In terms of quantity, there are likely more chargers available for NiMH batteries compared to Lithium-ion or Lithium-polymer batteries. If one were to opt for the latter options, integrating the charging circuit becomes necessary, increasing the complexity and cost of the battery pack. By keeping the batteries separate from the enclosure, the option to use Li-ion or LiPo batteries remains open. Fortunately, their initial voltage (around 3.6–3.7 volts) aligns closely with the combined voltage of three NiMH batteries in series. Additionally, employing a separate battery holder allows for the convenient replacement of discharged batteries with fresh ones, eliminating the need to charge first or open the enclosure. This flexibility proves invaluable, especially in situations where urgent charging is required, and you find yourself in a remote location with limited resources for recharging.
How many batteries to use?
The circuit has to produce 5 V and be able to deliver up to 1 A of output current. Four freshly charged NiMH batteries can have a voltage well above 5 V, so it seems prudent to keep the number at three. However, in USB speak ‘5 V’ is nominal, the actual range being 4.35 V to 5.40 V. Although that’s in favor of four batteries again, we still wish to produce a 5.00 V supply that’s accurate, if only because some designers use the USB voltage as a reference (keep that limited to non-critical applications). Three.
Boost converter TPS61030
The lower voltage of three batteries implies the use of a smaller battery holder but also the need for a boost converter. There’s an excellent device available from Texas Instruments, the TPS61030. It’s a synchronous boost converter with an internal 4-amp switch and an efficiency of 96 % (dependent on input voltage and output current of course). The converter also has an (optional) Low Battery Comparator to prevent deep discharging of the batteries. An extra undervoltage lockout (1.6 V) prevents the converter from malfunctioning.
The internal reference voltage is 0.5 V, making it easy to calculate the voltage divider for the correct output voltage. Here 1.8 MΩ is used for R3, and 200 kΩ for R4. According to the datasheet, only if R4 is significantly lower than 200 kΩ then an extra capacitor for stability is necessary in parallel with R3. Here a 10 pF cap is used just to be sure. Resistor R2 should be low enough to eliminate the input current of the comparator (about 10 nA). A value of 500 kΩ is recommended. The comparator level is around 500 mV with a hysteresis of 10 mV. A threshold of 1.1 V was chosen to define one fully drained battery.
Values of 1.8 MΩ for R1 and 330 kΩ for R2 result in a theoretical threshold of 3.23 V. If the total battery voltage drops below this threshold the output of the comparator goes Low (LBO). This output is used to disable the output circuit.
Decoupling Voltage for Optimal Operation
Proper decoupling of the input voltage is executed in accordance with the recommendations provided in the datasheet. The decoupling of the output voltage is contingent on the maximum allowable ripple. Ideally, a few millivolts of ripple is desired, but in practice, the equivalent series resistance (ESR) of the capacitors, as well as the board layout, can result in a higher value. Theoretically, the ripple should hover around 1 mV when using a 220 µF output buffer capacitor.
Reducing Ripple Voltage: Techniques and Considerations
When subjected to a 1 A load at 3.50 V, a measurement revealed an approximately 60 mV ripple across capacitor C5. Although C5 boasts a 20 mΩ equivalent series resistance (ESR) at 100 kHz, the significantly higher switching frequency of 600 kHz and the augmented switching current contribute to this heightened ripple voltage. To counteract switching noise, a ferrite bead (L2) is incorporated in series with the output circuit, further curtailing the ripple voltage. Another component addressing this issue is the ultimate output capacitor (C8), which additionally aids in diminishing the ripple voltage.
Inductor Selection and Operational Modes
To arrive at the suitable inductor value, a 10% deviation from the maximum average inductor current was taken into account. At 3.20 V, the average inductor current hovers around 2 A. Referring to the formula outlined in the datasheet (SLUS534E), this corresponds to an inductor value of roughly 10 µH. The Sync pin presents diverse converter operation modes. Opting for Power Save mode by grounding the Sync pin enhances efficiency under lighter loads, as the device operates intermittently. In this configuration, the converter functions exclusively when the output voltage dips below a predetermined threshold. However, this mode introduces a minor uptick in output ripple voltage. Under no load, an 80-mV sawtooth pattern emerged at a 150-ms interval, yet this pattern swiftly ameliorated with an increase in loading.
TPS2511: glue for USB:
A special IC type TPS2511 is used for controlling the output. Texas Instruments calls it a USB Dedicated Charging Port Controller and Current Limiting Power Switch but we still liked it. Here’s why. It’s often not enough to just put 5 V on a USB connector and get a device to work.
It’s great that phone & gizmo manufacturers increasingly fit their devices with USB connectors as the charge port, but chargers are unlikely to be compatible between fruit and non-fruit platforms, and different manufacturers. For example, some devices expect specific voltages on the data lines, or simply a connection (resistor) between the data lines to recognize a charger (Dedicated Charger Port or DCP). The TPS2511 supports three of the most common protocols:
- USB Battery Charging Specification, Revision 1.2 (BC1.2);
- Chinese Telecommunications Industry Standard YD/T 1591-2009;
- Divider Mode.
Maximizing TPS2511 Functionality
For a comprehensive overview of the TPS2511’s capabilities, it is advised to consult the official datasheet provided by Texas Instruments (SLUSB18). The TPS61030 is capable of delivering 2 A at a battery voltage of 3.3 V, a current load that the TPS2511 can efficiently handle. However, it’s crucial to note that even at 1 A output current and 3.33 V input voltage, the converter already draws 1.7 A from the batteries. With an increase to 2 A output current, this figure will more than double due to amplified losses.
Additionally, higher output currents result in decreased battery capacity. To address these challenges, the TPS2511 is configured to function as a 5-watt charger. The DP pin is linked to the D– line, and the DM pin is connected to the D+ line of the USB connector. The current limit is marginally set higher than required (R6 = 47 kΩ) to prevent premature output voltage limitations by the TPS2511.
Creative Use of Current Sensing Report Pin
In an unconventional approach, the Current Sensing Report pin is utilized differently. Instead of compensating for voltage loss by adjusting the converter feedback (a measure typically unnecessary at a 1 A maximum output current), this pin drives an LED (D1). When D1 illuminates, it indicates that more than half of the maximum output current is being utilized. The LED draws slightly over 1 mA. As previously mentioned, the Low Battery Comparator output controls the EN (Enable) pin of the TPS2511. By employing R5, the comparator output, which is in a high impedance state when inactive, ensures proper functionality.
Polarity and overvoltage guard circuits:
The battery pack connection to the PCB is by way of a screw header (0.15’’ lead spacing). So in practice it’s possible for the batteries to be connected the wrong way around. To prevent damage to the circuit and still have virtually no losses when properly connected, a small n-channel power MOSFET (T1) is used, purposely the wrong way around. When connecting the batteries with the proper polarity, the body diode is in the conducting direction, and the MOSFET is fully turned on, its gate positive with respect to the source through R12.
Ensuring Proper MOSFET Functionality
When the current flows from source to drain, there are no issues. In the event of incorrect battery polarity connection, the gate becomes negative, causing the MOSFET to turn off, effectively blocking the battery voltage through the body diode. The MOSFET employed has a maximum allowable gate voltage of 12 V, which also serves as the circuit’s survival limit.
Smart Overvoltage Protection Design
To avoid the need for an expensive heavy-duty on-off switch, the overvoltage protection is cleverly integrated with a smaller, more economical switch. The protection mechanism is straightforward: when the supplied voltage exceeds the acceptable level, a zener diode (D2) triggers an n-p-n transistor (T2), thereby cutting off the gate voltage of MOSFET T3. The 5.1-V zener diode begins conducting below the specified voltage, with a current of approximately 12 µA at a battery voltage of 3.60 V, rising to over 30 µA at 4.25 V. This current can be measured across R9, preventing excessive current flow through the zener diode when the input voltage surpasses around 5.70 V.
Adjusting Overvoltage Protection Threshold
In situations where the overvoltage protection triggers prematurely due to potential zener diode tolerance, R10 can be adjusted. It’s essential to note that a lower resistance value increases the threshold. This overvoltage safeguard becomes crucial if an AC power adapter, hopefully set below 12 V output, or a 9-V battery is connected. The TPS61030 can handle up to 7.00 V (absolute maximum, 5.50 V recommended). The challenge with the boost converter lies in the output voltage increase when the input voltage exceeds the regulated output voltage, set here at 5.00 V nominal.
The PCB is specifically designed for a Hammond Manufacturing enclosure (see parts list). It’s cheap and easy to adapt to our application. The PCB is fixed with four self-tapping screws, and the top and bottom halves with two longer ones. The front and back are separate panels. In one panel only, three holes have to be drilled. The holes for the USB connector and switch should be aligned with the parts on the PCB. The same is true for two holes for the LEDs in the top cover. The exact placement of the hole for the two wires to the external battery holder is not that critical—there’s a large margin to play with. It can be located anywhere else for that matter. A power jack may also be used—there’s more than enough room in the other panel.
Avoid any extra contact resistance where possible. As it will reduce the efficiency of the device as a whole. The holes for fixing the PCB are also used to connect the board’s top and bottom power planes. Be aware that the hole next to screw header K2 is not connected to ground but the net between T1 and T3. Connecting this net to ground will not cause any damage but simply turn on the circuit. The other three holes are connected to ground. However, the hole next to IC2 is specifically output ground. It’s assumed the PCB is placed in the above mentioned hard plastic (ABS) enclosure. Finally, do not touch junction R3/C3/R4 with the circuit in operation. This is a high impedance point and any hum introduced here may destroy IC1.