Texas Instruments TPS3420 Button Controller Solution Introduction

Many modern portable devices are shipped with a battery installed so that customers can immediately turn on the device without the need for battery installation or charging. If the components connected to the battery “leak” excess current, the device may be dead by the time it reaches the customer. All components have leakage currents, and although IC components are the main culprit, capacitors, board dirt, and humidity also have unpredictable levels of leakage.

Author: Michael Hartshorne

Many modern portable devices are shipped with a battery installed so that customers can immediately turn on the device without the need for battery installation or charging. If the components connected to the battery “leak” excess current, the device may be dead by the time it reaches the customer. All components have leakage currents, and although IC components are the main culprit, capacitors, board dirt, and humidity also have unpredictable levels of leakage.

Solving this problem is no small matter. The maximum idle time is obtained when the load is completely disconnected from the battery, but any power-up detection circuits require a battery connection to function. In addition, PCB area is also very important in many battery powered applications, and it is difficult to adjust the space for a single latch switch circuit.

A simple and straightforward method is to use a simple P-type MOSFET (PMOS) and N-type MOSFET (NMOS) latch to disconnect the battery.

Texas Instruments TPS3420 Button Controller Solution Introduction

However, this seemingly simple circuit can lead to unreliable performance. Any disturbance on the switch will open the latch. Also, if the output voltage jumps to positive, or if the capacitive divider created by the CGS of the PMOS and the CDS of the NMOS turns the PMOS device on, the latch may automatically turn on when a battery is inserted. Of course, this problem can be solved by adding some other resistors and capacitors, but it will quickly increase the size and complexity of the design for this basic function.

A better design is given below, which starts the load with a 7.5-second switch touch to avoid interference from the basic latch, allowing the switch to be turned on using a Texas Instruments (TI) TPS3420 push-button controller.

Texas Instruments TPS3420 Button Controller Solution Introduction

The TPS3420 is an ultra-low Iq push button controller. It has two button inputs, this solution uses only one. When a button is pressed (any button already in the system can be used), one half of the dual Schottky diodes can be connected to ground, which in turn can pull the PB1 input of the TPS3420 low. 7.5 seconds after the PB1 input is pulled low, the open-drain output pin RST of the TPS3420 will pull low on the gate of the PMOS switch, connecting the load to the battery. The other half of the dual Schottky diodes will provide a latching mechanism so that when the RST pin goes low, the PB1 pin is also held low, allowing the RST pin to remain low until the battery is fully discharged or removed. This solution uses a tiny CSD23381 (1mm x 0.6mm) PMOSFET as the disconnect switch between the battery and the load.

Texas Instruments TPS3420 Button Controller Solution Introduction

With the system off and less than 1µA total current consumption, the solution can extend battery idle time for significant periods of time, avoiding the risk of customers being disappointed to find their new device needs charging.

The Links:   LTM170E8-L02 NL8060BC2635AD