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LT1304-5 Datasheet(PDF) 6 Page - Linear Technology |
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LT1304-5 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LT1304/LT1304-3.3/LT1304-5 OPERATIO The LT1304’s operation can best be understood by exam- ining the block diagram in Figure 1. Comparator A1 monitors the output voltage via resistor divider string R3/R4 at the FB pin. When VFB is higher than the 1.24V reference, A2 and the timers are turned off. Only the reference, A1 and A3 consume current, typically 120 µA. As VFB drops below 1.24V plus A1’s hysteresis (about 6mV), A1 enables the rest of the circuit. Power switch Q1 is then cycled on for 6 µs, or until current comparator A2 turns off the ON timer, whichever comes first. Off-time is fixed at approximately 1.5 µs. Q1’s switching causes cur- rent to alternately build up in inductor L1 and discharge into output capacitor C2 via D1, increasing the output voltage. As VFB increases enough to overcome C1’s hys- teresis, switching action ceases. C2 is left to supply current to the load until VOUT decreases enough to force A1’s output high, and the entire cycle repeats. If switch current reaches 1A, causing A2 to trip, switch ON time is reduced. This allows continuous mode opera- tion during bursts. A2 monitors the voltage across 7.2 Ω resistor R1, which is directly related to the switch current. Q2’s collector current is set by the emitter-area ratio to 0.5% of Q1’s collector current. R1’s voltage drop exceeds 36mV, corresponding to 1A switch current, A2’s output goes high, truncating the ON time part of the switch cycle. The 1A peak current can be reduced by tying a resistor between the ILIM pin and ground, causing a voltage drop to appear across R2. The drop offsets some of the 36mV reference voltage, lowering peak current. A 22k resistor limits current to approximately 550mA. A capacitor con- nected between ILIM and ground provides soft start. Shut- down is accomplished by grounding the SHDN pin. The low-battery detector A3 has its own 1.17V reference and is always on. The open collector output device can sink up to 500 µA. Approximately 35mV of hysteresis is built into A3 to reduce “buzzing” as the battery voltage reaches the trip level. Inductor Selection Inductors used with the LT1304 must be capable of handling the worst-case peak switch current of 1.2A without saturating. Open flux rod or drum core units may be biased into saturation by 20% with only a small reduc- tion in efficiency. For the majority of 2-cell or 3-cell input LT1304 applications, a 22 µH or 20µH inductor such as the Sumida CD54-220 (drum) or Coiltronics CTX20-1 (toroid) will suffice. If switch current is reduced using the ILIM pin, smaller inductors such as the Sumida CD43 series or Coilcraft DO1608 series can be used. Minimizing DCR is important for best efficiency. Ideally, the inductor DCR should be less than 0.05 Ω, although the physical size of such an inductor makes its use prohibitive in many space conscious applications. If EMI is a concern, such as when sensitive analog circuitry is present, a toroidal inductor such as the Coiltronics CTX20-1 is suggested. A special case exists where the VOUT/VIN differential is high, such as a 2V to 12V boost converter. If the required duty cycle for continuous mode operation is higher than the LT1304 can provide, the converter must be designed for discontinuous operation. This means that the inductor current decreases to zero during the switch OFF time. For a simple step-up (boost) converter, duty cycle can be calculated by the following formula: DC = 1 – [(VIN – VSAT)/(VOUT + VD)] where, VIN = Minimum input voltage VSAT = Switch saturation voltage (0.3V) VOUT = Output voltage VD = Diode forward voltage (0.4V) If the calculated duty cycle exceeds the minimum LT1304 duty cycle of 76%, the converter should be designed for discontinuous mode operation. The inductance must be low enough so that current in the inductor reaches the peak current in a single cycle. Inductor value can be calculated by: L = (VIN – VSAT)(tON/1A) where, tON = Minimum on-time of LT1304 (4µs) One advantage of discontinuous mode operation is that inductor values are usually quite low so very small units can be used. Ripple current is higher than with continuous mode designs and efficiency will be somewhat less. |
Similar Part No. - LT1304-5_15 |
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Similar Description - LT1304-5_15 |
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