September 2010 Doc ID 14716 Rev 2 1/37
37
L6924U
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)
Applications
■PDAs, GPS and MP3 players
■USB powered devices
■Cellular phones
■Digital still cameras
■Standalone chargers
■Wireless appliances
VFQFPN16
Table 1. Device summary
Order codes Package Packaging
L6924U013 VFQFPN16 Tube
L6924U013TR Tape and reel
www.st.com
Contents L6924U
2/37 Doc ID 14716 Rev 2
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
L6924U Contents
Doc ID 14716 Rev 2 3/37
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
Description L6924U
4/37 Doc ID 14716 Rev 2
1 Description
The L6924U is a fully monolithic battery charger that safely charges single-cell Li-
Ion/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 2. Basic application schematic
Figure 1. Minimum size application board
L6924U Pin description
Doc ID 14716 Rev 2 5/37
2 Pin description
Figure 3. Pin connection (top view)
2.1 Pin description
Table 2. Pin functions
Pin I/O Name Pin description
1IV
IN Input pin of the power stage.
2IV
INSNS
Supply voltage pin of the signal circuitry.
The operating input voltage ranges from 2.5 V to 12 V, and the
start-up threshold is 4 V.
3 - 4 O ST2-ST1Open-collector status pins.
5IT
PRG
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.
6 - GND Ground pin.
7ISD
Shutdown pin.
When connected to GND enables the device; when floating
disables the device.
8ITH
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.
Pin description L6924U
6/37 Doc ID 14716 Rev 2
Pin I/O Name Pin description
9 I ISEL
Switches between high power USB (IUSB up to 500 mA) and low
power USB (IUSB/5) in USB mode. A low level sets the L6924U in
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.
10 I VOSNS
Output voltage sense pin.
It senses the battery voltage to control the voltage regulation loop.
11 O VOUT Output pin. (connected to the battery)
12 O VREF External reference voltage pin. (reference voltage is 1.8 V ± 2 %)
13 I/O IEND
Charge termination pin.
A resistor connected from this pin to GND sets the charge
termination current threshold IENDTH: if ICHG < IENDTH, the charge
process ends. The voltage across the resistor is proportional to the
current delivered to the battery (gas gauge function).
14 I MODE
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.
15 I IUSB
Charge current program pin in USB mode: a resistor connected
from this pin to ground sets the fast charge current value (IUSB up to
500 mA) with an accuracy of 7 %. The USB high power/low power
mode is selected with the ISEL pin.
16 I IAC
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 %.
Table 2. Pin functions (continued)
L6924U Maximum ratings
Doc ID 14716 Rev 2 7/37
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
VIN Input voltage -0.3 to 16 V
VINSNS, SD Input voltage -0.3 to VIN V
VOUT
, VOSNS Output voltage -0.3 to 5 V
ISEL, MODE Input voltage -0.3 to 6 V
ST1, ST2 Output voltage -0.3 to VIN V
Output current 30 mA
VREF
, TH, IEND,
IAC, IUSB, TPRG,
GND
-0.3 to 4 V
All pins
Maximum withstanding voltage range test condition:
CDFAEC-Q100-002- “human body model”
acceptance criteria: “normal performance’
± 2 kV
Table 4. Thermal data
Symbol Parameter Value Unit
R
thJA
Thermal resistance junction to ambient (1)
1. Device mounted on demonstration board
75 °C/W
T
STG
Storage temperature range - 55 to 150 °C
T
J
Junction temperature range - 40 to 125 °C
P
TOT
Power dissipation at T = 70 °C 0.67 W
Electrical characteristics L6924U
8/37 Doc ID 14716 Rev 2
4 Electrical characteristics
TJ = 25 °C, VIN = 5 V, unless otherwise specified.
Table 5. Electrical characteristics
Symbol Parameter Test condition Min. Typ. Max. Unit
VIN (1) Operating input voltage 2.5 12 V
Start up threshold 4.1 V
IIN (1) Supply current Charging mode (RPRG = 24 kΩ)1.82.5mA
Shutdown mode (RPRG = 24 kΩ)6080µA
ISINK Current flowing from VOUT
Shutdown mode (RPRG = 24 kΩ)500nA
Stand by mode (RPRG = 24 kΩ)
(VIN = 2.5 V < VBATTERY)500 nA
VOUT (1) Battery regulated voltage 4.16 4.2 4.24 V
IAC Charge current with AC
adapter input
MODE at GND, RPRG = 24 kΩ450 490 525 mA
MODE at GND, RPRG = 12 kΩ905 975 1045 mA
IUSB
Charge current with USB
input
MODE at HIGH, ISEL at HIGH,
RPRG-USB = 24 kΩ450 490 525
mA
MODE at HIGH, ISEL at LOW,
RPRG-USB = 2 4 kΩ86 96 105
IPRE_AC
Pre-charge current with AC
input
MODE at GND,
RAC = 24 kΩ41 49 56 mA
IPRE_USB
Pre-charge current with USB
input (high power mode)
MODE at HIGH, ISEL at HIGH
RUSB = 24 kΩ41 49 56 mA
Pre-charge current with USB
input (low power mode)
MODE at HIGH, ISEL at LOW
RUSB = 24 kΩ7.6 9.6 11.4 mA
VPRETH Pre-charge voltage threshold 2.9 3.0 3.1 V
IENDTH Termination current REND = 3.3 kΩ12 16 20 mA
TMAXCH (2) Maximum charging time CTPRG = 10 nF
R[IPRG] = 24 kΩ3 hours
TMAXCH (2) Maximum charging time
accuracy
CTPRG = 5.6n F
RPRG = 24 kΩ10 %
SDTH
Shutdown threshold high 2 V
Shutdown threshold low 0.4 V
ST1,2 Output status sink current Status on 10 mA
MODETH
MODE threshold high 1.3 V
MODE threshold low 0.4 V
ISELTH
ISEL threshold high 1.3 V
ISEL threshold low 0.4 V
L6924U Electrical characteristics
Doc ID 14716 Rev 2 9/37
Symbol Parameter Test condition Min. Typ. Max. Unit
RDS(on)
Power MOSFET resistance
(3) Charge current = 500 mA 280 380 mΩ
TH
NTC pin hot threshold
voltage 10 12.5 15 %VREF
NTC pin cold threshold
voltage 40 50 60 %VREF
1. TJ from -40 °C to 125 °C
2. Guaranteed by design
3. Device working in quasi pulse mode
Table 5. Electrical characteristics (continued)
Block diagram L6924U
10/37 Doc ID 14716 Rev 2
5 Block diagram
Figure 4. Block diagram
OSC
BG
ANALOG
PRE.
NTC/PTC
MANAG.
VDD
VDD
VDD
VOSNS
VREF
4.2V
VREF
Logic
SD
VIN
VINS
ST1
ST2TPRG
TH
VPRE
VREF
VDD
LOGIC
BODY
CONTROL
Logic
VBG
IAC
IUSB
POWER MOS
Mos
Driver
Charge
Control CA-VA-TA
REG
UVLO
THERMAL
CONTROL
GND
Logic
IEND
Logic
VOUT
Logic Logic
I FAULT I DETECT
Gas Gauge
ISEL
MODE
L6924U Operation description
Doc ID 14716 Rev 2 11/37
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 fast-
charge current).
●Fast-charge constant current: in this phase the device charges the battery with the
maximum current (IAC for AC adapter mode, IUSB for USB mode).
●Constant voltage: when the battery voltage reaches the selected output voltage, the
device starts to reduce the current, until the charge termination is done.
The full flexibility is provided by:
●Programmable fast-charging current (IAC or IUSB) (Chapter 7.3 on page 18).
●Programmable end of charge current threshold (IENDTH) (Chapter 7.4 on page 20).
●Programmable end of charge timer (TMAXCH) (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 IEND and GND gives information about the
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 VOUT
. If VOSNS doesn’t detect the
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 (R1 or R2 = 26 kΩ).
Operation description L6924U
12/37 Doc ID 14716 Rev 2
Figure 5. Li-Ion charging profile
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).
VADP is the output voltage of the upstream AC-DC adapter that is, in turn, the input voltage
of the L6924U. If the battery voltage is lower than the default pre-charge voltage (VPRETH),
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 VPRETH, the battery is charged with the fast
charge current (IUSB or IAC according to the selection of the MODE pin).
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 (IENDTH) or when the
charging timer expires.
Figure 6 shows the different phases.
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
0.450
0.500
0 200 400 600 800 1000 1200
Charging time (sec)
Ichg (A)
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
Vbatt (V)
Ichg
Vbatt
L6924U Operation description
Doc ID 14716 Rev 2 13/37
Figure 6. Typical charge curves in linear mode
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
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.
End
Charge
Voltage-Regulation
Phase
Power dissipation
Pre-Charge
Phase
Fast-Charge
Phase
IPRETH
ICHG
VOPRGTH Battery Voltage
Charge Current
Adapter Voltage
VADP
VPRETH
CHGBATADPDIS IVVP
×
−
=
)(
Operation description L6924U
14/37 Doc ID 14716 Rev 2
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
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
Adapter Voltage
Charge Current
Power dissipation
Battery Voltage
End
Charge
Voltage Regulation
Phase
Pre-Charge
Phase
Fast-Charge
Phase
ILIM
Ilim x Rdson
IPRETH
ICHG
VPRETH
VOPRGTH
VADP
MOSBATADPIN VVVV
Δ
+
=
=
L6924U Operation description
Doc ID 14716 Rev 2 15/37
Where ΔVMOS is given by:
Equation 3
Where,
ILIM = current limit of the wall adapter, and RDS(on) = resistance of the power MOSFET.
The difference between the programmed charge current and the adapter limit should be
high enough to minimize the RDS(on) value (and the power dissipation). This makes the
control loop completely unbalanced and the power element is fully turned on.
Figure 8 shows the RDS(on) values for different output voltages and charging currents for an
adapter current limit of 500 mA.
Figure 8. RDS(on) curves vs. charging current and output voltage
Neglecting the voltage drop across the charger (ΔVMOS) when the device operates in this
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
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 VADP
. This is the worst case in power
dissipation:
IRV LIM)ON(DSMOS ×=
Δ
2
LIM)on(DSCH IRP ×=
Operation description L6924U
16/37 Doc ID 14716 Rev 2
Equation 5
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.
LIMBATADPDIS I)VV(P ×−=
L6924U Applications information: charging process
Doc ID 14716 Rev 2 17/37
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.
Applications information: charging process L6924U
18/37 Doc ID 14716 Rev 2
The battery is considered deeply discharged when its voltage is lower than a threshold
(VPRETH), internally set to 3 V.
During the pre-charge phase, the current (IPRECH) has a default value equal to 10 % of the
fast-charge current.
A safety timer is also present. If the battery voltage does not rise over VPRETH within this
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 VPRETH, the
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 IAC pin (Chapter 7.3: Fast charge phase), regardless of the resistor
connected to IUSB.
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 IUSB pin (Chapter 7.3: Fast
charge phase), regardless of the resistor connected to IAC.
Figure 10. MODE pin selection
7.3 Fast charge phase
When the battery voltage reaches the pre-charge voltage threshold (VPRETH), the L6924U
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 IAC pin (AC adapter mode selected) or to IUSB pin (USB
mode) with an accuracy of 7 %.
In AC adapter mode (MODE pin low), the resistor RAC can be calculated as:
I
AC
R
AC
L6924U
MODE
AC adapter mode
R
USB
I
AC
R
AC
L6924U
MODE
USB mode
R
USB
I
USB
V
IN
I
USB
Sets the fast charge current Sets the fast charge current
I
AC
R
AC
L6924U
MODE
AC adapter mode
R
USB
I
AC
R
AC
L6924U
MODE
USB mode
R
USB
I
USB
V
IN
I
USB
Sets the fast charge current Sets the fast charge current
L6924U Applications information: charging process
Doc ID 14716 Rev 2 19/37
Equation 6
Where VBG is the internal reference equal to 1.23 V, whereas KPRG is a constant equal to
9500.
Figure 11. IAC pin connection
In USB mode (MODE pin high), the RUSB resistor can be selected as:
Equation 7
Where VBG and KPRG have the same meaning and value above mentioned.
The charge current in USB mode depends on RUSB as well as the state of the ISEL pin.
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 (Section 7.7: Maximum charging time).
Figure 12. IUSB pin connection
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.
PRG
AC
BG
AC K
I
V
R⋅
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
PRG
USB
BG
USB K
I
V
R⋅
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
Applications information: charging process L6924U
20/37 Doc ID 14716 Rev 2
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, IENDTH. This current can be selected by an
external resistor connected between the IEND pin and GND Figure 13, whose value can be
calculated as:
Equation 8
Figure 13. IEND pin connection
Where KEND is 1050 and VMIN is 50 mV.
When the charge current goes below IENDTH, after a deglitch time, the status pins notify the
end of charge and the charge process ends.
This de-glitch time is expressed as:
Equation 9
where TMAXCH is the maximum charging time. (Chapter 7.7 on page 21)
IEND pin is also used to monitor the charge current, because the current injected in REND is
proportional to the charge current. The voltage across REND can be used by a
microcontroller to check the charge status like a gas gauge.
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×=
ENDTH
END
MINEND
I
K
VR
E
220
MAXCH
DEGLITCH
T
T=
L6924U Applications information: charging process
Doc ID 14716 Rev 2 21/37
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 (VRCH), the device goes back in charging state. The value of the recharge
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 TPRG pin and GND. CTPRG is the
external capacitor (in nF) and is given by the following equation:
Equation 10
Note: The maximum recommended CTPRG value must be less than 50 nF.
9
10×
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛×
=
REF
PRG
BG
T
MAXCH
TPRG V
R
V
K
T
C
E
Applications information: charging process L6924U
22/37 Doc ID 14716 Rev 2
Figure 15. TPRG pin connection
Where,
RPRG = resistor which sets the current (RUSB or RAC)
VREF = 1.8 V,
KT = 279 x 105,
VBG = 1.23 V, and
TMAXCH is the charging time given in seconds.
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
If this timer expires and the battery voltage is still lower than VPRETH, a fault signal is
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 RUSB).
MAXCHMAXPRECH TT ×= 8
1
L6924U Application information: monitoring and protection
Doc ID 14716 Rev 2 23/37
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.
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 VREF
. When the voltage on the TH pin exceeds the minimum or maximum
voltage threshold (internal window comparator), the device stops the charge process, and
indicates a fault condition through the status pin.
Figure 16. Power dissipation in both linear and quasi pulse modes with thermal loop
Application information: monitoring and protection L6924U
24/37 Doc ID 14716 Rev 2
When the voltage (and thus, the temperature), returns to the window range, the device re-
starts the charging process. Moreover, there is a hysteresis for both the upper and lower
thresholds, as shown in Figure 18.
Note: TBAT = OK when the battery temperature is between 0 °C and 50 °C
When the TH pin voltage rises and exceeds the VMINTH = 50 % of VREF (900 mV typ), the
L6924U stops the charge, and indicates a fault by the status pins. The device re-starts to
Figure 17. Battery temperature control flow chart
Figure 18. Voltage window with hysteresis on TH
Figure 19. Pin connection
V
MI NTH
V
MAXTH
V
MINTH_HYS
V
MAXTH_HYS
900mV
780mV
225mV
248mV
Voltage
Variation on TH pin Charge disable
Charge enable
L6924U Application information: monitoring and protection
Doc ID 14716 Rev 2 25/37
charge the battery, only when the voltage at the TH pin goes under VMINTH_HYS = 780 mV
(typ).
For what concerns the high temperature limit, when the TH pin voltage falls under the
VMAXTH = 12.5 % of VREF (225 mV Typ.), the L6924U stops the charge until the TH pin
voltage reaches the VMAXTH_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
Where RUP is the pull-up resistor, VREF is equal to 1.8 V, and RNTC0°C is the value of the
NTC at 0 °C. Since at the low temperature limit VMINTH = 900 mV:
Equation 13
It follows that:
Equation 14
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 RNTC50°C is the value of the NTC at 50 °C. Considering VMAXTH = 225 mV it follows
that:
Equation 16
Consequently:
Equation 17
CNTCUP
CNTC
REFMINTH RR
R
VV
°
°
+
×=
0
0
CNTCUP
CNTC
RR
R
°
°
+
×=
0
0
8.19.0
UPCNTC RR
=
°0
CNTCUP
CNTC
REFMAXTH RR
R
VV
°
°
+
×=
50
50
CNTCUP
CNTC
RR
R
°
°
+
×=
50
50
8.1225.0
7
50
UP
CNTC
R
R=
°
Application information: monitoring and protection L6924U
26/37 Doc ID 14716 Rev 2
Based on Equation 14 and Equation 17, it derives that:
Equation 18
The temperature hysteresis can be estimated by the equation:
Equation 19
Where VTH is the pin voltage threshold on the rising edge, VTH_HYS is the pin voltage
threshold on the falling edge, and NTCT (- %/°C) is the negative temperature coefficient of
the NTC at temperature (T) expressed in % resistance change per °C. For NTCT values, see
the characteristics of the NTC manufacturers (e.g. the 2322615 series by VISHAY). At low
temperature, the hysteresis is approximately:
Equation 20
Obviously at high temperature hysteresis is:
Equation 21
Considering typical values for NTC0°C and NTC50°C, the hysteresis is:
Equation 22
And:
Equation 23
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 RUP
.
In this case, the voltage at the TH pin is always inside the voltage window, and the charge is
always enabled.
7
50
0=
°
°
CNTC
CNTC
R
R
TTH
HYSTHTH
HYS NTCV
VV
T×
−
=_
CNTCmV
mVmV
TCHYS °×
−
=
°0900
780900
0
CNTCmV
mVmV
TCHYS °×
−
=
°50225
248225
50
C
mV
mVmV
TCHYS
o
5.2
051.0900
780900
0≅
×
−
=
°
C
mV
mVmV
TCHYS
o
5.2
039.0225
248225
50 −≅
×
−
=
°
L6924U Application information: monitoring and protection
Doc ID 14716 Rev 2 27/37
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 IENDTH.
At the end of the de-glitch time, a detection current IDETECT
, equal to 1 mA, is sunk from the
output for a time of TDETECT
. The device checks the voltage at the output. If it is below the
VPRETH, a current equal to IDETECT is injected in the output capacitor for a TDETECT
, and it is
checked to see if the voltage on the output goes higher than VRCH (4.05 V). If the battery
voltage changes from VPRETH to VRCH and vice versa in a TDETECT time, it means that no
battery is connected to the charger. The TDETECT is expressed by:
Equation 24
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.
Figure 20. Battery absence detection flow chart
3
1054×
=MAXCH
DETECT
T
T
BATTERY
ABSENT
Detect Low Absent
VBAT
>
VPRETH FAST CHARGE
Detect High Absent
VBAT
>
VRCH PRE CHARGE
NO
YES
YES
NO
DETECT LOW ABSENT = a ISINK is sunk for a TDET from the battery
DETECT HIGH ABSENT = a IINJ is injected for a TDET in the battery
TDET = 100ms (Typ.)
ISINK = IINJ = 1mA (Typ.)
Application information: monitoring and protection L6924U
28/37 Doc ID 14716 Rev 2
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, VREF is turned OFF.
Figure 21. ST1 and ST2 connection with LEDs or microcontroller
Table 6. Status LEDs Indications
Charge condition Description ST1 ST2
Charge in progress When the device is in pre-charge or fast-
charge status ON OFF
Charge done When the charging current goes below the
IENDTH
OFF ON
Stand by mode When the input voltage goes under
VBAT + 50 mV OFF OFF
Bad battery temperature
When the voltage on the TH pin is out of
the programmable window, in accordance
with the NTC or PTC thermistor
ON ON
Battery absent When the battery pack is removed ON ON
Over time When TMAXCH or TMAXPRECH expires ON ON
L6924U Additional applications information
Doc ID 14716 Rev 2 29/37
9 Additional applications information
9.1 Selecting the input capacitor
In most applications, a 1 µF ceramic capacitor, placed close to the VIN and VINSN pins can
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 VOUT and VOUTSN pin is enough to
keep voltage control loop stable. This ensures proper operation of battery absent detection
in removable battery pack applications.
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 ICHG. However, in order to maximize the current capability, it is important to
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. Figure 22 shows the thermal
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
Where the RthJA of the device mounted on board is 75 °C/W, the power dissipated is 1 W,
and the ambient temperature is 25 °C.
In this case the junction temperature is:
Equation 26
AMBDISSATHJJ TPRT
+
×
=
−
CTJ
o
10025175 =+×=
Additional applications information L6924U
30/37 Doc ID 14716 Rev 2
The VOSNS pin can be used as a remote sense; it should be therefore connected as closely
as possible to the battery. The demonstration board layout and schematic are shown in
Figure 23, Figure 24 and Figure 25.
Figure 22. Thermal image of the demonstration board
Figure 23. Demonstration board layout, top side
Figure 24. Demonstration board layout, bottom side
L6924U Additional applications information
Doc ID 14716 Rev 2 31/37
Figure 25. Demonstration board schematic
Table 7. Demonstration board components description
Name Value Description
R1 24 kΩAC mode fast-charge current resistor. Used to set the charging current in AC mode.
R2 24 kΩUSB mode fast-charge current resistor. Used to set the charging current in USB
mode.
R3 3.3 kΩEnd of Charge current resistor. Used to set the termination current and, as a “gas
gauge” when measuring the voltage across on it.
R4 1 kΩPull up resistor. Connected between VREF and TH pin.
R5 1 kΩPull up resistor. To be used when the ST1 is connected to a LED
R6 1 kΩPull up resistor. To be used when the ST2 is connected to a LED.
RT1 470 ΩIf 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 TMAX capacitor. Used to set the maximum charging time.
C4 1 nF VREF filter capacitor.
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.
Application idea: dual input management with AC priority L6924U
32/37 Doc ID 14716 Rev 2
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 VIN pin through the diode D1.
The voltage at the VIN pin is:
A correct choice of this diode is important to limit Vdiode and keeping VIN as close as
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 RAC.
When only the USB power source is available, both Q1 and Q2 switch on and the pin VIN is
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 IUSB (RUSB).
The voltage of the VIN pin is given by:
The voltage drop across the MOSFETs must be kept as low as possible to avoid reducing
too much the voltage of the VIN pin.
When both sources are present, this circuit gives the priority to the AC adapter. In fact, for
VAC ≥ 5 V, surely both Q1 and Q2 are off and VIN is connected to the AC adapter through
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.
diodeACIN VVV
−
=
(
)
USB2Q_DSon1Q_DSonUSBIN IRRVV
⋅
+
−
=
L6924U Application idea: dual input management with AC priority
Doc ID 14716 Rev 2 33/37
Figure 26. Dual input management
L6924U
VIN
MODE
AC
USB
RG
RM
Q1 Q2
D1
IAC
IUSB
RAC RUSB
VOUT
Li-Ion
battery
L6924U
VIN
MODE
AC
USB
RG
RM
Q1 Q2
D1
IAC
IUSB
RAC RUSB
VOUT
Li-Ion
battery
Package mechanical data L6924U
34/37 Doc ID 14716 Rev 2
11 Package mechanical data
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK®
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK® is an ST trademark.
L6924U Package mechanical data
Doc ID 14716 Rev 2 35/37
Table 8. VFQFPN16 (3 x 3 mm.) mechanical data
Dim.
mm.
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
L6924U
Doc ID 14716 Rev 2 37/37
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