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ISL6292-2CR3Z-T Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6292-2CR3Z-T Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 20 page  11 FN9105.8 November 14, 2006 maximum power the IC is capable of dissipating is dependent on the thermal impedance of the printed-circuit board (PCB). Figure 17 shows, with dotted lines, two cases that the charge currents are limited by the maximum power dissipation capability due to the thermal foldback. When using a current-limited adapter, the thermal situation in the ISL6292 is totally different. Figure 18 shows the typical charge curves when a current-limited adapter is employed. The operation requires the IREF to be programmed higher than the limited current ILIM of the adapter, as shown in Figure 18. The key difference of the charger operating under such conditions occurs during the CC mode. The Block Diagram, Figure 16, aids in understanding the operation. The current loop consists of the current amplifier CA and the sense MOSFET QSEN. The current reference IR is programmed by the IREF pin. The current amplifier CA regulates the gate of the sense MOSFET QSEN so that the sensed current ISEN matches the reference current IR. The main MOSFET QMAIN and the sense MOSFET QSEN form a current mirror with a ratio of 100,000:1, that is, the output charge current is 100,000 times IR. In the CC mode, the current loop tries to increase the charge current by enhancing the sense MOSFET QSEN, so that the sensed current matches the reference current. On the other hand, the adapter current is limited, the actual output current will never meet what is required by the current reference. As a result, the current error amplifier CA keeps enhancing the QSEN as well as the main MOSFET QMAIN, until they are fully turned on. Therefore, the main MOSFET becomes a power switch instead of a linear regulation device. The power dissipation in the CC mode becomes: where rDS(ON) is the resistance when the main MOSFET is fully turned on. This power is typically much less than the peak power in the traditional linear mode. The worst power dissipation when using a current-limited adapter typically occurs at the beginning of the CV mode, as shown in Figure 18. The Equation 1 applies during the CV mode. When using a very small PCB whose thermal impedance is relatively large, it is possible that the internal temperature can still reach the thermal foldback threshold. In that case, the IC is thermally protected by lowering the charge current, as shown with the dotted lines in the charge current and power curves. Appropriate design of the adapter can further reduce the peak power dissipation of the ISL6292. See the Application Information section for more information. Figure 19 illustrates the typical signal waveforms for the linear charger from the power-up to a recharge cycle. More detailed Applications Information is given below. Applications Information Power on Reset (POR) The ISL6292 resets itself as the input voltage rises above the POR rising threshold. The V2P8 pin outputs a 2.8V voltage, the internal oscillator starts to oscillate, the internal timer is reset, and the charger begins to charge the battery. The two indication pins, STATUS and FAULT, indicate a LOW and a HIGH logic signal respectively. Figure 19 illustrates the start up of the charger between t0 to t2. The ISL6292 has a typical rising POR threshold of 3.4V and a falling POR threshold of 2.4V. The 2.4V falling threshold guarantees charger operation with a current-limited adapter to minimize the thermal dissipation. Charge Cycle A charge cycle consists of three charge modes: trickle mode, constant current (CC) mode, and constant voltage (CV) mode. The charge cycle always starts with the trickle mode until the battery voltage stays above VMIN (2.8V typical) for 15 consecutive cycles of the internal oscillator. If the battery voltage drops below VMIN during the 15 cycles, the 15-cycle counter is reset and the charger stays in the trickle mode. The charger moves to the CC mode after verifying the battery voltage. As the battery-pack terminal voltage rises to the final charge voltage VCH, the CV mode begins. The terminal voltage is regulated at the constant VCH in the CV mode and the charge current is expected to decline. After the charge current drops below IMIN (programmable for the 4x4 and 5X5 package and programmed to 1/10 of IREF for the 3x3 package, see End-of-Charge Current for more detail), the ISL6292 indicates the end-of-charge (EOC) with the STATUS pin. The charging actually does not terminate until the internal timer completes its length of TIMEOUT in order to bring the battery to its full capacity. Signals in a charge cycle are illustrated in Figure 19 between points t2 to t5. P CH R DS ON () ICHARGE 2 ⋅ = (EQ. 2) FIGURE 19. OPERATION WAVEFORMS VIN V2P8 STATUS FAULT VBAT I CHARGE 15 Cycles to 1/8 TIMEOUT 15 Cycles POR Threshold 2.8V V MIN V RECHRG t 0 t 1 t2 t3 t 4 t 5 t 6 t7 t 8 Charge Cycle Charge Cycle I MIN ISL6292 |
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