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TK65428MTL Datasheet(PDF) 11 Page - TOKO, Inc |
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TK65428MTL Datasheet(HTML) 11 Page - TOKO, Inc |
11 / 28 page September 1999 TOKO, Inc. Page 11 TK654xx ADVANCED INFORMATION THEORY OF OPERATION (CONT.) cycle will be dependent upon the duration of the preceding "on" cycle and the sensed input and the output voltages. The approximate duration of the "off" cycle can be predicted as follows: T OFF = TON X (VIN - VOUT) / VOUT During the majority of the "off" cycle, a power switch is turned on. The turn-on point of the switch is synchronized with the initiation of the "off" cycle. The turn-off point is near the end of the "off" cycle. The addition of this switch reduces the voltage drop across the external rectifying device and provides significant improvements in converter efficiency. BATTERY MONITOR SECTION In a typical voltage monitor, which uses an external resistive divider for setting the voltage monitor threshold, the input bias current to the monitor pin is essentially zero. In this type of scenario, the voltage on the monitor input would be a resistively divided version of the battery voltage. The Battery Monitor of the TK654xx introduces a small feedback current (-I LBI) which introduces a “plateau” into the transfer characteristics between the battery voltage and the voltage monitoring pin. The width of this plateau is dependent upon the current range of the feedback current (-I LBI) and the values of the external resistor network. Figure 2 illustrates the typical relationship between the battery voltage (V IN), the feedback current (-ILBI) and the voltage on the monitoring pin (V LBI). In selecting a resistor divider network, there are typically two degrees of freedom when selecting values. The first criteria in selecting the divider is the ratio of the two resistors. Selecting the ratio defines the upper threshold of the voltage monitor. The second degree of freedom when selecting the resistor divider is the absolute resistance values. This second degree of freedom can be utilized to set a secondary monitoring threshold (V OFF) lesser than the first. Typically, when the battery voltage is relatively high, the voltage on the LBI input pin will be a resistively divided version of the battery voltage. As the battery voltage drops, the voltage on the V LBI pin will drop proportionately. When the voltage on the LBI input pin passes through approximately 1.22 V, the output of the first comparator (LBO) will transition from a high to a low state and the synchronous rectifier N-channel MOSFET will stop being switched. There is about 50 mV of hysteresis between the LBO trip point and its reset point. Shortly after the LBO output has been asserted, as the input voltage continues to drop, the magnitude of the current out of the V LBI pin will begin to flow and proportionately increase as the input voltage decreases. An op-amp feedback loop internal to the Battery Monitor of the TK654xx will attempt to maintain the voltage on the V LBI pin at a constant value of approximately 1.22 V (thus, the plateau). As the battery voltage continues to drop, there comes a point where the feedback current stops increasing at about 720 nA. At this point, the voltage on the LBI pin will resume a proportional drop with the input voltage and the TK654xx converter will turn off. For details on how to properly select the resistor divider, refer to the “Design Considerations” section. FIGURE 2: BATTERY MONITOR OPERATION GRAPHS VLBI LBO OFF 1.22 V VLBO VOFF VIN 720 nA 0 nA -ILBI |
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