With a maximum of 20V across the ZXCT1010 and 1V output, transistor TR1 will have to drop 139V across it (ignoring output short circuit conditions). Therefore, the first task is to establish the maximum current drawn from the output. Although the ZXTP05120HFF is rated up to 1.5W, this will only be realizable under very tight conditions. In most cases, after allowing for ambient temperature rise and derating the device, the expected transistor current range would be 400mW to 750mW depending on factors such as mounting, ambient temperature, ventilation, etc. Assuming 400mW for the maximum dissipation in this example, the maximum current that can be drawn will be 400mW divided by the voltage drop across transistor TR1 (139V), or 2.87mA.