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BQ24725ARGRT Datasheet(PDF) 19 Page - Texas Instruments |
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BQ24725ARGRT Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 47 page  o o o 1 = 2 L C p ¦ bq24725A www.ti.com SLUSAL0A – SEPTEMBER 2011 – REVISED AUGUST 2014 Device Functional Modes (continued) 8.4.8 Charge Timeout The bq24725A 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 keep unchanged but charge is suspended. Write ChargeVoltage() or write ChargeCurrent() commands must be re-sent to reset watchdog timer and resume charging. The watchdog timer can be disabled, or set to 44s, 88s or 175s via SMBus command (ChargeOption() bit[14:13]). After watchdog timeout write ChargeOption() bit[14:13] to disable watchdog timer also resume charging. 8.4.9 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 gives a characteristic resonant frequency (1) The resonant frequency fo 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 give a resonant frequency of 10–20 kHz nominal for the best performance. Suggest component value as charge current of 750kHz default switching frequency is shown in Table 1. Ceramic capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is applied across a ceramic capacitor, as on the output capacitor of a charger. The effect may lead to a significant capacitance drop, especially for high output voltages and small capacitor packages. See the manufacturer's data sheet about the performance with a dc bias voltage applied. It may be necessary to choose a higher voltage rating or nominal capacitance value in order to get the required value at the operating point. Table 1. Suggest Component Value as Charge Current of Default 750kHz Switching Frequency Charge Current 2A 3A 4A 6A 8A Output Inductor Lo (µH) 6.8 or 8.2 5.6 or 6.8 3.3 or 4.7 3.3 2.2 Output Capacitor Co (µF) 20 20 20 30 40 Sense Resistor (m Ω) 10 10 10 10 10 The bq24725A has three loops of regulation: input current, charge current and charge voltage. The three loops are brought together internally at the error amplifier. The maximum voltage of the three loops appears at the output of the error amplifier EAO. An internal saw-tooth ramp is compared to the internal error control signal EAO (see Figure 10) to vary the duty-cycle of the converter. The ramp has offset of 200mV in order to allow 0% duty- cycle. When the battery charge voltage approaches the input voltage, EAO signal is allowed to exceed the saw-tooth ramp peak in order to get a 100% duty-cycle. If voltage across BTST and PHASE pins falls below 4.3V, a refresh cycle starts and low-side n-channel power MOSFET is turned on to recharge the BTST capacitor. It can achieve duty cycle of up to 99.5%. 8.4.10 Continuous Conduction Mode (CCM) With sufficient charge current the bq24725A’s inductor current never crosses zero, which is defined as continuous conduction mode. The controller starts a new cycle with ramp coming up from 200mV. As long as EAO voltage is above the ramp voltage, the high-side MOSFET (HSFET) stays on. When the ramp voltage exceeds EAO voltage, HSFET turns off and low-side MOSFET (LSFET) turns on. At the end of the cycle, ramp gets reset and LSFET turns off, ready for the next cycle. There is always break-before-make logic during transition to prevent cross-conduction and shoot-through. During the dead time when both MOSFETs are off, the body-diode of the low-side power MOSFET conducts the inductor current. During CCM mode, the inductor current is always flowing and creates a fixed two-pole system. Having the LSFET turn-on keeps the power dissipation low, and allows safely charging at high currents. Copyright © 2011–2014, Texas Instruments Incorporated Submit Documentation Feedback 19 Product Folder Links: bq24725A |
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