L6924D013TR - STMicroelectronics, Inc.

September 2010 Doc ID 11908 Rev 9 1/38

L6924D

Battery charger system with integrated

power switch for Li-Ion/Li-Polymer

Features

■Fully integrated solution, with a power

MOSFET, reverse blocking diode, sense

resistor, and thermal protection

■Ideal for coke and graphite anode single-cell LI-

ION packs

■Both linear and quasi-pulse operation

■Closed loop thermal control

■USB BUS-compatible

■Programmable charge current up to 1 A

■Programmable pre-charge current

■Programmable end-of-charge current

■Programmable pre-charge voltage threshold

■Programmable charge timer

■Programmable output voltage at 4.1 V and 4.2

V, with ± 1 % output voltage accuracy

■(NTC) or (PTC) thermistor interface for battery

temperature monitoring and protection

■Flexible charge process termination

■Status outputs to drive LEDs or to interface

with a host processor

■Small VFQFPN 16-leads package (3 x 3 mm)

Applications

■PDAs

■Handheld devices

■Cellular phones

■Digital cameras

■Standalone chargers

■USB-powered chargers

VFQFPN16

Table 1. Device summary

Order code Package Packaging

L6924D VFQFPN16 Tube

L6924D013TR Tape and reel

www.st.com

Contents L6924D
2/38 Doc ID 11908 Rev 9
Contents
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Pins description and connection diagrams  . . . . . . . . . . . . . . . . . . . . . . 4
1 Pin description  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
2 Thermal data   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
Electrical specifications  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
Block diagram  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Operation description  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1 Linear mode  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
2 Quasi-pulse mode   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
Applications information: charging process   . . . . . . . . . . . . . . . . . . . . 16
1 Charging process flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
2 Pre-charge current  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
3 Pre-charge voltage  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
4 Fast charge current   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
5 End-of-charge current   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
6 Recharge flow chart  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
7 Recharge threshold  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
8 Maximum charging time  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
9 Termination modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
Application information: monitoring and protection  . . . . . . . . . . . . . . 23
1 NTC thermistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  23
2 Battery absence detection  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
3 Status pins  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  27

L6924D Contents
Doc ID 11908 Rev 9 3/38
4 Shutdown  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
Additional applications information   . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1 Selecting the input capacitor   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
2 Selecting the output capacitor  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
3 Layout guidelines and demonstration board description   . . . . . . . . . . . . .  30
Application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
1 USB battery charger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
Package mechanical data  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Revision history   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Description L6924D

4/38 Doc ID 11908 Rev 9

1 Description

The L6924D is a fully monolithic battery charger dedicated to single-cell Li-Ion/Polymer

battery packs. It is the ideal solution for space-limited applications, like PDAs, handheld

equipment, cellular phones, and digital cameras. It integrates all of the power elements (the

power MOSFET, reverse blocking diode and the sense resistor) in a small VFQFPN16 (3 x 3

mm) package. When an external voltage regulated wall adapter is used, the L6924D 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. The L6924D allows the user to program

many parameters, such as pre-charge current, fast-charge current, pre-charge voltage

threshold, end-of-charge current threshold, and charge timer. The L6924D 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 L6924D also provides very

flexible control of the charge process termination and Gas Gauge capability, as well as other

functions, such as checking for battery presence, and monitoring and protecting the battery

from unsafe thermal conditions.

Figure 2. Basis application schematic

Figure 1. Minimum application size

L6924D Pins description and connection diagrams

Doc ID 11908 Rev 9 5/38

2 Pins description and connection diagrams

Figure 3. Pins connection (top view)

2.1 Pin description

OSNS

IEND

INSNS

GND SD

ST1

ST2

OPRG

IPRG

TPRG TH

OSNS

IPRE IEND

REF

OUT

VPRE

Table 2. Pin functions

Pin I/O Name Pin description

1I V

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 7.8: Maximum charging time on page 20

6 - GND Ground pin.

7I SD

Shutdown pin.

When connected to GND enables the device; when floating disables the device.

8I TH

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.

9IV

OPRG

Output voltage selection pin.

VOUT = 4.1 V if left floating. VOUT = 4.2 V if connected to GND.

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)

Pins description and connection diagrams L6924D

6/38 Doc ID 11908 Rev 9

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 fixes the charge termination current threshold

IENDTH: if I < IENDTH, the charger behaves according to the VPRE status (see Chapter 7.5:

End-of-charge current on page 19). The voltage across the resistor is proportional to the

current delivered to the battery (Gas Gauge function).

14 I VPRE

Multifunction pin.

A resistor connected to GND allows the user to adjust the pre-charge voltage threshold

VPRETH.

If the pin is floating, VPRETH = 2.8 V. If the voltage on VPRE pin is lower than 0.8 V, VPRETH =

2.8 V and the charge is not automatically terminated when I < IENDTH.

If the voltage on VPRE goes lower than 0.5 V (edge sensitive), the maximum charging time is

reset.

15 I IPRG

Charge current program pin.

A resistor connected from this pin to GND, fixes the fast charge current value (ICHG), with an

accuracy of 7%.

16 I IPRE

Pre-charge current program pin.

If the pin is floating IPRETH is equal to 10% of ICHG. If IPRETH has to be programmed at a

different value, the pin has to be connected to GND or VREF

, through a resistor see

Chapter 7.2: Pre-charge current on page 17.

Table 2. Pin functions

L6924D Maximum ratings

Doc ID 11908 Rev 9 7/38

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.

3.1 Absolute maximum ratings

3.2 Thermal data

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

ST1, ST2 Output voltage –0.3 to 6 V

Output current 30 mA

VREF

, TH, IEND, IPRG,

VPRE, IPRE, VOPRG,

TPRG, GND

–0.3 to 4 V

ST1 and TH pins Maximum withstanding voltage range test condition:

CDFAEC-Q100-002 (Normal “human body model”

acceptance criteria performance)

±1.5 kV

Other pins ±2 kV

Table 4. Thermal data

Symbol Parameter Value Unit

thJA

Thermal resistance junction to ambient (1)

1. Device mounted on demonstration board

75 °C/W

STG

Storage temperature range –55 to 150 °C

Junction temperature range –40 to 125 °C

TOT

Power dissipation at T= 70 °C TBD W

Electrical specifications L6924D

8/38 Doc ID 11908 Rev 9

4 Electrical specifications

4.1 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 = 24kΩ)1.82.5mA

Shutdown mode (RPRG = 24kΩ)6080µA

ISINK Current flowing from VOUT

Shutdown mode (RPRG = 24kΩ)500nA

Stand by mode (RPRG = 24kΩ)

(VIN = 2.5V < VBATTERY)500 nA

VOUT(1) Battery regulated voltage VOPRG at VIN 4.06 4.1 4.14 V

VOPRG at GND 4.16 4.2 4.24 V

ICHG Charge current RPRG = 24kΩ450 490 525 mA

RPRG = 12kΩ905 975 1045 mA

IPRECH

Pre-Charge current

[default value = 10% ICHG]

IPRE floating

RPRG = 24kΩ41 49 56 mA

IPRECH Pre-Charge current RPRE = 62kΩ to GND;

RPRG = 24kΩ57 67 78 mA

IPRECH Pre-Charge current RPRE = 39kΩ to VREF;

RPRG = 24kΩ29.5 35 40.1 mA

VPRETH

Pre-Charge voltage threshold

[default] VPRE = VPRETHDefault = Floating 2.7 2.8 2.9 V

VPRETH Pre-Charge voltage threshold RVPRE = 13kΩ; RPRG = 12kΩ2.87 3.03 3.19 V

VPRETH

Pre-Charge voltage threshold

[default]. Charge termination

disabled

2.7 2.8 2.9 V

IENDTH Termination current REND = 3K3 12 16 20 mA

TMAXCH(2) Maximum charging time CTPRG = 10nF

R[IPRG] = 24kΩ3 hours

TMAXCH

(2) Maximum charging time accuracy CTPRG = 5.6nF

RPRG = 24kΩ10%

SDTH

Shutdown threshold high 2 V

Shutdown threshold low 0.4 V

ST1,2 Output status sink current Status on 10 mA

RDS(on) Power MOSFET resistance RDS(on)@ICHG = 500mA 280 380 mΩ

L6924D Electrical specifications

Doc ID 11908 Rev 9 9/38

TH NTC pin hot threshold voltage 10.625 12.5 14.375 %VREF

NTC pin cold threshold voltage 45 50 55 %VREF

1. TJ from –40°C to 125°C.

2. Guaranteed by design.

Table 5. Electrical characteristics (continued)

Symbol Parameter Test condition Min Typ Max Unit

Block diagram L6924D

10/38 Doc ID 11908 Rev 9

5 Block diagram

Figure 4. Block diagram

L6924D Operation description

Doc ID 11908 Rev 9 11/38

6 Operation description

The L6924D 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. However, thanks to its very low minimum input voltage (down to 2.5 V) the L6924D

can also work as a Quasi-Pulse charger when powered from a current limited adapter. To

work in this condition, is enough to set the device’s charging current higher than the

adapter’s one (Chapter 7.4 on page 18). 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 the upstream adapter. However, the advantage of the Quasi-Pulse approach is that

the power dissipated inside the portable equipment is dramatically reduced.

The L6924D 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.

●Fast-Charge constant current: in this phase the device charges the battery with the

maximum current.

●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 pre-charging current and voltage thresholds (IPRETH and VPRETH)

(Chapter 7.2 on page 17, Chapter 7.3 on page 17).

●Programmable fast-charging current (ICHG) (Chapter 7.4 on page 18).

●Programmable end of charge current threshold (IENDTH) (Chapter 7.5 on page 19).

●Programmable end of charge timer (TMAXCH) (Chapter 7.8 on page 20).

If the full flexibility is not required and a smaller number of external components is preferred,

default values of IPRETH and VPRETH are available leaving the respective pins floating.

●If a PTC or NTC resistor is used, the device can monitor the battery temperature in

order to protect the battery from operating in 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 µC

ADC.

Operation description L6924D

12/38 Doc ID 11908 Rev 9

When the VPRE pin is not used to program the Pre-Charge voltage threshold, it has two

different functions:

●If the voltage across VPRE pin is lower than 0.8 V, when I < IENDTH, the end of charge is

notified by the status pin, but the charging process is not disabled. The charge process

ends when the maximum charging time expires.

●If the voltage at VPRE pin false under 0.5 V the timer is reset on the falling edge.

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 shows the real charging profile of a Li-Ion battery, with a fast charge current of 450

mA (RPRG = 26 kΩ),

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 L6924D. If the battery voltage is lower than a set pre-charge voltage (VPRETH), the

pre-charge phase takes place. The battery is pre-charged with a low current IPRE

(Chapter 7.2 on page 17).

When the battery voltage goes higher than VPRETH, the battery is charged with the fast

charge current ICHG, set through an external resistor (Chapter 7.4 on page 18).

Finally, when the battery voltage is close to the regulated output voltage VOPRGTH (4.1 V or

4.2 V), the voltage regulation phase takes place and the charging current is reduced. The

Charging profile

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

Vb a tt

L6924D Operation description

Doc ID 11908 Rev 9 13/38

charging process is usually terminated when the charging current reaches a set value or

when a charging timer expires (Chapter 7.9 on page 22).

Figure 6 shows the different phases.

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, this 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. 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 L6924D

14/38 Doc ID 11908 Rev 9

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, ICHG must be set higher than the

current limit of the adapter. In this mode, the L6924D 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 ICHG 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

Where ΔVMOS is given by:

Equation 3

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

VLIMONDS

MOS ×=

)(

L6924D Operation description

Doc ID 11908 Rev 9 15/38

Where,

ILIM = current limit of the wall adapter, and RDS(on) = resistance of the power MOSFET.

The difference between the set 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 voltage 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:

Equation 5

In conclusion, the advantage of the linear charging approach is that the designer has the

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.

)( LIMonDSCH IRP ×=

LIMBATADPDIS IVVP

−

)(

Applications information: charging process L6924D

16/38 Doc ID 11908 Rev 9

7 Applications information: charging process

7.1 Charging process flow chart

Figure 9. Charging process flow chart

L6924D Applications information: charging process

Doc ID 11908 Rev 9 17/38

7.2 Pre-charge current

The L6924D allows pre-charging the battery with a low current when the battery voltage is

lower than a specified threshold (VPRETH). The Pre-charge current has a default value equal

to 10% of the fast-charge current. However it can be adjusted by connecting a resistor from

the IPRE pin to GND or VREF (see Figure 10). When the resistor is connected between IPRE

pin and GND, the current is higher than the default value. The RPRE value is given by:

Equation 6

Figure 10. IPRE pin connection

When RPRE is connected to VREF

, the current is lower than the default value. VREF is the

external reference equal to 1.8 V, VBG is the internal reference equal to 1.23 V and KPRE is a

constant equal to 950. See Figure 11.

The relationship is shown in the equation 7:

Equation 7

Figure 11. IPRE pin connection

7.3 Pre-charge voltage

If the VPRE pin is floating, a default value of VPRETH is set, equal to 2.8 V (VPRETHDefault).

Otherwise, the device offers the possibility to program this value, with a resistor connected

between the VPRE pin and GND (see Figure 12). In this case, the RVPRE is given by the

equation 8:

PRG

PRE

PRECH

PRE

−

L6924D

IPRE

L6924D

IPRE

PRE

PRECH

PRG

REF

PRE

−

VREF

L6924D

IPRE

VREF

L6924D

IPRE

Applications information: charging process L6924D

18/38 Doc ID 11908 Rev 9

Equation 8

Figure 12. VPRE pin connection

Where RVPRE is the resistor between VPRE and GND, and RPRG is the resistor used to set

the charge current (see Section 7.4: Fast charge current), and VPRETH is the selected

threshold.

A safety timer is also present. If the battery voltage doesn't rise over VPRETH, before this

time is expired, a fault is given (see Section 7.8: Maximum charging time). If at the beginning

of the charge process, the battery voltage is higher than the VPRETH, the Pre-Charge phase

is skipped.

7.4 Fast charge current

When the battery voltage reaches the Pre-charge voltage threshold (VPRETH), the L6924D

starts the Fast-charge Phase. In this phase, the device charges the battery with a constant

current, ICHG, programmable by an external resistor that sets the charge current with an

accuracy of 7% Figure 13. The equation used to select the RPRG as follows:

Equation 9

Figure 13. IPRG pin connection

Where KPRG is a constant, equal to 9500.

During this phase, the battery voltage increases until it reaches the programmed output

voltage. A safety timer is also present. If this time expires, a fault is given (Section 7.8:

Maximum charging time).

⎟

⎠

⎞

⎜

⎝

⎛

×=

ltPRETHDefau

PRETH

PRGVPRE V

VPRE

L6924D

RPRE

VPRE

L6924D

RPRE

⎟

⎠

⎞

⎜

⎝

⎛

×=

CHG

PRG

BGPRG I

L6924D Applications information: charging process

Doc ID 11908 Rev 9 19/38

7.5 End-of-charge current

When the charge voltage approaches the selected value (4.1 V or 4.2 V), the voltage

regulation phase takes place. The charge current starts to decrease until it goes lower than

a programmable end value, IENDTH, depending on an external resistor connected between

the IEND pin and GND (see Figure 14). The equation that describes this relation as follows:

Equation 10

Figure 14. IEND pin connection

Where KEND is 1050; and VMIN is 50 mV.

Typically, this current level is used to terminate the charge process. However, it is also

possible to disable the charge termination process based on this current level (Chapter 7.9

on page 22).

This pin is also used to monitor the charge current, because the current injected in REND is

proportional to ICHG. The voltage across REND can be used by a microcontroller to check the

charge status like a gas gauge.

⎟

⎠

⎞

⎜

⎝

⎛

×=

ENDTH

END

MINEND I

Applications information: charging process L6924D

20/38 Doc ID 11908 Rev 9

7.6 Recharge flow chart

Figure 15. Recharge flow chart

7.7 Recharge threshold

When, from an end-of-charge condition, the battery voltage goes lower than the recharging

threshold (VRCH), the device goes back in charging state. The value of the recharge

threshold is VOPRG–150 mV.

7.8 Maximum charging time

To avoid the charging of a dead battery for a long time, the L6924D has the possibility to set

a maximum charging time starting from the beginning of the fast-charge phase. This timer

can be set with a capacitor, connected between the TPRG pin and GND. The CTPRG is the

external capacitor (in nF) and is given by the following equation:

Equation 11

Note: The maximum recommended CTPRG value must be less than 50 nF.

END of CHARGE

VBAT

VRCH

Detect Low

VBAT

VPRETH

FAST CHARGE

Detect High

VBAT

VRCH

BATTERY

ABSENT

PRE CHARGE

FAULT

IND FAULT

VBAT

VRCH

Detect High Fault

VBAT

VABS

Detect Low Fault

VBAT

VPRETH

BATTERY

ABSENT

YES

DETECT LOW = a ISINK is sunk for a TDET from the battery

DETECT HIGH = a IINJ is injected for a TDET in the battery

DETECT LOW FAULT = a ISINK is sunk for a TDET from the battery

DETECT HIGH FAULT = a IINJ is injected for a TDET in the battery

VABS = VOPRG – 50mV

VRCH = VOPRG – 150mV

TDET = 100ms (Typ.)

ISINK = IINJ = 1mA (Typ.)

RETURN TO CHARGING PROCESS

FLOW CHART

GO TO BATTERY ABSENT

FLOW CHART

FROM CHARGING PROCESS FLOW CHART

10×

⎟

⎠

⎞

⎜

⎝

⎛×

REF

PRG

MAXCH

TPRG V

L6924D Applications information: charging process

Doc ID 11908 Rev 9 21/38

Figure 16. TPRG pin connection

Where,

VREF = 1.8V,

KT = 279 x 105,

VBG = 1.23V, 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 12

If this timer expires and the battery voltage is still lower than VPRETH, a fault signal is

generated, and the charge process is terminated.

TPRG

L6924D

CTPRG

MAXCHMAXPRECH TT ×= 8

Applications information: charging process L6924D

22/38 Doc ID 11908 Rev 9

7.9 Termination modes

As shown in Figure 14, it is possible to set an end of charge current IENDTH connecting a

resistor between the IEND pin and GND. When the charge current goes down to this value,

after a de-glitch time, the status pins notify that the charge process is complete. This de-

glitch time is expressed as:

Equation 13

However, the termination of the charger process depends on the status of the VPRE pin:

●If the voltage at the VPRE pin is higher than 0.8 V, the charge process is actually

terminated when the charge current reaches IENDTH.

●If the voltage at VPRE pin goes lower than 0.8 V, the charge process does not terminate,

and the charge current can go lower than IENDTH. The status pins notify the end-of-

charge as a fault condition, but the device continues the charge. When the TMAXCH is

elapsed, the charge process ends, and a fault condition is issued.

●If the voltage on VPRE pin is lower than 0.8 V during the Pre-charge Phase, the device

sets the VPRETHDefault automatically.

●If the voltage at the VPRE pin goes lower than 0.5 V (edge sensitive), the timer is reset,

both in pre-charge and in fast-charge phase.

Figure 17. Charge termination flow chart

220

MAXCH

DEGLITCH

L6924D Application information: monitoring and protection

Doc ID 11908 Rev 9 23/38

8 Application information: monitoring and protection

The L6924D uses a VFQFPN 3 mm 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 L6924D 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 L6924D 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.

The Figure 18 shows how the thermal loop acts (with the 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 18. Power dissipation both linear and quasi pulse mode with thermal loop

Application information: monitoring and protection L6924D

24/38 Doc ID 11908 Rev 9

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 20.

Note: TBAT = OK when the battery temperature between 0°C and 50°C

Figure 19. Battery temperature control flow chart

Figure 20. Voltage window with hysteresis on TH

Figure 21. Pin connection

MI NTH

MAXTH

MINTH_HYS

MAXTH_HYS

900mV

780mV

225mV

248mV

Voltage

Variation on TH pin Charge disable

Charge enable

L6924D

VREF

NTC

L6924D Application information: monitoring and protection

Doc ID 11908 Rev 9 25/38

When the TH pin voltage rises and exceeds the VMINTH = 50% of VREF (900 mV typ), the

L6924D stops the charge, and indicates a fault by the status pins. The device re-starts to

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 L6924D stops the charge until the TH pin

voltage rises to 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 14

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 15

It follows that:

Equation 16

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 17

Where RNTC50°C is the value of the NTC at 50°C. Considering VMAXTH = 225 mV it follows

that:

Equation 18

Consequently:

Equation 19

CNTCUP

CNTC

REFMINTH RR

×=

CNTCUP

CNTC

×=

8.19.0

UPCNTC RR

°0

CNTCUP

CNTC

REFMAXTH RR

×=

CNTCUP

CNTC

×=

8.1225.0

CNTC

Application information: monitoring and protection L6924D

26/38 Doc ID 11908 Rev 9

Based on Equation 16: and Equation 19: , it derives that:

Equation 20

The temperature hysteresis can be estimated by the equation:

Equation 21

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 the

low temperature, the hysteresis is approximately:

Equation 22

Obviously at the high temperature hysteresis is:

Equation 23

Considering typical values for NTC0°C and NTC50°C, the hysteresis is:

Equation 24

And:

Equation 25

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.

CNTC

TTH

HYSTHTH

HYS NTCV

T×

−

CNTCmV

mVmV

TCHYS °×

−

°0900

780900

CNTCmV

mVmV

TCHYS °×

−

°50225

248225

mVmV

TCHYS

5.2

051.0900

780900

0≅

−

mVmV

TCHYS

5.2

039.0225

248225

50 −≅

−

L6924D Application information: monitoring and protection

Doc ID 11908 Rev 9 27/38

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 VABS (the value is VOPRGTH-50

mV). If the battery voltage changes from VPRETH to VABS and vice versa in a TDETECT time,

it means that no battery is connected to the charger. The TDETECT is expressed by:

Equation 26

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 to an

external power source, by a resistor, or to communicate to a host processor. These pins

must never be connected to the VIN when it exceeds their absolute value (6 V).

Figure 22. Battery absent detection flow chart

1054×

=MAXCH

DETECT

BATTERY

ABSENT

Detect Low Absent

VBAT

VPRETH FAST CHARGE

Detect High Absent

VBAT

VRCH PRE CHARGE

YES

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 L6924D

28/38 Doc ID 11908 Rev 9

8.4 Shutdown

The L6924D has a shutdown pin (SD) that allows enabling or disabling the device.

If the SD pin voltage is below 0.4 V (e.g. pin connected to GND), the device is enabled,

whereas if the SD pin voltage exceeds 2 V (e.g. the shutdown pin is left floating) the device

is disabled.

When the device enters the shutdown mode, the current consumption is reduced to 60 μA

typ. In this condition, VREF is turned off.

The Figure 24 clarifies the SD pin behavior.

Figure 23. 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 lower than 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 is expired ON ON

L6924D Application information: monitoring and protection

Doc ID 11908 Rev 9 29/38

Figure 24. Shutdown

TH,high

TH,low

device enabled

device disabled

pin voltage

0.4V

TH,high

TH,low

device enabled

device disabled

pin voltage

0.4V

Additional applications information L6924D

30/38 Doc ID 11908 Rev 9

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, 1 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 description

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 25 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 27

Where the RTH J-A 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 28

AMBDISSATHJJ TPRT

−

CTJ

10025175 =+×=

L6924D Additional applications information

Doc ID 11908 Rev 9 31/38

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 26, Figure 27 and Figure 28.

Figure 25. Thermal image of the demonstration board

Figure 26. Demonstration board layout, top side

Figure 27. Demonstration board layout, bottom side

Additional applications information L6924D

32/38 Doc ID 11908 Rev 9

Figure 28. Demonstration board schematic

SHDN GND VOPRG IPRE

TPRG

VPRE

IPRG

IEND

VOSNS

VOUT

VREF

VINSNS

VIN

ST1

ST2

L6924D

CHARGER

BATTERY

Vref

R1 R2

R7 R8

LD1 LD2

μC

NTC

J3 J4

R10

L6924D Additional applications information

Doc ID 11908 Rev 9 33/38

Table 7. Demonstration board components description

Name Value Description

R1 1k Pull up resistor. To be used when the ST1 is connected to a LED.

R2 1k Pull up resistor. To be used when the ST1 is connected to a LED.

R3 1k Pull up resistor. Connected between VREF and TH pin.

R4 3k3 End of charge current resistor. Used to set the termination current and, as a “Gas

Gauge” when measuring the voltage across on it.

R5 24k Fast-charge current resistor. Used to set the charging current.

R6 N.M. VPRETH resistor. Used to set programmable pre-charge voltage threshold. If not

mounted, the VPRETHDefault, equal to 2.8V, is set.

R7 N.M. IPRETH resistor. Used to set the programmable pre-charge current threshold below

the default one. If not mounted, the IPRETHDefault is set.

R8 68k IPRETH resistor. Used to set the programmable pre-charge current threshold above

the default one. If not mounted, the IPRETHDefault is set.

R9 470R If a NTC is not used, a half value of R3 must be mounted to keep the TH voltage in

the correct window.

R10 N.M. It has the same function of R6. Moreover, if it is replaced with a short-circuit, when

J5 is closed, the timer is reset (falling edge).

C1 1µF Input capacitor.

C2 10nF TMAX capacitor. Used to set the maximum charging time.

C3 4.7µF Output capacitor.

C4 1nF VREF filter capacitor.

LD1 GREEN ST1 LED.

LD2 RED ST2 LED.

J1 ST1 jumper. Using to select the LED or the external µC.

J2 ST2 jumper. Using to select the LED or the external µC.

J3 SD jumper. If open, the device is in shutdown mode; when closed, the device starts

to work.

J4 VOPRG jumper. If closed, the 4.2V output voltage is set; if open, the 4.1V is set.

J5 VPRE jumper. If closed with R10 in short-circuit with GND, resets the timer.

Application ideas L6924D

34/38 Doc ID 11908 Rev 9

10 Application ideas

10.1 USB battery charger

With a voltage range between 4.75 V and 5.25 V, and a maximum current up to 500 mA, the

USB power bus is an ideal source for charging a single-cell Li-Ion battery. Since it is not

possible to rely on the USB current limit to charge the battery, a linear approach must be

adopted. Therefore, it is only necessary to set the ICHG with a maximum value lower than

500 mA, and the device will charge the battery in Linear mode.

Figure 29 shows an example of USB charger application schematic.

Figure 29. USB charger application

L6924D

VIN

VINSNS

TPRG

ST1

ST2 SD GND

VOPRG IPRE

VPRE

IPRG

VOSNS

VOUT

IEND

VREF

R2 R3

R4 R5

BATTERY

PACK

VBUS

GND

D- D+

SYSTEM

AND

USB

CONTROLLER

L6924D Package mechanical data

Doc ID 11908 Rev 9 35/38

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.

Package mechanical data L6924D

36/38 Doc ID 11908 Rev 9

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 30. Package dimensions

7185330_G

L6924D Revision history

Doc ID 11908 Rev 9 37/38

12 Revision history

Table 9. Document revision history

Date Revision Changes

16-Dec-2005 1 First draft

20-Dec-2005 2 Package dimensions updated

10-Jan-2006 3 Few updates

14-Feb-2006 4 Part number updated

03-Jul-2006 5 Updates to equation in page 22, updated block diagram Figure 4.

07-Sep-2006 6 Added Note: on page 20, updated value CTPRG page 8

29-Jun-2007 7 Updated capacitor values C2, C3 in Table 7 on page 33

05-Jul-2010 8 Updated Table 5 on page 8 and Section 8.4 on page 28

22-Sep-2010 9 Updated Ta bl e 8 and Figure 30 on page 36. Minor changes.

L6924D

38/38 Doc ID 11908 Rev 9

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