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LT1301CN8 Datasheet(PDF) 5 Page - Linear Technology |
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LT1301CN8 Datasheet(HTML) 5 Page - Linear Technology |
5 / 12 page 5 LT1301 TEST CIRCUITS Oscillator Test Circuit 2V 100 µF VIN SEL SENSE GND PGND SHDN SW LT1301 IL 100 Ω 5V LT1301 TC fOUT OPERATION Operation of the LT1301 is best understood by referring to the Block Diagram in Figure 2. When A1’s negative input, related to the Sense pin voltage by the appropriate resis- tor-divider ratio is higher that the 1.25V reference voltage, A1’s output is low. A2, A3 and the oscillator are turned off, drawing no current. Only the reference and A1 consume current, typically 120 µA. When A1’s negative input drops below 1.25V, overcoming A1’s 6mV hysteresis, A1’s out- put goes high enabling the oscillator, current comparator A2, and driver A3. Quiescent current increases to 2mA as the device prepares for high current switching. Q1 then turns on in controlled saturation for (nominally) 5.3 µs or until comparator A2 trips, whichever comes first. After a fixed off-time of (nominally) 1.2 µs, Q1 turns on again. The LT1301’s switching causes current to alternately build up in L1 and dump into output capacitor C2 via D1, increasing the output voltage. When the output is high enough to cause A1’s output to go to low, 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. Figure 4 details relevant waveforms. A1’s cycling causes low-to-mid-frequency ripple voltage on the output. Ripple can be reduced by making the output capacitor large. The 33 µF unit specified results in ripple of 100mV to 200mV on the 12V output. A 100 µF capacitor will decrease ripple to 50mV. If operating at 5V ouput a 0.1 µF ceramic capacitor is required at the Sense pin in addition to the electrolytic. If switch current reaches 1A, causing A2 to trip, switch on- time is reduced and off-time increases slightly. This allows continuous mode operation during bursts. A2 monitors the voltage across 3 Ω resistor R1 which is directly related to the switch current. Q2’s collector current is set by the emitter-area ratio to 0.6% of Q1’s collector current. When R1’s voltage drop exceeds 18mV, corresponding to 1A switch current, A2’s output goes high, truncating the on- time portion of the oscillator cycle and increasing off-time to about 2 µs as shown in Figure 3, trace A. This pro- grammed peak current can be reduced by tying the ILIM pin to ground, causing 15 µA to flow through R2 into Q3’s collector. Q3’s current causes a 10.4mV drop in R2 so that only an additional 7.6mV is required across R1 to turn off the switch. This corresponds to a 400mA switch current as shown in Figure 3, trace B. The reduced peak switch current reduces I2R loses in Q1, L1, C1 and D1. Efficiency can be increased by doing this provided that the accom- panying reduction in full load current is acceptable. Lower peak currents also extend alkaline battery life due to the alkaline cell’s high internal impedance. Figure 3. Switch Pin Current With ILIM Floating or Grounded TRACE A 500mA/DIV ILIM PIN OPEN TRACE B 500mA/DIV ILIM PIN GROUNDED 20 µs/DIV |
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