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
2/14
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
3/14
STC4054 description AN2370
Figure 3. Top component demo board
Figure 4. Top layer layout Figure 5. Bottom layer layout
4/14
AN2370 STC4054 description
Figure 6. Complete charge cycle (750 mA/h battery)
Figure 7. Application circuit
5/14
Stability considerations AN2370
2 Stability considerations
The STC4054 contains two control loops: constant voltage and constant current. The
constant-voltage loop is stable without any compensation when a battery is connected with
low impedance leads. Excessive lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1 µF from BAT to GND. Furthermore, a 4.7 µF
capacitor with a 0.2 Ω to 1 Ω series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
In constant-current mode, the PROG pin is in the feedback loop, instead of the battery.
Because of the additional pole created by PROG pin capacitance, capacitance on this pin
must be kept to a minimum. With no additional capacitance on the PROG pin, the charger is
stable with program resistor values as high as 12 kΩ. However, additional capacitance on
this node reduces the maximum allowed program resistor. Therefore, if the PROG pin is
loaded with a capacitance, C
maximum resistance value for R
Equation 1
, the following equation should be used to calculate the
PROG
PROG
R
PROG
:
---------------------------------------------------------
≤
2 π 510•
1
5
••
C
PROG
Average, rather than instantaneous, battery current may be of interest to the user. For
example, if a switching power supply operating in low current mode is connected in parallel
with the battery, the average current being pulled out of the BAT pin is typically of more
interest than the instantaneous current pulses. In such a case, a simple RC filter can be
used on the PROG pin to measure the average battery current as shown in Figure 6 A 20 kΩ
resistor has been added between the PROG pin and the filter capacitor to ensure stability
(C1 = 100 nF).
Figure 8. Isolating capacitive load on prog pin and filtering
6/14
AN2370 Board layout considerations
3 Board layout considerations
Because of the small size of the ThinSOT package, it is very important to use a good
thermal PC board layout to maximize the available charge current. The thermal path for the
heat generated by the IC is from the die to the copper lead frame, through the package
leads, (especially the ground lead) to the PC board copper. The PC board copper is the heat
sink. The footprint copper pads should be as wide as possible and expand out to larger
copper areas to spread and dissipate the heat to the surrounding ambient. Feed through
vias to inner or backside copper layers are also useful in improving the overall thermal
performance of the charger. Other heat sources on the board, not related to the charger,
must also be considered when designing a PC board layout because they will affect overall
temperature rise and the maximum charge current.
Tab le 1 lists thermal resistance for several different board sizes and copper areas.
Figure 3 shows the complete assembly board.
Table 1. Measured thermal resistance (2-layer board)
Copper area
Top side Dow nside
2500 mm
1000 mm
225 mm
100 mm
50 mm
2
2
2
2
2
2500 mm
2500 mm
2500 mm
2500 mm
2500 mm
Board area
2
2
2
2
2
2500 mm
2500 mm
2500 mm
2500 mm
2500 mm
2
2
2
2
2
Junction-to-ambient
Thermal resistance
125 °C/W
125 °C/W
130 °C/W
135 °C/W
150 °C/W
7/14
External components AN2370
4 External components
This application requires few external components: two ceramic capacitors (CIN = 1 µF,
C
= 4.7 µF), and one resistor (R
OUT
For input and output capacitors, the use of ceramic capacitors with low ESR is
recommended. For good stability of devices supplied with a low input voltage of 4.25 V at
maximum output ratings, the use of 1 µF/6.3 V input capacitor (minimum value) and
4.7 µF/6.3 V output capacitor (minimum value) is recommended.
Table 2. Bill of materials
Symbol Parameter Type Qty Supplier Value Unit
PROG
).
C
IN
C
BAT
Rusb Usb current set Thick film type 1% CRG0603J2K0 1 THCO 2 KΩ
Rdc DC current set Thick film type 1% CRG0603J10K0 1 THCO 10 KΩ
N-Pmos Nmos-Pmos IC STS7C4F30 1 ST
D1 SCHOTTKY STPS1L40M 1 ST
Limit LED current Thick film type 1% CRG0603J1K0 1 THCO 1 KΩ
R
LED
Rpull Pull down resistor Thick film type 1% CRG0603J1K0 1 THCO 1 KΩ
Rf Resistance filter Thick film type 1% CRG0603J20K0 1 THCO 20 KΩ
Cf Capacitor filter Ceramic Low ESR GRM188r71e104ka01 1 Murata 100 nF
LED charge LED 1.8 mm-GREEN LED L-2060GD 1 KINGBRIGHT
USB In Connector USB Mini B 54819-0572 1 Molex
Input cap. Ceramic low ESR GRM155F50J105ZE01 1 Murata 1 µF
Output cap. Ceramic low ESR GRM188r60J475ke19 1 Murata 4.7 µF
8/14
AN2370 Power dissipation
5 Power dissipation
The conditions that cause the STC4054 to reduce charge current through thermal feedback
can be approximated by considering the power dissipated in the IC. For high charge
currents, the STC4054 power dissipation is approximately:
Equation 2
P
VCCV
D
–()I
BAT
•=
BAT
where P
voltage and I
is the power dissipated, VCC is the input supply voltage, V
D
is the current charge current. It is not necessary to perform any worst-case
BAT
is the battery
BAT
power dissipation scenarios because the STC4054 will automatically reduce the charge
current to maintain the die temperature at approximately 120°C.
However, the approximate ambient temperature at which the thermal feedback begins to
protect the IC is:
Equation 3
TA120° CPDθ
–=
JA
Equation 4
120° CVCCV
T
A
–=
–()I
•θ
BAT
BAT
JA
Example: Consider an STC4054 operating from a 5 V wall adapter providing 400 mA to a
3.7 V Li-Ion battery. The ambient temperature above which the STC4054 will begin to
reduce the 400 mA charge current is approximately:
Equation 5
C
T
120° C5V3.7V–()400mA()• 150°
A
-----
• 42° C=–=
W
The STC4054 can be used above 42°C, but the charge current will be reduced from
400 mA. The approximate current at a given ambient temperature can be calculated:
Equation 6
120° CT
–
I
BAT
------------------------------------------------ -=
V
–()θ
CCVBAT
A
•
A
Using the previous example with an ambient temperature of 65°C, the charge current will be
reduced to approximately:
Equation 7
120° C65° C–
I
-------------------------------------------------------- -
BAT
5V 3.7V–()150°
282m A==
C
-----
•
W
Furthermore, the voltage at the PROG pin will change proportionally with the charge current
as discussed in the Programming Charge Current section. It is important to remember that
STC4054 applications do not need to be designed for worst-case thermal conditions since
the IC will automatically reduce power dissipation when the junction temperature reaches
approximately 120 °C.
9/14
Automatic recharge AN2370
6 Automatic recharge
Once the charge cycle is terminated, the STC4054 continuously monitors the voltage on the
BAT pin using a comparator with a 2 ms filter time (t
RECHARGE
when the battery voltage falls below 4.05 V (which corresponds to approximately 80% to
90% battery capacity).
This ensures that the battery is kept at or near a fully charged condition and eliminates the
need for periodic charge cycle initiations. CHRG output enters a strong pulldown state
during recharge cycles.
Figure 9. State diagram of a typical charge cycle
). A charge cycle restarts
POWER O N
RECONNECTED
PROG
OR
UVLO
CONDITION
STOPS
SHUTDOWN MODE
CHRG: Hi-Z IN UVLO
WEAK PULL-DOWN
OTHERWISE
ICC DROPS TO <25µA
PROG FLOATED
OR
UVLO
CON DITION
Bat <2.9V
TRICKLE CHARGE
MODE
1/10TH FULL
CURRENT
CHRG: STRONG
PULL-DOWN
Bat >2.9V
CHARGE MODE
FULL CURRENT
CHRG: STRONG
PULL-DOWN
STANDBY MODE
NO CHARGE
CURRENT
CHRG: WEAK
PULL-DOWN
Bat >2.9V
Prog<100mV
2.9V<Vbat<4.05V
10/14
AN2370 CHRG status output pin
7 CHRG status output pin
The CHRG pin can provide an indication that the input voltage is greater than the
undervoltage lockout threshold level. A weak pull-down current of approximately 20 µA
indicates that sufficient voltage is applied to V
battery is connected to the charger, the constant current portion of the charge cycle begins
and the CHRG pin pulls to ground. The CHRG pin can sink up to 10mA to drive an LED that
indicates that a charge cycle is in progress. When the battery is nearing full charge, the
charger enters the constant-voltage portion of the charge cycle and the charge current
begins to drop. When the charge current drops below 1/10 of the programmed current, the
charge cycle ends and the strong pull-down is replaced by the 20 µA pull-down, indicating
that the charge cycle has ended. If the input voltage is removed or drops below the
undervoltage lockout threshold, the CHRG pin becomes high impedance.
Figure 10 shows that by using two different value pull-up resistors, a microprocessor can
detect all three states from this pin. To detect when the STC4054 is in charge mode, force
the digital output pin (OUT) high and measure the voltage at the CHRG pin. The N-channel
MOSFET will pull the pin voltage low even with the 2 k pull-up resistor. Once the charge
cycle terminates, the N-channel MOSFET is turned off and a 20 µA current source is
connected to the CHRG pin. The IN pin will then be pulled high by the 2 k pull-up resistor. To
determine if there is a weak pull-down current, the OUT pin should be forced to a high
impedance state. The weak current source will pull the IN pin low through the 800 k resistor;
if CHRG is high impedance, the IN pin will be pulled high, indicating that the part is in a
UVLO state.
to begin charging. When a discharged
CC
Figure 10. Using a microprocessor to determine CHRG state
11/14
USB and wall adapter power AN2370
8 USB and wall adapter power
Although the STC4054 allows charging from a USB port, a wall adapter can also be used to
charge Li-Ion batteries.
Figure 11 shows an example of how to combine wall adapter and USB power inputs. A P-
channel MOSFET is used to prevent back conducting into the USB port when a wall adapter
is present and a Schottky diode is used to prevent USB power loss through the 1 k pulldown resistor.
Typically a wall adapter can supply significantly more current than the 500 mA limited USB
port. Therefore, an N-channel MOSFET and an extra program resistor are used to increase
the charge current to 850 mA when the wall adapter is present.
Figure 11. Combining wall adapter and USB power
12/14
AN2370 Programming charge current
9 Programming charge current
The charge current is programmed using a single resistor from the PROG pin to ground.
The battery charge current is 1000 times the current out of the PROG pin. The program
resistor value is calculated using the following equation:
Equation 8
1.00 V
R
PROG
1000
--------------- -
•=
I
BAT
The charge current out of the BAT pin can be determined at any time by monitoring the
PROG pin voltage using the following equation:
Equation 9
V
PROG
BAT
------------------ -
R
PROG
1000•=
I
10 Revision history
Table 3. Document revision history
Date Revision Changes
3-Aug-2006 1 Initial release
07-Sep-2006 2 Minor text changes
19-Sep-2006 3 Table 2 changed
05-Oct-2007 4 Changed Figure 1, Figure 3, Figure 4 and Figure 5
13/14
AN2370
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