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SI9731 Datasheet(PDF) 8 Page - Vishay Siliconix |
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SI9731 Datasheet(HTML) 8 Page - Vishay Siliconix |
8 / 11 page www.vishay.com 8 Document Number: 71321 S09-2249-Rev. C, 26-Oct-09 Vishay Siliconix Si9731 DETAIL OPERATIONAL DESCRIPTION Si9731 is a chemistry independent battery charger designed for use with a system processor. For example, Si9731 can be integrated within a cellular phone whereby the fast charge and trickle (slow) charge modes can be software controlled by the DSP. The device is designed to charge 1-cell Li-ion or 1-cell to 3-cell NiCd/NiMH batteries. A regulated or unregulated external dc power source such as a wall adapter rated at typically 4.5 V to 12 V is connected to Si9731’s VCHARGER input pin. Note that a typical low cost wall adaptor is comprised of a transformer, bridge rectifier and a reservoir capacitor. The wall adaptor’s output voltage decreases linearly with increase in output current. When Si9731 is fast charging the battery, the wall adaptor’s output voltage tracks the battery voltage plus the voltage drop across Q1 (charging current times MOSFET Q1’s RDS(on), see Figure 1). The key features of Si9731 are described below. Since the under voltage lock out (UVLO) point of Si9731 is 2.6 V (typical), it is essential to keep the charger voltage above this level under all conditions, especially for fast charging of single cell NiCd/NiMH. One simple solution is adding a external resistor between VBAT+ pin and the battery, which creates extra voltage drop to elevate the charger voltage. The value of the resistor is affected by the output V-I characteristic of the ac charger. Trickle Charge The charge path is via N-Channel MOSFETs Q1 or Q2 (see applications circuit of Figure 1). Si9731 defaults to trickle (slow) charge mode if the battery voltage is too low to power the main processor. With the main processor unable to drive the MAINCHARGEEN pin as well as the TRICKLECHARGEEN pins, Q1 is turned "OFF" preventing fast charging. Meanwhile N-Channel MOSFET Q2 turns "ON" and establishes a trickle charge path from the external power source VCHARGER to the battery. The trickle charge current is set by an external current limiting resistor, Rext, and is approximately ITRICKLE = (VCHARGER - VBAT+)/Rext. Once the battery voltage charges up to minimum battery operating voltage 3.24 V, the internal latch is triggered and the CHARGERPOWER_ON output changes state to wake up the processor. The processor is now able to disable trickle charge mode by taking the TRICKLECHARGEEN pin high while taking control of fast charging via the MAINCHARGEEN pin. Fast Charge Fast charging is accomplished by the low "ON" resistance MOSFET, Q1. The application microprocessor is able to "Pulse Charge" the battery via the MAINCHARGEREN control input of Si9731. The processor monitors the battery voltage via the system A/D converter and varies the pulse charging duty cycle accordingly to maintain fast charging. Note that even though charging current may be sufficiently high, pulse charging with short "ON" time and long "OFF" time ensures that heat generation due to thermal heating is reduced. In the case of NiCd or NiMH batteries, one of several charge termination schemes may be used to terminate charge. For example, the processor may disable fast charging by sensing ΔV or dV/dt at the V BAT+ output or by monitoring the temperature differential ΔT of the battery. Following fast charge, trickle charge may be enabled to "top off" the battery. When charging a 1-cell Li-ion battery, fast charging will operate in two modes, constant current mode followed by constant voltage mode. In the constant current mode, a discharged Li-ion battery is charged with constant current available from the external dc source. The MOSFET pass transistor (Q1) may be pulsed "ON" and "OFF" at varying duty cycle by the control signal present at the MAINCHARGEEN input pin. Once the battery voltage reaches it’s termination voltage of 4.1 V or 4.2 V (depending on the connection of the 4.1 VTAP), Si9731 may be placed in the "Constant Voltage" charging mode by taking the CVMODE pin high. Taking CVMODE pin high disables trickle charging and enables the internal battery voltage divider by turning ON Q5. Then the error amplifier will compare divided VBAT+ voltage against an internal precision 1.3 V bandgap reference voltage (see Figure 1). The output of the error amplifier drives the pass transistor Q1 to maintain VBAT+ at the regulated termination voltage. This operation is same as a linear regulator. True Load Disconnect Both the fast charge FET (Q1 in Figure 1) and trickle charge FET (Q2 in Figure 1) incorporate a floating body diode. In their "OFF" state both FETs block current bidirectionally. Note that because of the reverse blocking switches, a Schottky diode in series with the external VCHARGER power supply is not required. |
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