How have current-limited power switches helped you in your designs?


Written by: Cornel Pazara, Product Marketing Engineer

With the advent of new handset and tablet devices in smaller and smaller footprints, engineers are facing the arduous task of designing systems that fit into these smaller form factors. Also, consumers are placing more value on the battery life of their portable devices by seeking longer operation time between charges.

One of the standards that has become a “must have” for these devices is the USB port that enables a user to connect various accessories to their device. However, power transfer from a device to a downstream accessory needs to be carefully managed to avoid inrush current and/or voltage transient events upon connection. The accessory typically sends a handshake signal to indicate that it is ready to accept downstream power transfer. Only then should the device’s CPU or system controller authorize current flow. Well-defined overload protection is also mandated to reliably manage capacitive loads or accessory fault modes.

Offered in a tiny 6-bump 1.2 mm x 0.8 mm micro SMD package, Texas Instruments has introduced two new current-limited power switches, LM34902 and LM34904, rated for 300mA and 500mA, respectively. Figure 1 demonstrates how the LM34902 and LM34904 switch connects to a device CPU and an accessory load. Table 1 presents the LM34902 and LM34904 truth table.

 LM34902 and LM34904 schematic

Figure 1: Schematic

 

 

 Truth Table Operation

Table 1: Truth Table Operation

 

When the accessory is connected, it asserts an active-low accessory detect (ACC_DET) control input. The CPU can then selectively turn on (or off) the switch using the ENABLE input, providing power to the accessory (or conserving power when needed). An open-drain output (POK) indicates switch status to the CPU. With a low ON resistance, the high-side PFET switch power loss is minimized, translating into lower temperature increase―a critical feature in space constrained applications―and longer battery life. Using an integrated sense resistor, the LM34902 and LM34904 provides accurate brickwall current limit even when the ACC_PWR output is shorted to ground. To assure reliability, thermal shutdown with hysteresis is provided if the junction temperature exceeds 135 degrees C.

Try this out on your design and tell us how it worked by ordering samples of the LM34902 and LM34904.


How have current-limited power switches helped you in your designs?