AN2407 Application note 800mA standalone linear Li-Ion battery charger with thermal regulation Introduction One way to minimize the size and complexity of a battery charger is to use a linear-type charger. The linear charger drops the AC adapter voltage down to the battery voltage. The number of external components is low: linear chargers require input and output bypass capacitors, and sometimes need an external pass transistor, and resistors for setting voltage and current limits. The main pitfall of a linear charger is power dissipation. The charger simply drops the AC adapter voltage down to the battery voltage. In the case of an 800mA charger, a 5V±10% regulated AC adapter voltage, and battery voltage that varies between 4.2V and 2.5V, the power dissipation can range from 0.6W to 2.0W. This type of charger is simpler than the switch-mode type, mainly because the passive LC filter is not required. It dissipates the most power when the battery voltage is at its minimum, since the difference between the fixed input voltage and the battery voltage is greatest during this condition. Application diagram September 2006 Rev 2 1/14 www.st.com Contents AN2407 Contents 1 STBC08 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Stability considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 Board layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 External components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6 Automatic recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7 CHRG and Power-on status output pins . . . . . . . . . . . . . . . . . . . . . . . . . 9 8 USB and wall adapter power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9 Charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9.1 Programming charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9.2 Maximum charge current in temperature . . . . . . . . . . . . . . . . . . . . . . . . . 11 Appendix A Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2/14 AN2407 1 STBC08 description STBC08 description The STBC08 is a constant current/constant voltage charger for single cell Li-Ion battery. No external sense resistor or blocking diode is required and its MLPD 3x3mm² 6L package make it ideally suited for portable applications. The STBC08 is designed to comply with USB power specifications. An internal block regulates the current when the junction temperature increases in order to protect the device when it operates in high power or high ambient temperature. The maximum power dissipation occurs when VBAT is 2.9V with the maximum charge current. The charge voltage is fixed at 4.2V, and the charge current limitation can be programmed using a single resistor connected between pins PROG and GND. The charge cycle finishes when the current flowing to the battery is 1/10 of the programmed value. If the external adaptor is removed, the STBC08 switches off and only 2µA can flow from the battery to the device. The device can be put into Shutdown Mode, reducing the supply current to 25µA. Figure 1. Block diagram 3/14 Stability considerations 2 AN2407 Stability considerations The STBC08 contains two control loops: constant voltage and constant current. The constant-voltage loop is stable without any compensation when a battery is connected with low impedance leads. Excessive lead length, however, may add enough series inductance to require a bypass capacitor of at least 1µF from BAT to GND. Furthermore, a 4.7µF capacitor with a 0.2Ω to 1Ω series resistor from BAT to GND is required to keep ripple voltage low when the battery is removed. High value capacitors with very low ESR (especially ceramic) reduce the constant-voltage loop phase margin. Ceramic capacitors up to 22µF may be used in parallel with a battery, but larger ceramics should be decoupled with 0.2Ω to 1Ω of series resistance. In constant-current mode, the PROG pin is in the feedback loop, not the battery. Because of the additional pole created by PROG pin capacitance, capacitance on this pin must be kept to a minimum. With no additional capacitance on the PROG pin, the charger is stable with program resistor values as high as 12k. However, additional capacitance on this node reduces the maximum allowed program resistor.Therefore, if the PROG pin is loaded with a capacitance, CPROG, the following equation should be used to calculate the maximum resistance value for RPROG: Equation 1 1 R PROG ≤---------------------------------------------------------5 2π • 5 • 10 • C PROG Average, rather than instantaneous, battery current may be of interest to the user. For example, if a switching power supply operating in low current mode is connected in parallel with the battery, the average current being pulled out of the BAT pin is typically of more interest than the instantaneous current pulses. In such a case, a simple RC filter can be used on the PROG pin to measure the average battery current as shown in Figure 2. This design includes a 20kΩ resistor between the PROG pin and the filter capacitor to ensure stability (CFILTER = 100nF). Figure 2. 4/14 Isolating capacitive load on PROG pin and filtering AN2407 3 Board layout considerations Board layout considerations Due to the small size of the MLP package, it is very important to use a good thermal PC board layout to maximize the available charge current. The thermal path for the heat generated by the IC is from the die to the copper lead frame, through the package leads, (especially the ground lead) to the PC board copper. The PC board copper is the heat sink. The footprint copper pads should be as wide as possible and expand out to larger copper areas to spread and dissipate the heat to the surrounding ambient. Feed-through vias to inner or backside copper layers are also useful in improving the overall thermal performance of the charger. Other heat sources on the board, not related to the charger, must also be considered when designing a PC board layout because they will affect overall temperature rise and the maximum charge current. Table 1 lists thermal resistance for several different board sizes and copper areas. Table 1. Measured thermal resistance (2-layer board) Copper area Board area Thermal resistance junction-to-ambient Top Bottom 2500mm2 2500mm2 2500mm2 125°C/W 1000mm2 2500mm2 2500mm2 125°C/W 225mm2 2500mm2 2500mm2 130°C/W 2 2500mm2 2500mm2 135°C/W 2500mm2 2500mm2 150°C/W 100mm 50mm2 Appendix A: Board layout on page 12 contains an illustration of the complete assembly board. 5/14 External components 4 AN2407 External components This application requires few external components: two ceramic capacitors (CIN = 1µF, COUT = 4.7µF) and one resistor (RPROG). For input and output capacitors, ST recommends using ceramic capacitors with low ESR. For good stability of device supplied from low input voltage 2.6V at maximum ratings of output, ST recommends using 1µF/6.3V as a minimum value for the input capacitor and 4.7µF/6.3V as a minimum value for the output capacitor. Table 2. Bill of materials Symbol Parameter Cin Input Cap. Cbat Output Cap. Rusb Qty Supplier Value Unit Ceramic Low ESR GRM155F50J105ZE01 1 Murata 1 µF Ceramic Low ESR GRM188r60J475ke19 1 Murata 4.7 µF Usb Current Set Thick film type 1% CRG0603J2K0 1 THCO 2 KOhm Rdc DC Current Set Thick film type 1% CRG0603J10K0 1 THCO 10 KOhm N-Pmos Nmos-Pmos IC STS7C4F30 1 ST D1 SCHOTTKY STPS1L40M 1 ST Rled Limit Led Current Thick film type 1% CRG0603J1K0 2 THCO 1 KOhm Rpull Pull down resistor Thick film type 1% CRG0603J1K0 1 THCO 1 KOhm Rf Resistance Filter Thick film type 1% CRG0603J20K0 1 THCO 20 KOhm Cf Capacitor Filter Ceramic Low ESR GRM188r71e104ka01 1 Murata 100 nF Led Power Led 1.8mm-RED LED L-2060ID 1 KINGBRIGHT Led Charge Led 1.8mm-GREEN LED L-2060GD 1 KINGBRIGHT USB In Connector USB Mini B 54819-0572 1 Molex 6/14 Type AN2407 5 Power dissipation Power dissipation The conditions that cause the STBC08 to reduce charge current through thermal feedback can be approximated by considering the power dissipated in the IC. For high charge currents, the STBC08 power dissipation is approximately: Equation 2 P D = ( V CC – V BAT ) • I BAT where PD is the power dissipated, VCC is the input supply voltage, VBAT is the battery voltage and IBAT is the current charge current. It is not necessary to perform any worst-case power dissipation scenarios because the STBC08 will automatically reduce the charge current to maintain the die temperature at approximately 120° C. However, the approximate ambient temperature at which the thermal feedback begins to protect the IC is: Equation 3 T A = 120° C – P D θ JA Equation 4 T A = 120° C – ( V CC – V BAT ) • I BAT θ JA Example: Consider an STBC08 operating from a 5V wall adapter providing 400mA to a 3.7V Li-Ion battery. The ambient temperature above which the STBC08 will begin to reduce the 400mA charge current is approximately: Equation 5 C T A = 120° C – ( 5V – 3.7V ) • ( 400mA ) • 105° ----- = 42° C W The STBC08 can be used above 42°C, but the charge current will be reduced from 400mA. The approximate current at a given ambient temperature can be calculated: Equation 6 120° C – T A I BAT = ------------------------------------------------( V CC – V BAT ) • θ A Using the previous example with an ambient temperature of 65°C, the charge current will be reduced to approximately: Equation 7 120° C – 65° C I BAT = --------------------------------------------------------- = 282mA C ( 5V – 3.7V ) • 150° ----W Furthermore, the voltage at the PROG pin will change proportionally with the charge current as discussed in Section 9.1: Programming charge current. It is important to remember that STBC08 applications do not need to be designed for worst-case thermal conditions since the IC will automatically reduce power dissipation when the junction temperature reaches approximately 120°C. 7/14 Automatic recharge 6 AN2407 Automatic recharge Once the charge cycle is terminated, the STBC08 continuously monitors the voltage on the BAT pin using a comparator with a 2-ms filter time (tRECHARGE). A charge cycle restarts when the battery voltage falls below 4.05V (which corresponds to approximately 80% to 90% battery capacity). This ensures that the battery is kept at or near a fully-charged condition and eliminates the need for periodic charge cycle initiations. The CHRG output enters a strong pulldown state during recharge cycles. Figure 3. State diagram of a typical charge cycle POWER ON Bat <2.9V TRICKLE CHARGE MODE 1/10TH FULL CURRENT CHRG: PULL-DOWN PROG RECONNECTED OR UVLO CONDITION STOPS SHUTDOWN MODE CHRG: Hi-Z IN UVLO PULL-DOWN OTHERWISE ICC DROPS TO <25µA Bat >2.9V CHARGE MODE FULL CURRENT CHRG: PULL-DOWN Bat >2.9V Prog<100mV PROG FLOATED OR UVLO CONDITION 8/14 STANDBY MODE NO CHARGE CURRENT CHRG: HI-Z PULL-DOWN 2.9V<Vbat<4.05V AN2407 7 CHRG and Power-on status output pins CHRG and Power-on status output pins The POWER ON pin (open drain) is a flag that indicates the presence of the VCC, VUVLO<VCC<7.2V and VCC>VBAT. High impedance indicates that VCC< VUVLO, VCC>7.2V or VCC<VBAT. In this case VCC is insufficient. The CHRG pin (open drain) is a flag that indicates the status of the charge, if the pin is low the device works and the charge is going, when the pin is high impedance the charge is finished (constant Voltage and IPROG/10). Table 3. LEDs status CHRG Low High Low 00 01 High Not used 11 Power-ON The values in Table 3 correspond to the following modes: – 00 is Precharge mode (Trickle Charge mode) or Charge Mode. VCC is more than VUVLO and RPROG is present on the PROG pin. – 01 is Standby mode (completed charge) or Shutdown mode (RPROG not connected). – 11 is Supply (insufficient and unqualified.) Figure 4. Using a microprocessor to determine device state 9/14 USB and wall adapter power 8 AN2407 USB and wall adapter power Although the STBC08 allows charging from a USB port, a wall adapter can also be used to charge Li-Ion batteries. Figure 5 shows an example of how to combine wall adapter and USB power inputs. A P-channel MOSFET is used to prevent back conducting into the USB port when a wall adapter is present and Schottky diode is used to prevent USB power loss through the 1kΩ pull-down resistor. Typically, a wall adapter can supply significantly more current than the 500mA-limited USB port. Therefore, an N-channel MOSFET and an extra program resistor are used to increase the charge current to 850mA when the wall adapter is present. Figure 5. 10/14 Combining wall adapter and USB power AN2407 Charge current 9 Charge current 9.1 Programming charge current The charge current is programmed using a single resistor from the PROG pin to ground. The battery charge current is 1000 times the current out of the PROG pin. The program resistor and the charge current are calculated using the following equations: Equation 8 1.00V R PROG = 1000 • ---------------I BAT The charge current out of the BAT pin can be determined at any time by monitoring the PROG pin voltage using the following equation: Equation 9 V PROG I BAT = -------------------- • 1000 R PROG 9.2 Maximum charge current in temperature Initial conditions: VIN = 4.4V, VBAT = 3.1V, RPROG = 1kΩ and Air flow = 4 l/s. Figure 6. IBAT vs. temperature The 1A battery current set by RPROG is constant in the -40° to 25° C temperature range. For temperatures higher than 25° C, the current is lower due to the thermal limit of the device. 11/14 Board layout Appendix A Figure 7. Figure 8. 12/14 AN2407 Board layout Top component demo board Top layer layout Figure 9. Bottom layer layout AN2407 Revision history Revision history Table 4. Document revision history Date Revision Changes 7-Sept-2006 1 Initial release. 19-Sept-2006 2 Table 2 changed 13/14 AN2407 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein. UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. UNLESS EXPRESSLY APPROVED IN WRITING BY AN AUTHORIZED ST REPRESENTATIVE, ST PRODUCTS ARE NOT RECOMMENDED, AUTHORIZED OR WARRANTED FOR USE IN MILITARY, AIR CRAFT, SPACE, LIFE SAVING, OR LIFE SUSTAINING APPLICATIONS, NOR IN PRODUCTS OR SYSTEMS WHERE FAILURE OR MALFUNCTION MAY RESULT IN PERSONAL INJURY, DEATH, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE. ST PRODUCTS WHICH ARE NOT SPECIFIED AS "AUTOMOTIVE GRADE" MAY ONLY BE USED IN AUTOMOTIVE APPLICATIONS AT USER’S OWN RISK. Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any liability of ST. ST and the ST logo are trademarks or registered trademarks of ST in various countries. Information in this document supersedes and replaces all information previously supplied. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners. © 2006 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com 14/14