I've a modem which has GPRS and GPS Module interfaced with PIC24F micro controller. The modem input voltage range is 8-35v and 300mA.We have planned to fix that modem to car battery, so we designed that modem with input voltage range 8-35v. But now we changed our mind not to connect with car battery. We would like to connect that modem to a power bank. We bought a 4charge power bank@ link! . The power bank has two output usb port with 5v 1A and 5v 2A respectively. Since our modem input voltage range is 8-35v so we decided to put a step-up the voltage by booster(XL6009 DC-DC step up module) to boost up 5v to 12v.When we switch on the power bank ,the modem doesn't getting any supply but sometimes Modem is working (getting supply). What could be the problem in this scenario and suggest any idea to solve this issue?
Your power bank 99% likely already has a DC-DC SMPS converter inside. The internal converter is used to convert between ~4.2-5.5V USB power input down to current charge level of the contained (usually LiPO or LiIon) battery(s), then back up to 5v for its usb output.
Nearly all power banks use some form of control circuitry that's designed around the >=500mA current drawn by 'normal' usb-charged (not usb-run) devices. As pointed out by @Passerby, you're likely triggering a 'powersave timeout' on this circuit feature while your modem is either not running, or only drawing ~300mA for too long.
Most likely, your best solution would be to keep the original powerbank converter, but only use it for charging; then attach your step-up (boost) output converter directly to the batteries inside the powerpack, and use a switch to enable/disable your converter (every converter stabe you use has efficiency losses, so 1 is nearly always better than 2 in series, anywise). Ideally, you could use the same switch to ena le/disable the boost convsrter and the modem, thus improving eass-of-use & end-user transparency.