Battery Charger Design (1S): Key considerations and system design limitations

1. Battery Charger Design (1S): Key considerations and system design limitations Miguel Aguirre October, 2012

2. Agenda • Single Cell Charger Systems • Input Considerations and Limitations • Topology Options • Pros & Cons of Power Path Architecture • Thermal Issues • Market Trends Needs vs. Limitations • Charge Time Optimizer • Summary and Questions

3. Single Cell Systems Overview • Single Cell • Most Common Solution for Smartphones (1s) and Tablets (1sXp) today • Allows for simple, low voltage design on the system (Max Battery Voltage 4.35V on some lithium based chemistries) • Simple design to charge from a 5V supply as the charger will always operate in step down mode • Multiple cells in parallel allow for longer run times due to extra capacity • This will require a higher charge currents to maintain an acceptable charge time. Charge current will be a function of the current capability of the adapter. 1s configuration 1sXp configuration

4. Input Considerations • How many input connectors will the device have? • Single Input (i.e. Micro-USB, Proprietary Connector) • Multiple Inputs (i.e. Micro-USB and Dock Connector) • How many input sources will the product support? • USB charging only (Max current: 500mA for USB2.0, 900mA for USB3.0) • USB charging and/or adapter into single port • For Micro-USB port, maximum current supported by adapter is 1.8A • USB specifies maximum current of 1.5A • With a limit on the current, changing the input voltage allows you to increase your output current • USB Power Delivery (USBPD) will allow for more power available for charge solutions. Output current change based on input voltage (assume 90% efficiency and 3.6V Battery)

5. Adapter Power Limits • Adapter Power Limits Today • Most Smartphones: 5W – 8W • Most Popular Tablets: 10W – 15W

6. Current Capabilities of Adapters • Power sources have their limits • There are situations where the input power source does not have enough power to supply what the portable device demands • Becoming increasingly important with the standardization of input connectors such as the Micro-USB • Input current limits and Input Voltage Dynamic Power Management (VINDPM) provide the functions needed to solve this problem

7. Voltage Input Dynamic Power Management (VINDPM) • Utilizing full capacity of adapter – VIN Dynamic Power Management (VINDPM) • Loop continuously monitoring the input voltage to the charger • Without VINDPM the device can enter a hiccup mode between power up and “brown-out” condition • When input voltage drops, device will limit the input current Device hits VINDPM threshold and input current is reduced

8. Voltage Input Dynamic Power Management (VINDPM) • Utilizing full capacity of adapter – VIN Dynamic Power Management (VINDPM) • Loop continuously monitoring the input voltage to the charger • Without VINDPM the device can enter a hiccup mode between power up and “brown-out” condition • When input voltage drops, device will limit the input current VIN IVIN VBAT STAT Programmed Charge current higher than adapter capability Device hits VINDPM threshold and input current is reduced

9. Power-path vs. Non Power-path Topologies • Non Power-Path Topology • The system voltage is always equal to the battery voltage • No system startup for deeply discharged batteries • ICHARGE is always split between IBAT and ISYS • ICHARGE must be programmed to the maximum charge current for the battery cell • If ISYS > Termination current, then termination will not occur • IBAT is reduced for any system load • Reduced charge current extends charge time. Safety timers may expire prematurely • Power-Path Topology • ICONVERTER is set to maximize the current from the source. • More available current to system and battery charging for faster charge time • IBAT is set independent of ICONVERTER • For low system loads, ICONVERTER is reduced to maintain proper charge current • IBAT is always known by charger • Accurate termination current • Safety timer extended when charge current is less than programmed value ICONVERTER ISYS ICHARGE ISYS IBAT IBAT

10. Dynamic Power-Path Management (DPPM) • Function that monitors the input current, input voltage and output currents of a Power-Path device and automatically gives priority to the system when the adapter can not support the system load • See following example of DPPM function in a linear charger. Same principle allies for switching chargers. Assume 5W adapter (5V, 1A) ISYS=0.5A IIN≈1A IBAT=0.5A

11. Dynamic Power-Path Management (DPPM) • Function that monitors the input current, input voltage and output currents of a Power-Path device and automatically gives priority to the system when the adapter can not support the system load • See following example of DPPM function in a linear charger. Same principle allies for switching chargers. Assume 5W adapter (5V, 1A) ISYS=0.8A IIN≈1A IBAT=0.2A

12. VINDPM and DPPM working together VIN 5V Adapter rated for 750mA Adapter Voltage Falls due to Adapter Current Limit Input Current Reduced by VINDPM function to Prevent Adapter from Crashing IIN VSYS 750mA Charging 750mA Charging IBAT Supplement Mode 1.2A Load Step ISYS

13. Does VINDPM = DPPM? • No. VINDPM prevents the adapter from hitting a “brown-out” condition. However, the charger will not know how much current is going to the system and how much current is going to the battery. • A charger can have VINDPM and not have Power-path (DPPM) • Charge current and system current is combined and the charger does not know how much current is being delivered only to the battery • DPPM allows the charger to know exactly how much current is going to the battery. • With this information, the charger can reduce the charge current and extend the charging safety timer in the even the system demands higher currents • Which one is better? • Both topologies allow to charge the battery. Non DPPM chargers will require the host to measure exactly how much current goes to the battery for proper termination

14. VIN ICHG Tj = 1250C ICHG_THRM Time Thermal Regulation and Protection Loops PLOSS = (VIN – VBAT) * ICHG • Thermal management functions: • Regulate IC junction temperature by reducing charge current , AND • Turn off the charger when IC junction temperature is excessive • Slow down the safety timers when the charge current is reduced by the thermal loop, avoiding a false safety timer fault • Common implementations: • The IC junction temperature is regulated to a value just below the maximum operating junction temperature, 1250C typical • The charger is turned off when the Charger IC junction temperature is excessive, 1500C typical

15. Factors affecting thermal performance (effects on θJA) • Case Study: Thermal Effect of PCB design on θJA • Device Size: 2.1mm x 2mm, WCSP • High K board (no vias), θJA = 69 C/W • Using 2x2 vias, θJA = 45.4 C/W EIA/JESD 51-1 Standard

16. Product Thickness and Battery Size: Smartphones HTC One X: 1800mAh, 8.9mm Samsung Galaxy S3: 2100mAh, 8.6mm Samsung Galaxy Note: 2500mAh, 10.1mm HTC One X Battery → 4.4mm thick Galaxy S3 Battery → 5mm thick Galaxy Note Battery → 6mm thick Programmed Charge current higher than adapter capability Device hits VINDPM threshold and input current is reduced Source: TechInsights Teardowns (web)

17. Working around tradeoffs… • Increasing Battery Size → Larger Charge Current • Decreasing product thickness → Thinner Inductors → Lower Charge Currents • 2A – 3A charge current provides sweet spot of short charge times with acceptable inductor sizes • Design for 1.5uH converter stability. Tradeoff between efficiency and inductor size • Focus on charge time improvements (i.e. Charge Time Optimizer Feature) 3A Inductor 1.2mm height 2A Inductor 1.0mm height 4A Inductor 2mm height

18. Charge Cycle → No CTO 83mV Overlap Charge current reduces too early

19. Charge Time Optimizer in Action – bq2426x Charge Time Optimizer Sharp handoff of CC and CV. Approximately 6mV overlap – Best in industry Reduces charge time! For ITERM = 50mA → Total Charge Time ~4:30 hrs For ITERM = 250mA → Total Charge Time ~ 3:50 hrs For ITERM = 500mA (<0.1C) → Total Charge Time ~ 3:10 hrs

20. Extending Run Time on Power Path Chargers Only 11 mΩ Optional External FET Driver

21. Summary • Charger solutions greater than 3A on smartphones increases the thickness of the design. • Thermal management is a problem with charge currents greater than 3A. Not enough board space to extract the heat generated on the charger. • Focus on reducing charge times with Charge Time Optimizer on newer TI chargers (bq2425x, bq2426x) • Increasing run time by reducing battery discharge path impedance (11 mΩ on bq2426x).

22. Questions?