Using an nMOS Transistor with the DS2714
IntroductionThe DS2714 application example in the datasheet uses pnp transistors to switch the charge current on and off. This is the most efficient configuration for component count, but it is not necessarily the best for every application. Since VCHG is a current-regulated supply, the voltage drops to ~VCELL when charging. Having VCHG at VCELL will not generate a high enough VGS for a pMOS transistor to turn on. The drawback of using a pnp transistor is that the IC has to sink a current proportional to the charge current (ib = β × ic). Depending on the charge rate, the base current (ib), can be close to the limit of the DS2714. This application note provides an example that uses an inverter with nMOS transistors to avoid the problems associated with the pnp-transistor configuration.
ConnectionConnect the output of your CCx pin to the input of an inverter; then, connect the output of that inverter to the gate of an nMOS transistor. The nMOS drain should be connected to VCHG, and the nMOS source should be connected to the battery. Because CCx pins are open drain, a pullup resistor needs to be added from VCC to CCx.
Figure 1. A diagram of an example application for the DS2714 that uses nMOS, instead of pnp, transistors.