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Texas Instruments Battery Charger Overload Protection in Boost Mode Operation Application notes
Application Report
SLUA832 – August 2017
Battery Charger Overload Protection in Boost Mode
Operation
Gabriel Xu, Eric Zhao
ABSTRACT
1
2
3
4
5
Contents
Introduction ...................................................................................................................
Average and Cycle-by-Cycle Overcurrent Protection ...................................................................
OTG Mode Overload Protection Based on Average Current ..........................................................
Persistent Overload Reaction ..............................................................................................
Summary ......................................................................................................................
2
2
3
6
7
List of Figures
1
Switch Charger Operating in Boost Mode ................................................................................ 2
2
Q1 FET Equivalent Circuit in LDO Mode ................................................................................. 3
3
bq24157 LDO Mode Protection ............................................................................................ 3
4
Hiccup Mode Overload Protection of bq24296 .......................................................................... 4
5
Hiccup Mode Overload Protection of bq24296 (Zoomed In) ........................................................... 4
6
V-I Operating Characteristic of CC and CV Mode Overload Protection .............................................. 5
7
CC and CV Mode Overload Protection of bq2560x ..................................................................... 5
8
Equivalent Output Circuit in CC Mode .................................................................................... 6
9
Active VDPM Function in CC Mode ....................................................................................... 6
List of Tables
1
Comparison of Three Different Overload Protection Features ......................................................... 7
Trademarks
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1
Introduction
1
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Introduction
Many personal portable devices that are battery-powered are used to power external accessories. At the
time of this writing, smartphones or tablets are the most popular applications used to power up external
keyboards or USB storage devices with USB on-the-go (OTG) functionality. Another very similar
application is the power bank. A power bank battery is charged when the wall power is available, after
which it is used to charge external accessories when required. The most common topology of a battery
charger is the step-down buck converter during the charging stage. When a battery is discharging to
power up the external accessories, power flow is reversed and the converter operates as a boost
converter. Depending on the power flows, operating the converter in either buck or boost mode reduces
the total solution size and cost. The overload protection scheme is crucial to ensure that the battery
charger and battery operate safely. This application note discusses the overload protection schemes of a
battery charger operating in OTG boost mode. In addition to the cycle-by-cycle current limit, the average
output current protections are implemented in battery chargers. Three different boost mode overload
protections are analyzed based on the average output current and the implementation. This application
note also discusses and compares the advantages of each scheme.
2
Average and Cycle-by-Cycle Overcurrent Protection
Most chargers from Texas Instruments such as the bq2560x, bq2419x, and bq2589x provide different
levels of overcurrent protection depending on either the average current or switching cycle-by-cycle
current (see Figure 1) The charger senses the current flowing through the Q1 field-effect transistor (FET),
which is the average current of the boost output. Alternatively, the cycle-by-cycle current limit is
implemented by sensing the current flowing through the Q3 FET.
POUT = VBUS × IOUT
Q2
Q1
Inductor
PMID
Q4
Monitor Average Output
Current in Q1
Q3
Switching Charger
In Boost Mode
BAT
Monitor Inductor
Peak Current In Q3
Figure 1. Switch Charger Operating in Boost Mode
2
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OTG Mode Overload Protection Based on Average Current
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3
OTG Mode Overload Protection Based on Average Current
OTG boost output overloading can potentially cause both the external accessary and the charger to
overheat; therefore, different layers of overload protection schemes are implemented in a battery charger.
The overload detection is generally based on the average current and the most common schemes are:
LDO mode, hiccup mode, and constant current-mode protection.
3.1
LDO Mode
In low-dropout regulator (LDO) mode, the Q1 FET operates in the saturation region and functions like a
variable resistor. When the average output current hits the preset threshold, the overload condition is
detected. The control loop limits the output current in the preset value ILDO by changing the Q1 conduction
resistance (see Figure 2).
ILDO
VBUS
PMID
RDS
ROUT
Equivalent
resistance
Q1
Figure 2. Q1 FET Equivalent Circuit in LDO Mode
The overload output power and Q1 FET losses can be calculated in Equation 1 and Equation 2 as:
2
POVERLOAD = VBUS ´ ILDO = ILDO
´ ROUT
(1)
PLoss _ Q1 = (VPMID - ROUT ) ´ ILDO
(2)
Equation 1 indicates that the LDO mode can significantly limit the output power with a simple
implementation to prevent the device from thermal damage, even with an output short circuit. The major
drawback is that the voltage drop in Q1 FET is high in a severe overload condition and leads to a Q1 FET
high-power dissipation. This high-power dissipation has a risk of thermal damage and battery energy loss.
The time in which Q1 is operated in LDO mode must be controlled to prevent thermal damage of the Q1
FET. Figure 3 shows the overload protection waveform of the bq24157 LDO mode. The charger operates
in LDO mode for 32 ms until the input current hits the 1-A threshold, after which the converter shuts down.
LDO MODE
NORMAL PHASE
CONVERTER SHUNT DOWN
ILDO
Figure 3. bq24157 LDO Mode Protection
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OTG Mode Overload Protection Based on Average Current
3.2
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Hiccup Mode
To reduce the power dissipation of the Q1 FET, the hiccup mode turns off the Q1 FET when the overload
condition has been detected. The Q1 FET remains turned off for a certain period of time before it turns on
again. If the overload condition still exists after Q1 FET has been turned on, the Q1 FET repeats this
process repeatedly. Figure 4 and Figure 5 show the hiccup overload protection waveform. The overload
output power can be calculated in Equation 3 as:
POVERLOAD
æ
æ V2 ö ö
ç t Q1_ ON ´ ç BUS ÷ ÷
ç ROUT ÷ ÷
ç
è
øø
=è
t Q1_ ON + t Q1_OFF
(
)
(3)
The tQ1_ON must be kept much shorter than tQ1_OFF to limit the overload output power. Take bq24296 as an
example: tQ1_ON = 260 µs, tQ1_OFF = 32 ms, and the input current overload threshold is 1.5-A (default).
tQ1_ON = 260 µs
tQ1_OFF = 32 ms
Figure 4. Hiccup Mode Overload Protection of bq24296
Q1 OFF
Q1 ON
Q1 OFF
Figure 5. Hiccup Mode Overload Protection of bq24296 (Zoomed In)
The hiccup mode can limit the overload output power with less loss in the Q1 FET and provide selfrecovery function.
4
Battery Charger Overload Protection in Boost Mode Operation
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3.3
Constant Current (CC) Mode
Figure 6 shows the CC mode operating with the V-I characteristic. When the overload condition has been
detected, the converter output characteristic shifts from the equivalent voltage source to the equivalent
current source and keeps the output current constant (CC mode). In the meantime, the Q1 FET
completely turns on consistently during the CC mode. Therefore, the voltage drop in the Q1 FET is low
regardless of the overload level and does not cause extra loss in the Q1 FET. In CC mode, when the
output voltage decreases to hit the battery voltage, the converter can no longer maintain operation in
boost mode, so it shuts down. Figure 7 shows the bq2560x CC and CV mode overload protection
waveform.
6V
CV MODE
5V
CC MODE
4V
VBAT
3V
2V
VBOOST_BAT (MIN)
1V
0V
0.25 A
0.5 A
0.75 A
1A
1.25 A
1.5 A
Figure 6. V-I Operating Characteristic of CC and CV Mode Overload Protection
CV Mode
CC Mode
Converter
Shunt
Down
Figure 7. CC and CV Mode Overload Protection of bq2560x
The CC mode overload output power can be calculated in Equation 4 as:
2
POVERLOAD = ICC
´ ROUT
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(4)
Battery Charger Overload Protection in Boost Mode Operation
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5
Persistent Overload Reaction
4
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Persistent Overload Reaction
For a persistent overcurrent condition, all three protection schemes turn off the converter to eventually
achieve protection. The procedures and result are different. In general, the better solution is to inform the
external device that the power supply is in an overloaded condition and then make it reduce the charging
power rather than to turn off the converter directly. The device is typically being charged and does not
recognize the exact output capability of the power supply. To prevent overload and break down the power
supply, the VDPM) of the charger reduces the charging power when the input voltage drops to a certain
threshold. See Dynamic power management for faster, more efficient battery charging (SLYT546) for more
details.
ILOAD
ICC
Load
VBUS
IC
Charger in
Boost Mode
C
Figure 8. Equivalent Output Circuit in CC Mode
The time converter that operates in CC mode, tcc, is determined by ILOAD (see Figure 8); therefore, for
severe overloaded conditions, such as output short-circuit conditions, the converter is very quickly turned
OFF. For a slightly overloaded condition, the output voltage dropping is slower and leaves enough time for
the VDPM to function. Increased capacitance of the bus cap can provide more VDPM response time
margin to handle more severe overload conditions with higher battery voltage. Figure 9 shows the
response of the bq25601 active VDPM function in CC mode.
VINDPM
ILOAD
IBUS
Figure 9. Active VDPM Function in CC Mode
6
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Summary
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5
Summary
Each three kinds of overload protection can limit the output power efficiently, even in a short-circuit
condition. As Table 1 shows, the LDO mode is a simple and low-cost solution; however, LDO mode can
result in high losses and power dissipation in the Q1 FET before the converter shuts down. The Q1 FET
loss in hiccup mode is smaller than in LDO mode and provides a self-recovery function. CC mode does
not cause extra power loss in the Q1 FET and can enable VDPM functionality.
Table 1. Comparison of Three Different Overload Protection Features
FEATURE
LDO MODE
HICCUP MODE
CC/CV MODE
Limit overload power
✓
✓
✓
Without extra Q1 FET loss
X
✓
✓
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