Configuring Standalone Linear Power Path Chargers
Configuring Standalone Linear Power Path Chargers
In this application report, proper configuration of standalone power path linear chargers will be discussed as well as the different mode these chargers have. Power Path chargers have an advantage over Non Power Path chargers due to the extra FET between the SYS rail and the BAT pin. This gives priority to the System which allows for system startup with a depleted battery and proper battery termination.
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1 Introduction
The BQ2407X family of devices is a popular Power Path standalone linear charger used in applications that require 1.5A or less charging current. Since these devices are standalone, most of the important parameters like input current limit, charge current and charge timers can be configured using external resistors. The family also has different options depending on the application.
2 Configuring 1.2A Fast Charge
To configure the BQ2407x, a few calculations will need to be done to get the resistors that would be used to program the desired values. Parameters that need to be programmed include the input current limit RILIM, the fast charge current RISET, the fast charge timer RTMR, and in some devices like the BQ24074 where the termination current can be programmed, RITERM. Since the termination current depends on the fast charge current in the BQ24074, RITERM should be changed when RISET is changed to have the same termination current.
Figure 1. 1.2A Fast Charge Configuration - ILIM set to 1.5A with RILIM = KILIM / ILIM = 1610 / 1.5A = 1.07kΩ
- ICHG set to 1.2A with RISET = KISET / ICHG = 890 / 1.2 = 750Ω
- ITERM set to 0.05A with RITERM = (ITERM * RISET) / KITERM = (0.05 * 750) / 0.03 = 1.25kΩ
- Charge timer set to 6.65 hours; RTMR = (Time * 3600)/ (10 * KTMR) = (6.65 * 3600) / (10 * 48) = 49.9kΩ
- EN2 set High and EN1 set Low to set input current through ILIM
- KILIM, KISET, KITERM and KTMR are obtained from the data sheet
3 Configuring USB100 and USB500 Mode
To set the device in USB500 mode, the EN1 pin should be pulled high and the EN2 pin should be pulled low as shown in Figure 2. In this mode, the ILIM setting is ignored with the input current and the charge current limited to 500 mA. In USB100 mode, the input current limit is set to 100 mA. To set the device in USB100 mode, set both EN1 and EN2 low. In USB mode, the input voltage is monitored with the input current being dynamically reduced when the input voltage drops below the VIN-DPM threshold to prevent the input voltage from crashing below the threshold voltage. ITERM can be set with a resistor or left floating to set the termination current to the default of 10% ICHARGE.
Figure 2. USB500 Mode 4 BQ2407x Optional Functions
The BQ2407x family of devices have optional features on pin 15 of each device. These functions on pin 15 of the device allows the devices to be used in a variety of applications.
| Part Number | Optional Function | Use |
|---|---|---|
| BQ24072 | TD | Charge termination can be disabled by pulling the TD pin high. Charge termination is enabled when TD is connected to VSS. Note the TD is internally pulled down with a 285 kΩ resistor |
| BQ24073 | ||
| BQ24074 | ITERM | The ITERM pin can be used to adjust the charge termination current by connecting a 0Ω to 15 kΩ resistor. If ITERM is left unconnected, the termination current is set to 10% of the fast charge current. |
| BQ24075 | SYSOFF | The FET between BAT and the Output can be turned off by setting SYSOFF high. Setting SYSOFF low put the device in normal operation. SYSOFF is internally pulled up to BAT. |
| BQ24076 | ||
| BQ24078 | ||
| BQ24079 |
5 Common RILIM and RISET Values for BQ2407x
| Input Current Limit | RILIM | Fast Charge Current | RISET |
|---|---|---|---|
| - | - | 100 mA | 8.87kΩ |
| 200 mA | 7.68kΩ | 200 mA | 4.42kΩ |
| 300 mA | 5.11kΩ | 300 mA | 2.94kΩ |
| 400 mA | 3.83kΩ | 400 mA | 2.21kΩ |
| 500 mA | 3.24kΩ | 500 mA | 1.78kΩ |
| 600 mA | 2.67kΩ | 600 mA | 1.47kΩ |
| 700 mA | 2.32kΩ | 700 mA | 1.27kΩ |
| 800 mA | 2kΩ | 800 mA | 1.1kΩ |
| 900 mA | 1.78kΩ | 900 mA | 976Ω |
| 1000 mA | 1.62kΩ | 1000 mA | 887Ω |
| 1100 mA | 1.47kΩ | 1100 mA | 806Ω |
| 1200 mA | 1.33kΩ | 1200 mA | 750Ω |
| 1300 mA | 1.24kΩ | 1300 mA | 681Ω |
| 1400 mA | 1.15kΩ | 1400 mA | 634Ω |
| 1500 mA | 1.07kΩ | 1500 mA | 590Ω |
6 Common RILIM and RISET Values for BQ2423x
| Input Limit Current | RILIM | Fast Charge Current | RISET |
|---|---|---|---|
| - | - | 100 mA | 8.66k |
| 200 mA | 7.68k | 200 mA | 4.32k |
| 300 mA | 5.11k | 300 mA | 2.87k |
| 400 mA | 3.83k | 400 mA | 2.15k |
| 500 mA | 3.09k | 500 mA | 1.74k |
