3.8. Power

The Zumo chassis has an internal compartment for four AA batteries. We recommend using rechargeable AA NiMH cells, which results in a nominal voltage of 4.8 V (1.2 V per cell). You can also use alkaline cells, which would nominally give you 6 V.

The negative battery voltage is connected to GND. The positive battery voltage is designated VB. VB feeds into a circuit that provides reverse protection and power switching controlled by the on-board power switch. The output of this circuit is designated VBAT.

VBAT provides power to the motors through the DRV8838 motor drivers, so the motors can only operate if the batteries are installed and the power switch is in the “On” position.

The battery voltage on VBAT can be monitored through a voltage divider that is connected to analog pin 1 (PF6) by default. The divider outputs a voltage that is equal to one half of the battery voltage, which will be safely below the ATmega32U4’s maximum analog input voltage of 5 V as long as the battery voltage is less than 10 V. The readBatteryMillivolts() function in the Zumo32U4 library can be used to determine the battery voltage from this reading (see Section 6). The surface-mount jumper labeled “A1 = VBAT/2” can be cut to disconnect the voltage divider and free the pin for other uses.

5V regulator

VBAT supplies power to a 5V regulator based on the TPS63061 switching step-up/step-down (buck-boost) converter from Texas Instruments. The regulator works with a 2.7 V to 11.8 V input voltage (although the motor drivers limit the maximum VBAT voltage to 11 V) and has a typical efficiency of 80% to 90% for most combinations of input voltage and load. (We also make a standalone regulator based on this integrated circuit.) The 5V output of this regulator is designated VREG.

The regulator can be disabled by driving the regulator shutdown pin, SHDN, high.

Power selection

The Zumo 32U4 main board’s power selection circuit uses the TPS2113A power multiplexer from Texas Instruments to choose whether its 5 V logic supply (designated 5V) is sourced from USB or the batteries via the regulator, enabling the robot to safely and seamlessly transition between the two sources. The TPS2113A is configured to select regulated battery power (VREG) unless the regulator output falls below about 4.5 V. If this happens, it will select the higher of the two sources, which will typically be the USB 5 V bus voltage if the Zumo is connected to USB.

Consequently, when the Zumo 32U4 is connected to a computer via USB, it will receive 5 V logic power even when the power switch is off. This can be useful if you want to upload or test a program without drawing power from the batteries and without operating the motors. It is safe to have USB connected and battery power switched on at the same time.

The currently selected source is indicated by the STAT pin; this pin is an open-drain output that is low if the batteries are selected and high-impedance if the USB supply is selected. The current limit of the TPS2113A is set to about 1.9 A. For more information about the power multiplexer, see the TPS2113A datasheet (1k redirect).

3.3V regulator

The main board also has 3.3 V linear regulator. The inertial sensors and level shifters draw power from the 3.3 V line; the remainder (up to a few hundred milliamps) is available for powering external circuits or devices.

Alternative power sources

For users who want to experiment with alternative power sources like lithium batteries, the Zumo 32U4 can accept a battery input voltage from 2.7 V to 10 V. You can raise the maximum allowable voltage to the motor drivers’ limit of 11 V by disconnecting or modifying the battery voltage divider.

We do not recommend using a 3-cell lithium battery to power the Zumo 32U4. Even though such a battery is usually specified with a nominal voltage of 11.1 V, it can measure well over 12 V when fully charged.

Adding a power switch

You can add your own power switch to the Zumo 32U4 using the PSW pin. When it is in the on position, your switch should connect PSW to GND. In that case, VBAT will receive power when either your switch or the main board switch are on.