Thank you for the array of images. Your first thought was accurate: The browning is due to the blistering hot sun.
Maples need to be watered deeply, every 2 to 3 weeks, through our dry months. Apply the water from a foot or so beyond the dripline (the ends of the branches) to halfway toward the trunk. Probe with a trowel to determine if the soil is moist to 8 inches deep.
Your tree will also benefit from an organic mulch. Use either 2-inch deep bark dust or 4-inch deep bark chips. Spread the mulch on the soil surface beginning 6-inches from the trunk and extending outward to at least the dripline.
The numerous bright yellow leaves in the images reveal a possible fertilizer deficiency which is probably related to the lack of moisture. The roots are unable to absorb the needed fertilizer elements from the dry soil.
Even so, do not fertilize the tree while it is severely stressed.)
Thank you so much for your quick response . That was very helpful.
You're welcome. Enjoy your garden.