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Battery Charger Design (1S):
Key considerations and system design limitations
Miguel Aguirre
October, 2012
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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)
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TI Confidential – Selective Disclosure
Adapter Power Limits
• Adapter Power Limits Today
– Most Smartphones: 5W – 8W
– Most Popular Tablets: 10W – 15W
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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
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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
UVLO
1V/div
IIN
Device hits VINDPM
threshold and input
current is reduced
500mA / div
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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
Programmed Charge
current higher than
adapter capability
STAT
Device hits VINDPM
threshold and input
current is reduced
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
Power-path vs. Non Power-path Topologies
• Non Power-Path Topology
• Power-Path Topology
ICONVERTER
ICHARGE
ISYS
System
Load
System
Load
Charger
ISYS
Converter
IBAT
Charger
IBAT
• 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
BMS Deep Dive 2012
• 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
TI Confidential – Selective Disclosure
9
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)
IIN≈1A
ISYS=0.5A
IBAT=0.5A
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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)
IIN≈1A
ISYS=0.8A
IBAT=0.2A
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VINDPM and DPPM working together
Adapter Voltage Falls due
to Adapter Current Limit
VIN
5V Adapter
rated for 750mA
Input Current Reduced by VINDPM function
to Prevent Adapter from Crashing
IIN
VSYS
750mA Charging
IBAT
ISYS
BMS Deep Dive 2012
750mA Charging
Supplement Mode
1.2A Load Step
TI Confidential – Selective Disclosure
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
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BMS Deep Dive 2012
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Thermal Regulation and Protection Loops
Thermal management functions:
• Regulate IC junction temperature by reducing
PLOSS = (VIN – VBAT) * ICHG
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
VIN
ICHG
Common implementations:
• The IC junction temperature is regulated to a value just
below the maximum operating junction
temperature, 1250C typical
•
ICHG_THRM
The charger is turned off when the Charger IC junction
temperature is excessive, 1500C typical
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Tj = 1250C
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Time
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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
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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)
BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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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)
4A Inductor
3A Inductor
2A Inductor
2mm height
1.2mm height
1.0mm height
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Charge Cycle → No CTO
83mV Overlap
Charge current
reduces too early
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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
BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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Extending Run Time on Power Path Chargers
Only 11 mΩ
Optional External
FET Driver
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
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).
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure
Questions?
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BMS Deep Dive 2012
TI Confidential – Selective Disclosure