ANALOG DEVICES
charging current can be accommodated using a step-down converter in front of the charger to regulate its output voltage (see Figure 3). This also minimises dropout and therefore reduces power dissipation on the charging MOSFET (see Figure 4). Implementing a fuel gauge in the battery pack enables the battery to become smart, enabling advanced charging scenarios and capabilities. For example, the fuel gauge can store the charging profile suitable for the cell inside the battery pack in its nonvolatile memory.
Figure 2: A block diagram showing fuel gauge with a charging MOSFET regulation
This has the added benefit of offloading charging from the host microcontroller unit (MCU). Now the host MCU only needs to manage the ALRT signal coming from the battery pack to increase/decrease the output voltage of the step-down converter according to the alert type received. CP: heat limit ➝ decrease the voltage. CT: MOSFET temperature limit ➝ decrease the voltage. Dropout: ➝ increase the voltage CP is a flag that is set when the current flowing in the protection MOSFETs can compromise thermal dissipation. CT is a flag that is set when the MOSFET temperature is too high. Heat limit and MOSFET limit settings are configured using the nChgCfg1 register set.
A programmable step-down converter like the MAX20743 uses the PMBus®
to allow fine
regulation of the output current. Integrated MOSFETs in the step-down converter support charging currents up to 10A. In addition, since the PMBus uses I2C as its physical layer, a single I2C bus can be used to manage both the step-down converter and fuel gauge.
Figure 3: A block diagram for high voltage/high current fast charging system
battery pack (see Figure 1).
When implemented in the battery pack, the fuel gauge requires nonvolatile memory to store battery information. MOSFETs on the power path monitor charging/discharging currents and protect against dangerous conditions. A device like the MAX17330 from Analog Devices is a battery fuel gauge with built-in protection circuitry and battery charger capabilities (see Figure 2). The charging MOSFET can be regulated with fine granularity to implement a linear charger that can be used as a standalone device when the charging source is limited to 5V and the charge current is in the range of 500mA. Since lithium battery charging exceeds 3.6V for 99% of the charge curve, power dissipation is limited.
A high voltage charging source and high
Figure 4: Using a step-down converter to regulate output voltage to allow 10A charge current with high efficiency. Shown here is the MAX20743 step-down converter with VIN = 12V
JUNE 2023 | ELECTRONICS TODAY 17
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