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LTC4009CUF-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC4009CUF-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  11 4009fb LTC4009 LTC4009-1/LTC4009-2 OPERATION Overview The LTC4009 is a synchronous step-down (buck) current mode PWM battery charger controller. The maximum charge current is programmed by the combination of a charge current sense resistor (RSENSE), matched input resistors (RIN, Figure 1), and a programming resistor (RPROG) between the PROG and GND pins. Battery volt- age is programmed either with an external resistor divider between FBDIV and GND (LTC4009) or two digital battery voltage select pins (LTC4009-1/LTC4009-2). In addition, the PROG pin provides a linearized voltage output of the actual charge current. The LTC4009 family does not have any built-in charge termination and is flexible enough for charging any type of battery chemistry. These are building block ICs intended for use with an external circuit, such as a microcontroller, capable of managing the entire algorithm required for the specific battery being charged. Each member of the LTC4009 family features a shutdown input and various state indicator outputs, allowing easy and direct management by a wide range of external (digital) charge controllers. Due to the popularity of rechargeable lithium-ion chemistries, the LTC4009-1 and LTC4009-2 also offer internal precision resistors that can be digitally selected to produce one of four preset output voltages for simplified design of those charger types. Shutdown The LTC4009 remains in shutdown until DCDIV exceeds 1.2V, and SHDN is driven above 1.4V. In shutdown, current drain from the battery is reduced to the lowest possible level, thereby increasing standby time. When in shutdown, the ITH pin is pulled to GND and the CHRG, ICL, FET gate drivers and INTVDD output are all disabled. The ACP status output indicates sensed adapter input voltage during all LTC4009 states. Charging can be stopped at any time by forcing SHDN below 300mV. Soft-Start Exiting the shutdown state enables the charger and releases the ITH pin. When enabled, switching will not begin until CLP exceeds BAT by 100mV and ITH exceeds a threshold that assures initial current will be positive (about 5% to 25% of the maximum programmed current). To limit inrush current, soft-start delay is created with the compensation values used on the ITH pin. Longer soft-start times can be realized by increasing the filter capacitor on ITH, if reduced loop bandwidth is acceptable. The actual charge current at the end of soft-start will depend on which loop (current, voltage or adapter limit) is in control of the PWM. If this current is below that required by the ITH start-up threshold, the resulting charge current transient duration depends on loop compensation but is typically less than 100μs. Bulk Charge When soft-start is complete, the LTC4009 begins sourc- ing the current programmed by the external components connected to CSP, CSN and PROG. Some batteries may require a small conditioning trickle current if they are heavily discharged. As shown in the Applications Information sec- tion, the LTC4009 can address this need through a variety of low current circuit techniques on the PROG pin. Once a suitable cell voltage has been reached, charge current can be switched to a higher, bulk charge value. End-of-Charge and CHRG Output As the battery approaches the programmed output volt- age, charge current will begin to decrease. The open- drain CHRG output can indicate when the current drops to 10% of its programmed full-scale value by turning off the strong pull-down (open-drain FET) and turning on a weak 25μA pull-down current. This weak pull-down state is latched until the part enters shutdown or the sensed current rises to roughly C/6. C/10 indication will not be set if charge current has been reduced due to adapter input current limiting or DCIN/battery overvoltage. As the charge current approaches 0A, the PWM continues to operate in full continuous mode. This avoids generation of audible noise, allowing bulk ceramic capacitors to be used in the application. |
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