ST AN2370 Application note
AN2370
Application note
800 mA standalone linear Li-Ion
battery charger with thermal regulation
Introduction
One way to minimize the size and complexity of a battery charger is to use a linear-type
charger. The linear charger drops the AC adapter voltage down to the battery voltage. The
number of external components is low: linear chargers require input and output bypass
capacitors, and sometimes need an external pass transistor, and resistors for setting voltage
and current limits.
The main pitfall of a linear charger is power dissipation. The charger simply drops the AC
adapter voltage down to the battery voltage.
In the case of a 0.8 A charger, a 5 V±10% regulated AC adapter voltage, and battery voltage
that varies between 4.2 V and 2.5 V, the power dissipation can range from 0.64 W to 2.0 W.
This type of charger is simpler than the switch-mode type, mainly because the passive LC
filter is not required. It dissipates the most power when the battery voltage is at its minimum,
since the difference between the fixed input voltage and the battery voltage is greatest
during this condition.
Figure 1. STC4054 battery charge board
October 2007 Rev 4 1/14
www.st.com
Contents AN2370
Contents
1 STC4054 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Stability considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3 Board layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 External components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6 Automatic recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7 CHRG status output pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8 USB and wall adapter power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9 Programming charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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AN2370 STC4054 description
1 STC4054 description
The STC4054 is a constant current/constant voltage charger for single cell Li-Ion battery. No
external sense resistor or blocking diode is required and its ThinSOT package make it
ideally suited for portable applications.
Its power dissipation can range from 0.2 W to 1.68 W because of Trickle charge mode. In
fact, if V
The maximum power dissipation occurs when V
The STC4054 is designed to work within USB power specifications. An internal block
regulates the current when the junction temperature increases in order to protect the device
when it operates in high power or high ambient temperature environments.
The charge voltage is fixed at 4.2 V, and the charge current limitation can be programmed
using a single resistor connected between the PROG pin and GND. The charge cycle
finishes when the current flowing to the battery is 1/10 of the programmed value. If the
external adaptor is removed, the STC4054 turns off and just 2 µA can flow from the battery
to the device. The device can be put into Shutdown mode, reducing the supply current to 25
µA. The device is delivered in a TSOT23-5L ThinSOT package.
Figure 2. Block diagram
< 2.9 V, I
BAT
is only 0.8 A/10 = 80 mA.
BAT
is 2.9 V with maximum charge current.
BAT
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STC4054 description AN2370
Figure 3. Top component demo board
Figure 4. Top layer layout Figure 5. Bottom layer layout
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AN2370 STC4054 description
Figure 6. Complete charge cycle (750 mA/h battery)
Figure 7. Application circuit
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