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Posted on 01 March 2019

Understanding System Loads and Interfacing with Chargers

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System power must not exceed charger power without energy storage

Battery charging is a fairly simple concept when considering only a standalone charger for a given input voltage, charge level and pack size. However, charging is often done with a system load, which can complicate charging. This article highlights the potential startup and operating issues with a battery charger powering a system load without a battery, and how the charger output characteristics react with the system input characteristics.

By Charles Mauney, Senior Battery Charger Applications Engineer, Texas Instruments

 

It is becoming more common for the system to operate off the charger without a battery. This may occur during a normal application or during a manufacturer’s test. In Figure 1, there is no battery source to power the system during any transients or power-up conditions. If not designed properly, the charger can get latched in a short-circuit condition. To resolve these design issues, it is essential to understand the chargers’ output source specifications and input system load requirements.

Block diagram of charger power source and system loads

Operational Issues without a Battery

A Lithium-Ion (Li-Ion) charger is considered a current source that is clamped at a regulation voltage. Typically, the device has a battery pack attached and acts as an energy reservoir (large capacitor) to keep the system powered through transients. If the system load exceeds the source current for a short period of time, the battery will supplement the additional current. When a battery is not present, the system voltage drops quickly, if the system load current exceeds the charger’s source current. To complicate matters, the charger is a three-stage current source, short circuit, pre-charge and fast-charge. Exceeding the available current causes the system voltage to drop and possibly cause the charger to enter pre-charge and then shortcircuit where less current is available. On the contrary, if the load current is less than the charger current the system voltage rises until 4.2V regulation is reached. Then the charge current drops to equal the load current.

To operate without a battery, the charger and system must be designed such that the charger can always deliver the required current to the system. To determine this, the charger’s IV characteristic must be compared to the system load IV characteristic.

Output Characteristics of the Charger

We’ll be discussing a Li-Ion charger as it has several charge phases, and the concepts discussed can be easily applied to other charger chemistries. Figure 2 shows the charger’s current profile as it relates to the charger’s output voltage, VSYS. Initially the voltage is at 0V, if the battery is not present and the charger has not been powered. When power is applied to the charger, the charger’s output voltage starts to rise due to an internal pull-up (~500 Ohms) between the input and output. The short circuit mode is below one volt and designed to minimize power dissipation during a short on the OUT pin.

Li-Ion charge profile – Charge current and voltage outputs

Once above the short circuit threshold (0.8 to 1.4V), the charger enters pre-charge mode. Pre-charge recovers a deeply discharged battery or determines if the pack is damaged, and if so terminates charge. The pre-charge current is approximately one-tenth the fast charge current, but some chargers can program this level independently. The pre-charge mode transitions into the fast-charge constant current at ~3V, where the programmed fast charge constant current is supplied. At no time will the charger deliver more than this programmed current level. When the voltage reaches the constant voltage mode at 4.2V, the output is regulated and capable of providing up to the programmed current level. If the load current drops to the termination threshold, the charge is terminated unless termination is disabled.

The current sourced in each of these phases is shown in Table 1.

Charging modes and available current and power.

Now that it is understood how much current is available from the charger, an analysis of the system load is needed to confirm if the design is compatible with operation without a battery.

System Load Characteristics

A resistive load sinks current, which is proportional to the voltage applied and may be present during power-up. Resistances lower than 125 Ohms (ISINK = 1V/125 Ohms = 8mA) may prevent the charger from exiting short circuit mode on power-up without a battery.

Typically, a DC/DC buck converter is not enabled until its input voltage is near its regulated output voltage. Since the converter’s output voltage is fixed, its load is often constant, so its input current is inversely proportional to its voltage. Two of the curves in Figure 3A show input current into a 1.8V and 3.3V DC/DC converter versus input voltage. You can see that as the voltage increases the current decreases and visa-versa.

DC-DC converter input current versus input voltage, Power-up sequence with issues

Typically, capacitive loads are not an issue on the input side of the converter and slow down the power-up, unless a timed event expires causing a reset or further loading. Capacitive loads on the output of the converter may cause peak power demands when powering up and can be reduced, if the converter has a soft-start feature. Pulsed loads add to the other static loads and may happen at any time, so special attention should be paid to make sure peak loading does not exceed the available charger source current when operating without a battery.

Comparing Source Current to the System Load Current

There are two types of comparisons that should be considered: a static DC comparison and a real-time power-up and operational comparison. The DC comparison simply compares the system load current to the available charger’s source current at any given system voltage. Figure 3 shows the total load current and the available charger current as the system voltage changes. Initially on power-up, the resistive load currents are close to the available charger’s short circuit current. Therefore, the designer may want to ensure the output voltage can charge up to the pre-charge region. In pre-charge, when the 1.8V converter enables at 1.6V, the total current slightly exceeds the pre-charge current. A solution is to enable the converter at VSYS = 1.8V, where the load current is reduced as shown in Figure 3B. Similarly, the 3.3V converter is enabled at 2.8V. Delaying the turn on until VSYS has reached 3.1V will move the loading into the fastcharge region and prevent a loading issue. Now that the static issues have been analyzed, it would be good to follow with an operational test.

DC-DC converter input current versus input voltage, Power-up sequence corrected

The real-time operational comparison helps to understand the load transients timing and to make sure the peak loads do not exceed the available source current. A simple test can be implemented by connecting the system load to a lab supply. Insert a 100m Ohm resistor in the return and set the supply voltage to 4.2V. Connect the scope probes as shown in Figure 4, to capture the voltage and current. Set the scope for a single sequence trigger on the voltage waveform and power on the lab supply. This test can be repeated with a hot plug-in. A continuous operational test may be done by triggering off of the current (set just below the charger’s programmed current threshold), while running the system through the system’s different operational modes. This should be done over the complete VSYS operation range of the system. If the scope is triggered, examine the current pulse and determine if the load is excessive.

Setup to capture real time operational currents vs voltage waveforms

System: Operational, Cycling On/Off or Latched-Off (Crashed)

The desired mode of operation when the battery is absent, is where the available charger current is always more than the system load current, thus stable operation occurs. In this mode the system capacitance charges up to the regulation voltage and the fast charge current tapers to equal the system load current. The system remains in this steady state mode as long as the system current is less than the programmed fast charge current. The cycling or latched state is entered if the load current exceeds the available charge current, since the DC/DC converters demand higher current at lower system voltages. If the system voltage drops such that the converter is disabled, then the system voltage recovers until the next over-current load. This cycling mode is commonly known as hick-up mode.

Design Hints for Operating or Testing without a Battery

Construct a table similar to Table 1 or plot a charger current curve as in Figure 3 to define the system’s absolute maximum load boundary. Operate the system in all modes of operation over the system voltage range and define what systems can be enabled and when to stay below the maximum load boundary. The best solution is to enable the system, only after the charger is in fast-charge. Never have a load greater than the minimum-fast charge power available (for example, Fast-Charge Mode in Table 1 with 3 Watts). Since the charger output power and the system load power are both a function of VSYS, one can compare the power or the current to come to the same conclusion.

PCHGR-OUT = PDC/DC-IN
ICHGR-OUT*VSYS = IDC/DC-IN*VSYS
ICHGR-OUT = IDC/DC-IN

Therefore, the designer should keep the system power demand below the minimum charger power output or keep the peak system current demand below the programmed charger output current to guarantee continuous system operation.

Summary

Powering an electronic product with an adaptor and a battery is fairly simple since the battery always can be used as a back-up for any peak loads that may appear. The only concern is that the average charger current is larger than the average load current so the battery is not discharged. If operation is desired without a battery, then pay particular attention to the load currents not exceeding the charger source current. Otherwise, the system voltage will likely crash and get stuck in a low-power current limited state. Often short circuit and pre-charge modes are where issues occur. Avoiding full operation in these modes will solve most issues. 

 

 

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