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LT3484-1 Datasheet(PDF) 7 Page - Linear Technology |
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LT3484-1 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LT3484-0/LT3484-1/LT3484-2 7 3484012f The CHARGE pin gives full control of the part to the user. The charging can be halted at any time by bringing the CHARGE pin low. Only when the final output voltage is reached will the DONE pin go low. Figure 2 shows these various modes in action. When CHARGE is first brought high, charging commences. When CHARGE is brought low during charging, the part goes into shutdown and VOUT no longer rises. When CHARGE is brought high again, charging resumes. When the target VOUT voltage is reached, the DONE pin goes low and charging stops. Finally the CHARGE pin is brought low again so the part enters shutdown and the DONE pin goes high. Both VBAT and VIN have undervoltage lockout (UVLO). When one of these pins goes below its UVLO voltage, the DONE pin goes low. With an insufficient bypass capacitor on VBAT or VIN , the ripple on the pin is likely to activate the UVLO and terminate the charge. The application diagrams suggest values adequate for most applications. OPERATIO The LT3484-0/LT3484-1/LT3484-2 are designed to charge photoflash capacitors quickly and efficiently. The opera- tion of the part can be best understood by referring to Figure 1. When the CHARGE pin is first driven high, a one shot sets both SR latches in the correct state. The power NPN device, Q1, turns on and current begins ramping up in the primary of transformer T1. Comparator A1 monitors the switch current and when the peak current reaches 1.4A (LT3484-0), 1A(LT3484-2) or 0.7A (LT3484-1), Q1 is turned off. Since T1 is utilized as a flyback transformer, the flyback pulse on the SW pin will cause the output of A3 to be high. The voltage on the SW pin needs to be at least 40mV higher than VBAT for this to happen. During this phase, current is delivered to the photoflash capacitor via the secondary and diode D1. As the second- ary current decreases to zero, the SW pin voltage will begin to collapse. When the SW pin voltage drops to 40mV above VBAT or lower, the output of A3 (DCM Comparator) will go low. This fires a one shot which turns Q1 back on. This cycle will continue to deliver power to the output. Output voltage detection is accomplished via R2, R1, Q2, and comparator A2 (VOUT Comparator). Resistors R1 and R2 are sized so that when the SW voltage is 31.5V above VIN, the output of A2 goes high which resets the master latch. This disables Q1 and halts power delivery. NPN transistor Q3 is turned on pulling the DONE pin low, indicating that the part has finished charging. Power delivery can only be restarted by toggling the CHARGE pin. LT3484-2 VIN = 3.6V COUT = 50µF VOUT 100V/DIV VCHARGE 5V/DIV VDONE 5V/DIV 1s/DIV 3484 F02 Figure 2. Halting the Charging Cycle with the CHARGE Pin APPLICATIO S I FOR ATIO Choosing the Right Device (LT3484-0/LT3484-1/LT3484-2) The only difference between the three versions of the LT3484 is the peak current level. For the fastest possible charge time, use the LT3484-0. The LT3484-1 has the lowest peak current capability, and is designed for appli- cations that need a more limited drain on the batteries. Due to the lower peak current, the LT3484-1 can use a physi- cally smaller transformer. The LT3484-2 has a current limit in between that of the LT3484-0 and the LT3484-1. Transformer Design The flyback transformer is a key element for any LT3484-0/LT3484-1/LT3484-2 design. It must be de- signed carefully and checked that it does not cause exces- sive current or voltage on any pin of the part. The main parameters that need to be designed are shown in Table 1. The first transformer parameter that needs to be set is the turns ratio N. The LT3484-0/LT3484-1/LT3484-2 accom- plish output voltage detection by monitoring the flyback waveform on the SW pin. When the SW voltage reaches 31.5V higher than the VBAT voltage, the part will halt power delivery. Thus, the choice of N sets the target output |
Similar Part No. - LT3484-1_15 |
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Similar Description - LT3484-1_15 |
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