are production tested under the conditions given, and the max/min numbers are guaranteed. For example the OUT_ voltage is specified at VCC = 3.0V and sinking 70mA, and is 0.4V maximum. From that you can infer the equivalent maximum on-resistance of the nMOS device as R = 0.4V/0.07A = 5.7 Ohms maximum. The specification one line above guarantees a maximum of 6 Ohms maximum with VCC = 2.7V.
The typical operating curves on page 4 show the On Resistance vs Supply Voltage (at TA = +25°C) and also vs. temperature for various supply voltages.
Concerning the maximum current the OUT can sink, that information is given in the page 2 Absolute Maximum Ratings. For a single OUT channel on (sinking) the device can handle 300mA max continuous. If all eight OUT are sinking current then 150mA is the maximum continuous for each.
The MAX4820 does include thermal shutdown protection specified as +160°C typical. If you have one channel sinking 300mA, and assuming worst case on-resistance of 6Ω, then the power dissipated in the nMOS is about 540mW. The single channel 300mA max limit allows a safe operating area with no device damage. The eight channel 150mA max limit is pushing the overall power dissipation up to 135mW per channel x 8 = 1080mW. The continuous power dissipation for the packages is given in the page 2 table also. If ambient temperature rises and the die temperature rises, then eventually the thermal shutdown will be activated when the die temp approaches +160°C.