This paper presents a method for reducing electromagnetic interference (EMI) generated by DC-DC boost converters in portable charging applications of smartphones. Portable charging applications have appeared in the marketplace since the release of the iPhone 3G. A portable charging application is defined as a device being capable of charging a smartphone without a wall charger. Some portable charging applications more or less fully cover the backside of a smartphone much like wearing a back pack. For example, there is Juice Pack for iPhone from Mophie. The EMI noise mentioned in this paper is only limited to this type of portable charging application. This study presents the reason why DC-DC boost converters generate EMI noise, and also presents a test configuration that measures Total Isotropic Sensitivity (TIS) and EMI noise level. This study is particularly focused on TIS degradation for mobile communication frequency bands (GSM900, DCS1800 and WCDMA2100) affected by the EMI noise generated on DC-DC boost converters and is interested in improving TIS by using the proposed EMI noise reduction. The EMI noise reduction method is performed with the shielding effectiveness of a shield-can made of SUS-301 and a Printed Circuit Board (PCB) layout modification. This study verified that EMI noise is improved almost up to 15dBμV for mobile communication frequency bands with a SUS-301 shield-can and additionally, is improved almost up to 3dBμV for GSM900 with PCB layout modification. Finally, the combination of the each method for reducing EMI noise delivered the result such that the TIS for GSM900 and DCS1800 were improved up to almost 17dBm.