USB compatible battery charger system
with integrated power switch for Li-Ion/Li-Polymer
Features
■ Fully integrated solution, with power MOSFET,
reverse blocking diode, sense resistor, and
thermal protection
■ Charges single-cell Li-Ion batteries from
selectable AC adapter or USB input
■ Programmable charge current up to 1 A in AC
adapter mode
■ Programmable charging current in USB mode
for both high power and low power inputs
■ 4.2 V output voltage with ± 1 % accuracy
■ Linear or quasi-pulse operating mode
■ Closed loop thermal control
■ Programmable end-of-charge current
■ Programmable charge timer
■ (NTC) or (PTC) thermistor interface for battery
temperature monitoring and protection
■ Status outputs to drive LEDs or to interface
with a host processor
■ Small VFQFPN 16-leads package (3 x 3 mm)
L6924U
VFQFPN16
Applications
■ PDAs, GPS and MP3 players
■ USB powered devices
■ Cellular phones
■ Digital still cameras
■ Standalone chargers
■ Wireless appliances
Table 1. Device summary
Order codes Package Packaging
L6924U013
VFQFPN16
L6924U013TR Tape and reel
September 2010 Doc ID 14716 Rev 2 1/37
Tube
www.st.com
37
Contents L6924U
Contents
1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6 Operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1 Linear mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2 Quasi-pulse mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7 Applications information: charging process . . . . . . . . . . . . . . . . . . . . 16
7.1 Pre-charge phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2 AC or USB mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3 Fast charge phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.4 End-of-charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.5 Recharge flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.6 Recharge threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.7 Maximum charging time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8 Application information: monitoring and protection . . . . . . . . . . . . . . 22
8.1 NTC thermistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2 Battery absence detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.3 Status pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.4 Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9 Additional applications information . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1 Selecting the input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.2 Selecting the output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2/37 Doc ID 14716 Rev 2
L6924U Contents
9.3 Layout guidelines and demonstration board . . . . . . . . . . . . . . . . . . . . . . . 28
10 Application idea: dual input management with AC priority . . . . . . . . 31
11 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Doc ID 14716 Rev 2 3/37
Description L6924U
1 Description
The L6924U is a fully monolithic battery charger that safely charges single-cell LiIon/Polymer battery from either an USB power source or an AC adapter. In USB mode, the
L6924U supports both low power and high power mode. Alternatively the device can charge
from an AC wall adapter. The ideal solution for space-limited portable products integrates
the power MOSFET, reverse blocking diode, sense resistor and thermal protection into a
compact VFQFPN16 package. When an external voltage regulated adapter or USB port is
used, the L6924U works in linear mode, and charges the battery in a constant current
constant voltage (CC/CV) profile. Moreover, when a current-limited adapter is used, the
device can operate in quasi-pulse mode, dramatically reducing the power dissipation.
Regardless of the charging approach, a closed loop thermal control avoids device
overheating. The device has an operating input voltage ranging from 2.5 V to 12 V and it
allows the user to program many parameters, such as fast-charge current, end-of-charge
current threshold, and charge timer. The L6924U offers two open collector outputs for
diagnostic purposes, which can be used to either drive two external LEDs or communicate
with a host microcontroller. Finally, the L6924U also provides other features like gas gauge
function, check for battery presence, and monitors and protects the battery from unsafe
thermal conditions.
Figure 1. Minimum size application board
Figure 2. Basic application schematic
4/37 Doc ID 14716 Rev 2
L6924U Pin description
2 Pin description
Figure 3. Pin connection (top view)
2.1 Pin description
Table 2. Pin functions
Pin I/O Name Pin description
1IV
2IV
3 - 4 O ST
5IT
6 - GND Ground pin.
7ISD
8ITH
Input pin of the power stage.
IN
Supply voltage pin of the signal circuitry.
INSNS
2
PRG
The operating input voltage ranges from 2.5 V to 12 V, and the
start-up threshold is 4 V.
-ST1Open-collector status pins.
Maximum charging time program pin.
It must be connected with a capacitor to GND to fix the maximum
charging time, see Chapter 7.7: Maximum charging time on
page 21.
Shutdown pin.
When connected to GND enables the device; when floating
disables the device.
Temperature monitor pin.
It must be connected to a resistor divider including an NTC or PTC
resistor. The charge process is disabled if the battery temperature
(sensed through the NTC or PTC) is out of the programmable
temperature window see Chapter 8.1: NTC thermistor on page 23.
Doc ID 14716 Rev 2 5/37
Pin description L6924U
Table 2. Pin functions (continued)
Pin I/O Name Pin description
9 I ISEL
10 I V
11 O V
12 O V
13 I/O I
OSNS
OUT
REF
END
14 I MODE
15 I I
16 I I
USB
AC
Switches between high power USB (I
power USB (I
) in USB mode. A low level sets the L6924U in
USB/5
up to 500 mA) and low
USB
low power mode and a high level sets the L6924U in high power
mode. When the AC mode is selected, the ISEL pin must be
connected to ground or left floating.
Output voltage sense pin.
It senses the battery voltage to control the voltage regulation loop.
Output pin. (connected to the battery)
External reference voltage pin. (reference voltage is 1.8 V ± 2 %)
Charge termination pin.
A resistor connected from this pin to GND sets the charge
termination current threshold I
ENDTH
: if I
CHG
< I
ENDTH
, the charge
process ends. The voltage across the resistor is proportional to the
current delivered to the battery (gas gauge function).
Selects pin AC adapter or USB port input modes. A high level sets
the L6924U in USB mode while a low level sets the L6924U in the
AC adapter mode. When the AC adapter input is selected, the ISEL
pin status does not affect the current set.
Charge current program pin in USB mode: a resistor connected
from this pin to ground sets the fast charge current value (I
USB
up to
500 mA) with an accuracy of 7 %. The USB high power/low power
mode is selected with the ISEL pin.
Charge current program pin in AC mode: a resistor connected from
this pin to GND sets the fast charge current value (IAC up to 1 A)
with an accuracy of 7 %.
6/37 Doc ID 14716 Rev 2
L6924U Maximum ratings
3 Maximum ratings
Stressing the device above the rating listed in the “absolute maximum ratings” table may
cause permanent damage to the device. These are stress ratings only and operation of the
device at these or any other conditions above those indicated in the operating sections of
this specification is not implied. Exposure to absolute maximum rating conditions for
extended periods may affect device reliability. Refer also to the STMicroelectronics sure
program and other relevant quality documents.
Table 3. Absolute maximum ratings
Symbol Parameter Value Unit
V
V
V
IN
, SD Input voltage -0.3 to V
INSNS
, V
OUT
OSNS
Input voltage -0.3 to 16 V
IN
Output voltage -0.3 to 5 V
ISEL, MODE Input voltage -0.3 to 6 V
ST1, ST2 Output voltage -0.3 to V
IN
Output current 30 mA
V
REF
IAC, I
, TH, I
USB, TPRG
END
,
,
-0.3 to 4 V
GND
Maximum withstanding voltage range test condition:
All pins
CDFAEC-Q100-002- “human body model”
± 2 kV
acceptance criteria: “normal performance’
Table 4. Thermal data
Symbol Parameter Value Unit
R
thJA
T
STG
T
J
P
TOT
1. Device mounted on demonstration board
Thermal resistance junction to ambient
Storage temperature range - 55 to 150 °C
Junction temperature range - 40 to 125 °C
Power dissipation at T = 70 °C 0.67 W
(1)
75 °C/W
V
V
Doc ID 14716 Rev 2 7/37
Electrical characteristics L6924U
4 Electrical characteristics
TJ = 25 °C, V
Table 5. Electrical characteristics
= 5 V, unless otherwise specified.
IN
Symbol Parameter Test condition Min. Typ. Max. Unit
Operating input voltage 2.5 12 V
(1)
VIN
Start up threshold 4.1 V
IIN
I
SINK
V
OUT
I
AC
I
USB
I
PRE_AC
I
PRE_USB
V
PRETH
I
ENDTH
T
MAXCH
T
MAXCH
(1)
Supply current
Charging mode (R
Shutdown mode (R
Shutdown mode (R
Current flowing from V
OUT
Stand by mode (R
(VIN = 2.5 V < V
(1)
Battery regulated voltage 4.16 4.2 4.24 V
Charge current with AC
adapter input
MODE at GND, R
MODE at GND, R
MODE at HIGH, ISEL at HIGH,
Charge current with USB
input
Pre-charge current with AC
input
Pre-charge current with USB
input (high power mode)
Pre-charge current with USB
input (low power mode)
R
PRG-USB
MODE at HIGH, ISEL at LOW,
R
PRG-USB
MODE at GND,
R
AC
MODE at HIGH, ISEL at HIGH
R
USB
MODE at HIGH, ISEL at LOW
R
USB
= 24 kΩ
= 2 4 kΩ
= 24 kΩ
= 24 kΩ
= 24 kΩ
Pre-charge voltage threshold 2.9 3.0 3.1 V
Termination current R
(2)
Maximum charging time
Maximum charging time
(2)
accuracy
= 3.3 kΩ 12 16 20 mA
END
C
= 10 nF
TPRG
R[I
] = 24 kΩ
PRG
C
= 5.6n F
TPRG
= 24 kΩ
R
PRG
= 24 kΩ)1.82.5mA
PRG
= 24 kΩ)6080µA
PRG
= 24 kΩ)500nA
PRG
= 24 kΩ)
PRG
BATTERY
)
= 24 kΩ 450 490 525 mA
PRG
= 12 kΩ 905 975 1045 mA
PRG
500 nA
450 490 525
86 96 105
41 49 56 mA
41 49 56 mA
7.6 9.6 11.4 mA
3 hours
10 %
Shutdown threshold high 2 V
SD
TH
Shutdown threshold low 0.4 V
mA
ST1,2 Output status sink current Status on 10 mA
MODE threshold high 1.3 V
MODE
TH
MODE threshold low 0.4 V
ISEL threshold high 1.3 V
ISEL
TH
ISEL threshold low 0.4 V
8/37 Doc ID 14716 Rev 2
L6924U Electrical characteristics
Table 5. Electrical characteristics (continued)
Symbol Parameter Test condition Min. Typ. Max. Unit
R
DS(on)
Power MOSFET resistance
(3)
Charge current = 500 mA 280 380 mΩ
NTC pin hot threshold
voltage
TH
NTC pin cold threshold
voltage
1. TJ from -40 °C to 125 °C
2. Guaranteed by design
3. Device working in quasi pulse mode
10 12.5 15 %V
40 50 60 %V
REF
REF
Doc ID 14716 Rev 2 9/37
Block diagram L6924U
5 Block diagram
Figure 4. Block diagram
VINS
ISEL
MODE
IAC
IUSB
Logic
VIN
Logic
SD
ANALOG
PRE.
Logic
UVLO
Logic
BG
VDD
VBG
BODY
CONTROL
Logic
Charge
Control
POWER MOS
VDD
Mos
Driver
THERMAL
CONTROL
I FAULT I DETECT
Gas Gauge
CA-VA-TA
REG
VREF
VPRE
LOGIC
VREF
OSC
ST2TPRG
ST1
GND
VDD
VDD
NTC/PTC
MANAG.
Logic
VOUT
IEND
VOSNS
VREF
4.2V
TH
10/37 Doc ID 14716 Rev 2
L6924U Operation description
6 Operation description
The L6924U is a fully integrated battery charger that allows a very compact battery
management system for space limited applications. It integrates in a small package all the
power elements: power MOSFET, reverse blocking diode and the sense resistor.
It normally works as a linear charger when powered from an external voltage regulated
adapter or USB port.
However, thanks to its very low minimum input voltage (down to 2.5 V) the L6924U can also
work as a quasi-pulse charger when powered from a current limited adapter. To work in this
condition, it is enough to set the device’s charging current higher than the adapter’s one
(Chapter 6.2: Quasi-pulse mode on page 14). The advantage of the linear charging
approach is that the device has a direct control of the charging current and so the designer
needn’t to rely on power source. However, the advantage of the quasi-pulse approach is that
the power dissipated inside the portable equipment is dramatically reduced.
The L6924U charges the battery in three phases:
● Pre-charge constant current: in this phase (active when the battery is deeply
discharged) the battery is charged with a low current (internally set to 10 % of the fastcharge current).
● Fast-charge constant current: in this phase the device charges the battery with the
maximum current (I
● Constant voltage: when the battery voltage reaches the selected output voltage, the
device starts to reduce the current, until the charge termination is done.
for AC adapter mode, I
AC
for USB mode).
USB
The full flexibility is provided by:
● Programmable fast-charging current (I
● Programmable end of charge current threshold (I
● Programmable end of charge timer (T
or I
AC
MAXCH
) (Chapter 7.3 on page 18).
USB
) (Chapter 7.4 on page 20).
ENDTH
) (Chapter 7.7 on page 21).
If a PTC or NTC resistor is used, the device can monitor the battery temperature in order to
protect the battery from operating under unsafe thermal conditions.
Beside the good thermal behavior guaranteed by low thermal resistance of the package,
additional safety is provided by the built-in temperature control loop. The IC monitors
continuously its junction temperature. When the temperature reaches approximately 120 °C,
the thermal control loop starts working, and reduces the charging current, in order to keep
the IC junction temperature at 120 °C.
Two open collector outputs are available for diagnostic purpose (status pins ST1 and ST2).
They can be also used to drive external LEDs or to interface with a microcontroller. The
voltage across the resistor connected between I
and GND gives information about the
END
actual charging current (working as a gas gauge), and it can be easily fed into a
microcontroller ADC.
Battery disconnection control is provided thanks to the differentiated sensing and forcing
output pins. A small current is sunk and forced through V
OUT
. If V
doesn’t detect the
OSNS
battery, the IC goes into a standby mode.
Figure 5 on page 12 shows the real charging profile of a Li-Ion battery, with a fast charge
current of 450 mA (R
or R2 = 26 kΩ).
1
Doc ID 14716 Rev 2 11/37
Operation description L6924U
Figure 5. Li-Ion charging profile
0.500
4.500
0.450
0.400
0.350
0.300
Ichg (A)
0.250
0.200
0.150
0.100
0.050
0.000
0 200 400 600 800 1000 1200
6.1 Linear mode
When operating in linear mode, the device works in a way similar to a linear regulator with a
constant current limit protection.
It charges the battery in three phases:
● Pre-charging current ("pre-charge" phase).
● Constant current ("fast-charge" phase).
● Constant voltage ("voltage regulation" phase).
Charging time (sec )
Ichg
Vbatt
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
0.000
Vbatt (V)
V
is the output voltage of the upstream AC-DC adapter that is, in turn, the input voltage
ADP
of the L6924U. If the battery voltage is lower than the default pre-charge voltage (V
the pre-charge phase takes place. The battery is pre-charged with a low current, internally
set to 10 % of the fast charge current.
When the battery voltage goes higher than V
charge current (I
or IAC according to the selection of the MODE pin).
USB
PRETH
Finally, when the battery voltage is close to the regulated output voltage (4.2 V), the voltage
regulation phase takes place and the charging current is reduced. The charging process
ends when the charging current reaches the programmed value (I
charging timer expires.
Figure 6 shows the different phases.
12/37 Doc ID 14716 Rev 2
PRETH
, the battery is charged with the fast
) or when the
ENDTH
),
L6924U Operation description
×−=
Figure 6. Typical charge curves in linear mode
V
OPRGTH
V
I
V
ADP
PRETH
I
CHG
PRETH
Pre-Charge
Phase
Fast-Charge
Phase
Adapter Voltage
Voltage-Regulation
Phase
Battery Voltage
Charge Current
Power dissipation
End
Charge
The worst case in power dissipation occurs when the device starts the fast-charge phase. In
fact, the battery voltage is at its minimum value. In this case, there is the maximum
difference between the adapter voltage and battery voltage, and the charge current is at its
maximum value.
The power dissipated is given by the following equation:
Equation 1
IVVP
)(
CHGBATADPDIS
The higher the adapter voltage is, the higher the power dissipated is. The maximum power
dissipated depends on the thermal impedance of the device mounted on board.
Doc ID 14716 Rev 2 13/37
Operation description L6924U
Δ+=
=
6.2 Quasi-pulse mode
The quasi-pulse mode can be used when the system can rely on the current limit of the
upstream adapter to charge the battery. In this case, the fast charge current must be set
higher than the current limit of the adapter. In this mode, the L6924U charges the battery
with the same three phases as in Linear Mode, but the power dissipation is greatly reduced
as shown in Figure 7.
Figure 7. Typical charge curves in quasi pulse mode
V
OPRGTH
V
I
V
ADP
PRETH
I
CHG
I
LIM
PRETH
Pre-Charge
Phase
Fast-Charge
Phase
Ilim x R dso n
Voltage Regulation
Phase
Adapter Voltage
Battery Voltage
Charge Current
End
Charge
The big difference is due to the fact that the charge current is higher than the current limit of
the adapter. During the fast-charge phase, the output voltage of the adapter drops and goes
down to the battery voltage plus the voltage drop across the power MOSFET of the charger,
as shown in the following equation:
Equation 2
14/37 Doc ID 14716 Rev 2
Power dissipation
VVVV
MOSBATADPIN
L6924U Operation description
Δ
Where ΔV
is given by:
MOS
Equation 3
×=
IRV
LIM)ON(DSMOS
Where,
I
= current limit of the wall adapter, and R
LIM
= resistance of the power MOSFET.
DS(on)
The difference between the programmed charge current and the adapter limit should be
high enough to minimize the R
value (and the power dissipation). This makes the
DS(on)
control loop completely unbalanced and the power element is fully turned on.
Figure 8 shows the R
values for different output voltages and charging currents for an
DS(on)
adapter current limit of 500 mA.
Figure 8. R
curves vs. charging current and output voltage
DS(on)
Neglecting the voltage drop across the charger (ΔV
) when the device operates in this
MOS
condition, its input voltage is equal to the battery one, and so a very low operating input
voltage (down to 2.5 V) is required. The power dissipated by the device during this phase is:
Equation 4
2
IRP ×=
LIM)on(DSCH
When the battery voltage approaches the final value, the charger gets back the control of
the current, reducing it. Due to this, the upstream adapter exits the current limit condition
and its output goes up to the regulated voltage V
. This is the worst case in power
ADP
dissipation:
Doc ID 14716 Rev 2 15/37
Operation description L6924U
Equation 5
I)VV(P ×−=
LIMBATADPDIS
In conclusion, the advantage of the linear charging approach is that the designer has direct
control of the charge current, and consequently the application can be very simple. The
drawback is the high power dissipation.
The advantage of the quasi-pulse charging method is that the power dissipated is
dramatically reduced. The drawback is that a dedicated upstream adapter is required.
16/37 Doc ID 14716 Rev 2
L6924U Applications information: charging process
7 Applications information: charging process
Figure 9. Charging process flow chart
7.1 Pre-charge phase
The L6924U allows pre-charging the battery with a low current when the battery is deeply
discharged.
Doc ID 14716 Rev 2 17/37
Applications information: charging process L6924U
The battery is considered deeply discharged when its voltage is lower than a threshold
(V
During the pre-charge phase, the current (I
), internally set to 3 V.
PRETH
) has a default value equal to 10 % of the
PRECH
fast-charge current.
A safety timer is also present. If the battery voltage does not rise over V
time, a fault is given (
Chapter 7.7: Maximum charging time on page 21).
If at the beginning of the charge process, the battery voltage is higher than the V
pre-charge phase is skipped.
7.2 AC or USB mode
The L6924U can charge batteries from both an AC adapter and USB inputs.
The power supply type can be chosen by driving the MODE pin.
A low level sets the L6924U in AC mode. The fast charge current is determined by the
resistor connected to the I
connected to I
On the other hand, a high level at the MODE pin sets the L6924U in USB mode. The fast
charge current is determined by the resistor connected to the I
charge phase
Figure 10. MODE pin selection
.
USB
), regardless of the resistor connected to IAC.
Sets the fast charge current
Sets the fast charge current
I
I
USB
USB
within this
PRETH
PRETH
pin (Chapter 7.3: Fast charge phase), regardless of the resistor
AC
pin (Chapter 7.3: Fast
USB
Sets the fast charge current
Sets the fast charge current
I
I
USB
USB
, the
L6924U
L6924U
AC adapter mode
AC adapter mode
MODE
MODE
I
I
AC
AC
7.3 Fast charge phase
When the battery voltage reaches the pre-charge voltage threshold (V
enters the fast-charge phase.
In this phase the device charges the battery with a constant current, whose value can be set
by external resistors connected to I
mode) with an accuracy of 7 %.
In AC adapter mode (MODE pin low), the resistor R
L6924U
L6924U
R
R
AC
AC
R
R
USB
USB
V
V
IN
IN
pin (AC adapter mode selected) or to I
AC
can be calculated as:
AC
MODE
MODE
I
I
AC
AC
USB mode
USB mode
R
R
AC
AC
), the L6924U
PRETH
pin (USB
USB
R
R
USB
USB
18/37 Doc ID 14716 Rev 2
L6924U Applications information: charging process
Equation 6
⎞
⎛
V
BG
⎟
AC
⎜
=
⎜
⎝
R ⋅
K
PRG
⎟
I
AC
⎠
Where VBG is the internal reference equal to 1.23 V, whereas K
is a constant equal to
PRG
9500.
Figure 11. IAC pin connection
In USB mode (MODE pin high), the R
resistor can be selected as:
USB
Equation 7
USB
⎜
=
⎜
⎝
Where VBG and K
R ⋅
have the same meaning and value above mentioned.
PRG
The charge current in USB mode depends on R
I
USB
K
PRG
⎟
⎠
as well as the state of the ISEL pin.
USB
⎞
⎛
V
BG
⎟
When this pin is high, the “high-power” USB mode is selected and the charge current is
determined by the equation 7.
The charge current in USB mode should be set in accordance with the typical USB current
capability (up to 500 mA). If ISEL is low, the “low-power” USB mode is selected and the
charge current is a fifth of the high-power USB mode charge current (up to 100 mA)
During low power USB mode operation, since the charge current is reduced by one fifth, the
maximum charging time is proportionally increased (
Figure 12. I
pin connection
USB
Section 7.7: Maximum charging time).
Regardless of the operation mode (AC adapter or USB), during the fast-charge phase the
battery voltage increases until it reaches the programmed output voltage (4.2 V). A safety
timer is also present. If the Fast-charge phase does not finish within the programmed time
(see
Chapter 7.7: Maximum charging time on page 21), a fault is given.
Doc ID 14716 Rev 2 19/37
Applications information: charging process L6924U
E
7.4 End-of-charge current
When the charge voltage approaches the battery regulated voltage (internally set to 4.2 V),
the voltage regulation phase takes place. The charge current starts to decrease until it goes
below a programmable termination current, I
external resistor connected between the I
END
calculated as:
Equation 8
. This current can be selected by an
ENDTH
pin and GND Figure 13, whose value can be
VR
Figure 13. I
Where K
END
When the charge current goes below I
end of charge and the charge process ends.
This de-glitch time is expressed as:
Equation 9
pin connection
END
is 1050 and V
is 50 mV.
MIN
ENDTH
T =
DEGLITCH
END
K
ENDTH
⎞
⎟
⎟
⎠
⎛
⎜
MINEND
×=
⎜
I
⎝
, after a deglitch time, the status pins notify the
T
MAXCH
220
where TMAXCH is the maximum charging time. (Chapter 7.7 on page 21)
I
pin is also used to monitor the charge current, because the current injected in R
END
proportional to the charge current. The voltage across R
microcontroller to check the charge status like a gas gauge.
20/37 Doc ID 14716 Rev 2
can be used by a
END
END
is
L6924U Applications information: charging process
E
7.5 Recharge flow chart
Figure 14. Recharge flow chart
7.6 Recharge threshold
When, from an end-of-charge condition, the battery voltage goes below the recharging
threshold (V
), the device goes back in charging state. The value of the recharge
RCH
threshold is 4.05 V.
7.7 Maximum charging time
To avoid the charging of a dead battery for a long time, the L6924U has the possibility to set
a maximum charging time starting from the beginning of the fast-charge phase. This timer
can be set through a capacitor, connected between the T
external capacitor (in nF) and is given by the following equation:
Equation 10
C
TPRG
Note: The maximum recommended C
TPRG
pin and GND. C
PRG
⎛
T
MAXCH
⎜
⎜
=
⎜
⎜
⎝
×
K
T
V
REF
⎞
V
BG
⎟
R
PRG
⎟
9
10×
⎟
⎟
⎠
value must be less than 50 nF.
TPRG
is the
Doc ID 14716 Rev 2 21/37
Applications information: charging process L6924U
Figure 15. T
pin connection
PRG
Where,
R
= resistor which sets the current (R
PRG
V
= 1.8 V,
REF
K
= 279 x 105,
T
V
= 1.23 V, and
BG
T
is the charging time given in seconds.
MAXCH
USB
or RAC)
If the battery does not reach the end-of-charge condition before the timer expires, a fault is
issued.
Also during the pre-charge phase there is a safety timer, given by:
Equation 11
1
TT ×=
MAXCHMAXPRECH
8
If this timer expires and the battery voltage is still lower than V
, a fault signal is
PRETH
generated, and the charge process finishes.
Note: When the device is charged in low power USB mode, in order to take into account the
reduced charge current, the maximum charging time is proportionally increased (five times
the maximum charging time calculated with R
USB
).
22/37 Doc ID 14716 Rev 2
L6924U Application information: monitoring and protection
8 Application information: monitoring and protection
The L6924U uses a VFQFPN (3 x 3 mm) 16-pin package with an exposed pad that allows
the user to have a compact application and good thermal behavior at the same time. The
L6924U has a low thermal resistance because of the exposed pad (approximately
75 °C/W, depending on the board characteristics). Moreover, a built-in thermal protection
feature prevents the L6924U from having thermal issues typically present in a linear charger.
Thermal control is implemented with a thermal loop that reduces the charge current
automatically when the junction temperature reaches approximately 120 °C. This avoids
further temperature rise and keeps the junction temperature constant. This simplifies the
thermal design of the application as well as protects the device against over-temperature
damage.
Figure 16 shows how the thermal loop acts (dotted lines), when the junction temperature
reaches 120 °C.
Figure 16. Power dissipation in both linear and quasi pulse modes with thermal loop
8.1 NTC thermistor
The device allows designers to monitor the battery temperature by measuring the voltage
across an NTC or PTC resistor. Li-Ion batteries have a narrow range of operating
temperature, usually from 0 °C to 50 °C. This window is programmable by an external
divider which is comprised of an NTC thermistor connected to GND and a resistor
connected to V
voltage threshold (internal window comparator), the device stops the charge process, and
indicates a fault condition through the status pin.
. When the voltage on the TH pin exceeds the minimum or maximum
REF
Doc ID 14716 Rev 2 23/37
Application information: monitoring and protection L6924U
When the voltage (and thus, the temperature), returns to the window range, the device restarts the charging process. Moreover, there is a hysteresis for both the upper and lower
thresholds, as shown in
Figure 17. Battery temperature control flow chart
Figure 18.
Note: T
= OK when the battery temperature is between 0 °C and 50 °C
BAT
Figure 18. Voltage window with hysteresis on TH
V
MI N T H
V
MINTH_ HYS
V
MAXTH_HYS
V
MAXTH
Figure 19. Pin connection
900mV
780mV
248mV
Vol t ag e
Vari atio n on TH pi n
225mV
Charge disable
Charge enable
When the TH pin voltage rises and exceeds the V
L6924U stops the charge, and indicates a fault by the status pins. The device re-starts to
24/37 Doc ID 14716 Rev 2
MINTH
= 50 % of V
(900 mV typ), the
REF
L6924U Application information: monitoring and protection
=
charge the battery, only when the voltage at the TH pin goes under V
MINTH_HYS
= 780 mV
(typ).
For what concerns the high temperature limit, when the TH pin voltage falls under the
V
MAXTH
voltage reaches the V
= 12.5 % of V
(225 mV Typ.), the L6924U stops the charge until the TH pin
REF
MAXTH_HYS
= 248 mV (typ.).
When the battery is at the low temperature limit, the TH pin voltage is 900 mV. The correct
resistance ratio to set the low temperature limit at 0 °C can be found with the following
equation:
Equation 12
R
CNTC
°
VV
×=
REFMINTH
Where RUP is the pull-up resistor, V
is equal to 1.8 V, and R
REF
NTC at 0 °C. Since at the low temperature limit V
+
MINTH
0
RR
CNTCUP
°
0
= 900 mV:
is the value of the
NTC0°C
Equation 13
R
CNTC
°
8.19.0
×=
0
RR
+
CNTCUP
°
0
It follows that:
Equation 14
RR
UPCNTC
°0
Similarly, when the battery is at the high temperature limit, the TH pin voltage is 225 mV. The
correct resistance ratio to set the high temperature limit at 50 °C can be found with the
following equation:
Equation 15
Where R
NTC50°C
VV
REFMAXTH
R
×=
is the value of the NTC at 50 °C. Considering V
CNTC
°
50
RR
+
CNTCUP
°
50
= 225 mV it follows
MAXTH
that:
Equation 16
8.1225.0
R
×=
CNTC
°
50
RR
+
CNTCUP
°
50
Consequently:
Equation 17
R =
Doc ID 14716 Rev 2 25/37
R
50UPCNTC
°
7
Application information: monitoring and protection L6924U
−
−
−
−
−
Based on Equation 14 and Equation 17, it derives that:
Equation 18
R
0
CNTC
7
R
=
50
°°CNTC
The temperature hysteresis can be estimated by the equation:
Equation 19
VV
HYSTHTH
=
T
HYS
_
NTCV
×
TTH
Where VTH is the pin voltage threshold on the rising edge, V
threshold on the falling edge, and NTC
(- %/°C) is the negative temperature coefficient of
T
TH_HYS
the NTC at temperature (T) expressed in % resistance change per °C. For NTC
is the pin voltage
values, see
T
the characteristics of the NTC manufacturers (e.g. the 2322615 series by VISHAY). At low
temperature, the hysteresis is approximately:
Equation 20
mVmV
T
=
CHYS
°
0
780900
CNTCmV
°×
0900
Obviously at high temperature hysteresis is:
Equation 21
mVmV
T
Considering typical values for NTC
=
CHYS
°
50
and NTC
0°C
248225
CNTCmV
°×
50225
, the hysteresis is:
50°C
Equation 22
mVmV
T
=
CHYS
0
°
mV
780900
051.0900
×
o
C
5.2
≅
And:
Equation 23
mVmV
T
=
CHYS
50
°
mV
248225
039.0225
×
o
C
5.2
−≅
If a PTC connected to GND is used, the selection is the same as above, the only difference
is when the battery temperature increases, the voltage on the TH pin increases, and vice
versa. For applications that do not need a monitor of the battery temperature, the NTC can
be replaced with a simple resistor whose value is one half of the pull-up resistor R
UP
.
In this case, the voltage at the TH pin is always inside the voltage window, and the charge is
always enabled.
26/37 Doc ID 14716 Rev 2
L6924U Application information: monitoring and protection
8.2 Battery absence detection
This feature provides a battery absent detection scheme to detect the removal or the
insertion of the battery. If the battery is removed, the charge current falls below the I
At the end of the de-glitch time, a detection current I
output for a time of T
V
, a current equal to I
PRETH
DETECT
. The device checks the voltage at the output. If it is below the
DETECT
is injected in the output capacitor for a T
DETECT
checked to see if the voltage on the output goes higher than V
voltage changes from V
battery is connected to the charger. The T
PRETH
to V
and vice versa in a T
RCH
DETECT
is expressed by:
, equal to 1 mA, is sunk from the
(4.05 V). If the battery
RCH
DETECT
time, it means that no
DETECT
Equation 24
T
T
DETECT
Figure 20. Battery absence detection flow chart
MAXCH
=
3
1054×
ENDTH
, and it is
.
Detect Low Absent
Detect High Absent
YES
BATTERY
ABSENT
V
BAT
>
V
PRETH
NO
V
BAT
>
V
RCH
YES
NO
DETECT LOW ABSENT = a I
DETECT HIGH ABSENT = a I
= 100ms (Typ.)
T
DET
= I
I
SINK
= 1mA (Typ.)
INJ
FAST CHARGE
PRE CHARGE
is sunk for a T
SINK
is injected for a T
INJ
from the battery
DET
in the battery
DET
8.3 Status pins
To indicate various charger status conditions, there are two open-collector output pins, ST1
and ST2. These status pins can be used either to drive status LEDs, connected with an
external power source, by a resistor, or to communicate to a host processor.
Doc ID 14716 Rev 2 27/37
Application information: monitoring and protection L6924U
Figure 21. ST1 and ST2 connection with LEDs or microcontroller
Table 6. Status LEDs Indications
Charge condition Description ST1 ST2
Charge in progress
Charge done
Stand by mode
Bad battery temperature
Battery absent When the battery pack is removed ON ON
Over time When T
8.4 Shutdown
The L6924U has a shutdown pin; when the pin is connected to GND, the device is operating.
When the pin is left floating, the device enters the shutdown mode, the consumption from
the input is dramatically reduced to 60 µA (typ.). In this condition, V
When the device is in pre-charge or fastcharge status
When the charging current goes below the
I
ENDTH
When the input voltage goes under
V
+ 50 mV
BAT
When the voltage on the TH pin is out of
the programmable window, in accordance
with the NTC or PTC thermistor
MAXCH
or T
MAXPRECH
expires ON ON
ON OFF
OFF ON
OFF OFF
ON ON
is turned OFF.
REF
28/37 Doc ID 14716 Rev 2
L6924U Additional applications information
+×=
9 Additional applications information
9.1 Selecting the input capacitor
In most applications, a 1 µF ceramic capacitor, placed close to the VIN and V
be used to filter the high frequency noise.
9.2 Selecting the output capacitor
Typically, a 4.7 µF ceramic capacitor placed close to the V
keep
voltage control loop stable. This ensures proper operation of battery absent detection
in removable battery pack applications.
OUT
and V
9.3 Layout guidelines and demonstration board
The thermal loop keeps the device at a constant temperature of approximately 120 °C which
in turn, reduces I
ensure a good thermal path. Therefore, the exposed pad must be properly soldered to the
board and connected to the other layer through thermal vias. The recommended copper
thickness of the layers is 70 μm or more.
The exposed pad must be electrically connected to GND.
image of the board with the power dissipation of 1 W. In this instance, the temperature of the
case is 89 °C, but the junction temperature of the device is given by the following equation:
Equation 25
. However, in order to maximize the current capability, it is important to
CHG
Figure 22 shows the thermal
TPRT
−
AMBDISSATHJJ
INSN
pin is enough to
OUTSN
pins can
Where the R
and the ambient temperature is 25 °C.
In this case the junction temperature is:
Equation 26
of the device mounted on board is 75 °C/W, the power dissipated is 1 W,
thJA
J
Doc ID 14716 Rev 2 29/37
o
CT
10025175 =+×=
Additional applications information L6924U
Figure 22. Thermal image of the demonstration board
The V
pin can be used as a remote sense; it should be therefore connected as closely
OSNS
as possible to the battery. The demonstration board layout and schematic are shown in
Figure 23, Figure 24 and Figure 25.
Figure 23. Demonstration board layout, top side
Figure 24. Demonstration board layout, bottom side
30/37 Doc ID 14716 Rev 2
L6924U Additional applications information
Figure 25. Demonstration board schematic
Table 7. Demonstration board components description
Name Value Description
R1 24 kΩ AC mode fast-charge current resistor. Used to set the charging current in AC mode.
R2 24 kΩ
USB mode fast-charge current resistor. Used to set the charging current in USB
mode.
R3 3.3 kΩ
End of Charge current resistor. Used to set the termination current and, as a “gas
gauge” when measuring the voltage across on it.
R4 1 kΩ Pull up resistor. Connected between VREF and TH pin.
R5 1 kΩ Pull up resistor. To be used when the ST1 is connected to a LED
R6 1 kΩ Pull up resistor. To be used when the ST2 is connected to a LED.
RT1 470 Ω
If a NTC is not used, a half value of R4 must be mounted to keep the TH voltage in
the correct window.
C1 1 µF Input capacitor.
C2 4.7 µF Output capacitor.
C3 10 nF T
C4 1 nF V
capacitor. Used to set the maximum charging time.
MAX
filter capacitor.
REF
D1 GREEN ST1 LED.
D2 RED ST2 LED.
J1 ST2 jumper. Using to select the LED or the external microcontroller.
J2 ST1 jumper. Using to select the LED or the external microcontroller.
J3
SD jumper. If open, the device is in shutdown mode; when closed, the device starts
to work.
J4 Low power/ high power USB mode selection jumper.
J5 AC/USB mode selection jumper.
Doc ID 14716 Rev 2 31/37
Application idea: dual input management with AC priority L6924U
−
=
(
)
⋅+−
=
10 Application idea: dual input management with AC
priority
In some applications both AC adapter and USB power source may be available.
Figure 26 shows a possible schematic which provides the possibility to manage two power
sources (AC/USB) and gives the priority to AC adapter in case both sources are available at
the same time.
For simplicity, only the relevant pins of the L6924U for this application have been indicated.
If only the AC adapter is available, since the gates of Q1 and Q2 are connected to AC, both
MOSFETs are off. The AC adapter voltage is provided to the V
The voltage at the V
pin is:
IN
VVV
diodeACIN
pin through the diode D1.
IN
A correct choice of this diode is important to limit V
and keeping VIN as close as
diode
possible to AC.
In this condition the MODE pin is low. This sets the L6924U in AC mode and the battery is
charged with the current programmed by R
When only the USB power source is available, both Q1 and Q2 switch on and the pin V
AC
.
is
IN
connected to USB.
The MODE pin is connected to the drains of Q1 and Q2 and is high. Therefore the USB
mode for the L6924U is selected and the battery is charged with a current in accordance
with the resistor connected to the pin I
The voltage of the V
pin is given by:
IN
USB
(R
USB
).
IRRVV
USB2Q_DSon1Q_DSonUSBIN
The voltage drop across the MOSFETs must be kept as low as possible to avoid reducing
too much the voltage of the V
IN
pin.
When both sources are present, this circuit gives the priority to the AC adapter. In fact, for
V
≥ 5 V, surely both Q1 and Q2 are off and VIN is connected to the AC adapter through
AC
D1. The MODE pin is kept low and L6924U is set to AC mode.
The use of two P-channel MOSFETs connected as shown in
Figure 26 is particularly useful
in this case because they remove any path between the two power sources.
32/37 Doc ID 14716 Rev 2
L6924U Application idea: dual input management with AC priority
Figure 26. Dual input management
V
AC
AC
USB
USB
D1
D1
Q1 Q2
Q1 Q2
R
R
G
G
V
V
IN
IN
L6924U
L6924U
MODE
MODE
R
R
M
M
V
I
I
I
I
OUT
OUT
USB
USB
AC
AC
RACR
RACR
Li-Ion
Li-Ion
battery
battery
USB
USB
Doc ID 14716 Rev 2 33/37
Package mechanical data L6924U
11 Package mechanical data
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK
specifications, grade definitions and product status are available at:
ECOPACK
®
packages, depending on their level of environmental compliance. ECOPACK®
®
is an ST trademark.
www.st.com.
34/37 Doc ID 14716 Rev 2
L6924U Package mechanical data
Table 8. VFQFPN16 (3 x 3 mm.) mechanical data
mm.
Dim.
Min. Typ. Max.
A 0.80 0.90 1.00
A1 0.02 0.05
A2 0.65 1.00
A3 0.20
b 0.18 0.25 0.30
D 2.85 3.00 3.15
D2 1.45 1.60 1.75
E 2.85 3.00 3.15
E2 1.45 1.60 1.75
e 0.45 0.50 0.55
L 0.30 0.40 0.50
Figure 27. Package dimensions
7185330_G
Doc ID 14716 Rev 2 35/37
Revision history L6924U
12 Revision history
Table 9. Document revision history
Date Revision Changes
20-May-2008 1 First release
22-Sep-2010 2 Modified: Ta bl e 8 and Figure 27 on page 35. Minor changes.
36/37 Doc ID 14716 Rev 2
L6924U
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Doc ID 14716 Rev 2 37/37
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