Integration Manual
TSD-19090601 - R15 Contents Page 67 of 143
Table 20: Suggested components to reduce ripple / noise on the VCC
☞ The necessity of each part depends on the specific design, but it is recommended to provide all the
bypass capacitors illustrated in Figure 13 / Table 20 for noise-sensitive applications and/or if the end-
device integrates an internal antenna.
☞ The ESD sensitivity rating of the VCC supply pins is 1 kV (HBM as per JESD22-A114). A higher protection
level can be required if the line is externally accessible on the application board, e.g. if the accessible
battery connector is directly connected to the supply pins. A higher protection level can be achieved by
mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor) close to the accessible point.
2.2.1.7 Guidelines for the external battery charging circuit
TOBY-L3 series modules do not have an on-board charging circuit. Figure 14 provides an example of a
battery charger design which is suitable for applications that are battery powered with a Li-Ion (or Li-
Polymer) cell.
In the application circuit, a rechargeable Li-Ion (or Li-Polymer) battery cell, that features the correct pulse
and DC discharge current capabilities and the correct DC series resistance, is directly connected to the VCC
supply input of the module. Battery charging is completely managed by the STMicroelectronics L6924U
Battery Charger IC that, from a USB power source (5.0 V typ.), charges the battery as a linear charger, in
three phases:
Pre-charge constant current (active when the battery is deeply discharged): the battery is charged with
a low current, set to 10% of the fast-charge current.
Fast-charge constant current: the battery is charged with the maximum current, configured by the value
of an external resistor to a value suitable for USB power source (~500 mA).
Constant voltage: when the battery voltage reaches the regulated output voltage (4.2 V), the L6924U
starts to reduce the current until the charge termination is done. The charging process ends when the
charging current reaches the value configured by an external resistor to ~15 mA or when the charging
timer reaches the value configured by an external capacitor to ~9800 s.
Using a battery pack with an internal NTC resistor, the L6924U can monitor the battery temperature to
protect the battery from operating under unsafe thermal conditions.
The L6924U, as a linear charger, is more suitable for applications where the charging source has a relatively
low nominal voltage (~5 V), so that a switching charger is suggested for applications where the charging
source has a relatively high nominal voltage (e.g. ~12 V, see the following section 2.2.1.8 for the specific
design-in).