Here is a simplified circuit of a boost converter that convert a li-ion battery from 3.6-4.2V to 5V (feedback is not simulated here, thus voltage will swing wildly)
In TP5400 or TP5410, the switch will turn off during charging. You can either use a diode or use a PMOS, diode and pull down resistor to do load sharing, disconnecting the battery from the output while charging (5V USB is plugged in).
Using the PMOS load sharing circuit (top) has an advantage where the input 5V can drop by quite a lot (typically 0.5V less than the battery voltage) before the battery reconnects.
While in the case of just having one diode (bottom), the battery starts reconnecting when the 5V supply is around 50-200mV above the battery voltage.
It is unlikely the 5V USB will drop so low in voltage, but this is something worth considering.
