BQ24707 Datasheet, PDF (19/36 Pages) Texas Instruments – 1-4 Cell Li+ Battery SMBus Charge Controller With Independent Comparator and Advanced Circuit Protection

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bq24707
bq24707A
SLUSA78B – JULY 2010 – REVISED MARCH 2011
• ACOK is valid (See the Adapter Detect and ACOK Output section for details);
• V_SRN does not exceed BATOVP threshold;
• IC temperature does not exceed TSHUT threshold;
• Not in ACOC condition (see the Input Over Current Protection (ACOC) section for details);
One of the following conditions stops on-going charging:
• Charge is inhibited via SMBus (ChargeOption() bit[0] = 1);
• ILIM pin voltage lower than 75mV;
• One of three regulation limit DACs is set to 0 or out of range;
• ACOK is pulled high (see the Adapter Detect and ACOK Output section for details);
• V_SRN exceeds BATOVP threshold;
• TSHUT IC temperature threshold is reached;
• ACOC is detected (see the Input Over Current Protection (ACOC) section for details);
• Short circuit is detected (see the Inductor Short, MOSFET Short Protection section for details);
• Watchdog timer expires if watchdog timer is enabled (see the Charger Timeout section for details);
Automatic Internal Soft-Start Charger Current
Every time charge is enabled, the charger automatically applies soft-start on charge current to avoid any
overshoot or stress on the output capacitors or the power converter. The charge current starts at 128mA, and the
step size is 64mA in CCM mode for a 10mΩ current sensing resistor. Each step lasts around 240µs in CCM
mode, until it reaches the programmed charge current limit. No external components are needed for this function.
During DCM mode, the soft-start current step size is larger and each step lasts for a longer time period due to
the intrinsic slow response of DCM mode.
High Accuracy Current Sense Amplifier
As an industry standard, a high accuracy current sense amplifier (CSA) is used to monitor the input current or the
charge current, selectable via SMBus (ChargeOption() bit[5] = 0 selects the input current, bit[5] = 1 selects the
charge current) by the host. The CSA senses voltage across the sense resistor by a factor of 20 through the
IOUT pin. Once VCC is above UVLO and ACDET is above 0.6V, CSA turns on and the IOUT output becomes
valid. To lower the voltage on current monitoring, a resistor divider from IOUT to GND can be used and accuracy
over temperature can still be achieved.
A 100pF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional
RC filter is optional, if additional filtering is desired. Note that adding filtering also adds additional response delay.
Charge Timeout
The IC includes a watchdog timer to terminate charging if the charger does not receive a write ChargeVoltage()
or write ChargeCurrent() command within 175s (adjustable via ChargeOption() command). If a watchdog timeout
occurs all register values stay unchanged but charge is suspended. Write ChargeVoltage() or write
ChargeCurrent() commands must be re-sent to reset the watchdog timer and resume charging. The watchdog
timer can be disabled, or set to 44s, 88s, or 175s via a SMBus command (ChargeOption() bit[14:13]). After
watchdog timeout write ChargeOption() bit[14:13] to disable the watchdog timer also resume charging.
Converter Operation
The synchronous buck PWM converter uses a fixed frequency voltage mode control scheme and internal type III
compensation network. The LC output filter generates the following characteristic resonant frequency:
1
¦_o =
2p L_oC_o
(3)
The resonant frequency f_o is used to determine the compensation to ensure there is sufficient phase margin and
gain margin for the target bandwidth. The LC output filter should be selected to generate a resonant frequency of
10–20 kHz nominal for the best performance. The suggested component values per charge current with a
750kHz default switching frequency is shown in Table 7.
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