I designed a small breakout board for the LTC7004 gate driver to test some high-power MOSFETs. While initially powering the circuit using a 12V AC adapter, everything worked fine and the gate drive pins (TGUP/TGDN) would output 24V when the switch was on. I then moved the setup to run off of a 12V car battery and attached a 1/2 ohm dummy load in parallel with R2. At this point the LTC7004 stopped working and will no longer work even using the bench power supply. What might I have done wrong? There is no visible damaged to the chip or PCB and nothing got hot.
No decoupling caps, for one. You should always put decoupling caps in parallel with EVERY power-GND pin pair for EVERY IC unless you know specifically why you don't need to. There should also be a bulk decoupling capacitance.
Your problem is probably that high-side gate drivers runing off bootstrap capacitors can't work at continuous duty. They rely on the output node periodically going LO to allow a current path through the bootstrap diode then bootstrap capacitor to GND into order to refresh it. In a half-bridge this occurs whenever the low-side transistor turns on which happens in normal operation while commutating a motor or inverter.
In your case, you have it flowing through a 10K resistor which does a whole bunch of bad things. While it does provide a current path to recharge the bootstrap while the transistor is off, the current is severely limited, and while on, the boostrap current is drained.
WHen you put in a 1/2 Ohm dummy load, the charge current was probably too high and destroyed the internal bootstrap diode causing it to fail short.
What might I have done wrong?
Exceeded the voltage on INP, Vcc or some other pin. If you don't have good ESD procedures (wearing a wrist strap and have an ESD bench) its very possible that you could have zapped the part. (most of the pins can tolerate up to 15V, ESD can reach thousands of volts)
Another thing to check is if the mosfet is burned out, sometimes they fail and short the source to gate. The driver outputs TGUP TGDN can tolerate lots of voltage, but little or no current if the mosfet failed its possible that the gate driver is also dead:
Do not apply a voltage or current source to these pins. They must be connected to capacitive loads only; otherwise permanent damage may occur.
Source: Note 6 of the LT7004 datasheet