[Guide]Linear battery chargers are usually smaller and smaller than general switching chargers. Simple and cheap, but it has a major disadvantage: when the input voltage is high and the battery voltage is low (discharged battery), there will be too high power dissipation. In typical cases, these states are temporary (because the battery voltage increases with charging), but this worst-case scenario must be considered when determining the maximum allowable value of charging current and IC temperature.
Linear battery chargers are usually smaller, simpler and cheaper than general switching chargers, but they have a major disadvantage: excessive power dissipation occurs when the input voltage is high and the battery voltage is low (discharged battery). In typical cases, these states are temporary (because the battery voltage increases with charging), but this worst-case scenario must be considered when determining the maximum allowable value of charging current and IC temperature.
A simple way to solve the overheating problem is to reduce the charging current of the entire constant current part of the charging process. The problem with this method is that it increases the charging time accordingly. Another better choice is to use the LTC1733 Li-ion single-cell linear battery charger, which overcomes any overheating problems and can also maintain fast charging time. The unique thermal feedback loop inside the IC allows fast charging at full current under normal conditions and will not overheat in the worst case (including high ambient temperature, high input voltage or low battery voltage states).
Thermal feedback loop limits IC temperature
The thermal feedback loop limits the maximum junction temperature of the LTC1733 to around 105°C, which is much lower than the maximum allowable junction temperature of 125°C. When the junction temperature approaches 105°C, the temperature sensor in the chip begins to smoothly reduce the charging current to a level that limits the maximum junction temperature to 105°C (see Figure 1). Unlike ICs that simply shut down at 160°C to protect themselves, the LTC1733 can work in this temperature control mode for a long time. Devices with a thermal shutdown temperature of 160°C will start to turn on and off within the thermal limit, or they will not operate properly as a charger. Thermal shutdown is not a desirable way to operate. It is better to protect the IC from failure when it is overheated.
Figure 1: LTC1733 Li-ion battery charging cycle under high ambient temperature
Charging cycle with thermally limited operation
Figure 1 shows a typical lithium-ion single-cell battery charging cycle at the worst temperature. The curve shows the relationship between battery voltage, charging current and printed circuit board temperature and time.
When the input power is added to the connected battery and the programmable resistance is grounded, the charging cycle begins. The deeply discharged battery is slowly charged at 10% of the full current until the battery voltage reaches 2.48V and then the charger switches to full current charging.
When the charging cycle starts, the charging current quickly increases to the set value of 1.5A, causing the battery voltage to rise to 3.2V. Under the input voltage of 5.3V, the 3.2W power dissipation of LTC1733 makes the junction temperature rise to about 105°C, and the temperature of the 2″ × 2″ PCB (heat sink) reaches about 85°C in about 1.5 minutes. The thermal feedback loop reduces the charging current to limit any additional temperature rise. When the battery voltage increases, the LTC1733 temperature begins to drop, causing the charging current to increase again to the 1.5A set current level. Charging continues at a constant current of 1.5A until the battery voltage reaches 4.2V, at which point it enters the constant voltage section of the charging cycle. This situation continues and the charging current continues to decrease until the 3-hour timer ends the charging cycle (Figure 1 shows the situation in the first 90 minutes).
Packages with enhanced heat dissipation significantly improve power dissipation
The extremely small (1.1 mm) 10-pin MSOP package and exposed bottom metal pads allow the IC to be soldered directly to the copper layer of the PCB, which significantly reduces the thermal resistance from junction to case. The good thermal layout enables the LTC1733 to continuously dissipate up to 2.5W using a 2″ × 2″ 4-layer PCB board at an ambient temperature of 25°C.
A good thermal layout consists of the PCB copper layer under the package spreading directly to the entire copper area, and the copper layer passing through the thermal vias to the inside and back. For surface mount devices, the PCB copper layer becomes an effective heat sink.
It is also very important to solder the metal pad of the entire IC to the PCB board to ensure good heat conduction. Tests show that when a large 4.5W initial power is applied to the package, the poorly soldered package reaches the thermal feedback temperature in a few seconds, while the good soldering device takes more than one minute.
Fully independent charger
LTC1733 is a completely constant current, constant voltage, power-limited linear charger for single-cell lithium-ion batteries, as shown in Figure 2. The IC contains 1.5A power MOSFET, current detection resistor, settable charging current, settable timer, optional charging voltage and thermistor input to monitor the battery temperature for charging qualification. There are three status outputs that can drive LEDs to Display’AC power good’,’charging’ and’fault’. There is also an output that monitors the charging current. The input voltage requirements are 4.5V to 6.5V and can be manually shut down, and there is a micro-power sleep mode when the input voltage is disconnected. No blocking diode is required due to the internal MOSFET structure.
Figure 2: A complete 1.5A single-cell lithium battery charger for 4.1V or 4.2V batteries (no external MOSFET, blocking diode or sense resistor required)
LTC1733 is an independent linear charger IC for lithium-ion batteries, which allows the charging current to be set at the nominal VIN, VBATTERY and ambient temperature without excessive temperature under certain short-term charging conditions. This allows for higher charging currents (achieving faster charging) and ensures that the occasional worst-case scenario will not cause the system to overheat.