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LTC1144 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1144 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC1144 7 1144fa For more information www.linear.com/LTC1144 applicaTions inForMaTion theory will explain how the LTC1144 behaves. The loss, and hence the efficiency, is set by the output impedance. As frequency is decreased, the output impedance will eventually be dominated by the 1/(f × C1) term and power efficiency will drop. Note also that power efficiency decreases as frequency goes up. This is caused by internal switching losses which occurduetosomefinitechargebeinglostoneachswitching cycle. This charge loss per unit cycle, when multiplied by the switching frequency, becomes a current loss. At high frequency this loss becomes significant and the power efficiency starts to decrease. Figure 5. Power Conversion Efficiency and Output Resistance vs Oscillator Frequency SHDN (Pin 6) The LTC1144 has a SHDN pin that will disable the internal oscillator when it is pulled low. The supply current will also drop to 8µA. OSC (Pin 7) and Boost (Pin 1) The switching frequency can be raised, lowered or driven from an external source. Figure 6 shows a functional diagram of the oscillator circuit. By connecting the boost pin (pin 1) to V+, the charge and discharge current is increased, and hence the frequency is increased by approximately 10 times. Increasing the frequency will decrease output impedance and ripple for higher load currents. Loading pin 7 with more capacitance will lower the frequency. Using the boost (pin 1) in conjunction with external capacitance on pin 7 allows user selection of the frequency over a wide range. Driving the LTC1144 from an external frequency source can be easily achieved by driving pin 7 and leaving the boost pin open as shown in Figure 7. The output current from pin 7 is small, typically 4µA, so a logic gate is capable of driving this current. The choice of using a CMOS logic gate is best because it can operate over a wide supply voltage range (3V to 15V) and has enough voltage swing to drive the internal Schmitt trigger shown in Figure 6. For 5V applications, a TTL logic gate can be used by simply adding an external pull-up resistor (see Figure 7). Capacitor Selection External capacitors C1 and C2 are not critical. Matching is not required, nor do they have to be high quality or tight tolerance.Aluminumortantalumelectrolyticsareexcellent choices, with cost and size being the only consideration. Figure 6. Oscillator Figure 7. External Clocking OSCILLATOR FREQUENCY (kHz) 0.1 100 95 90 85 80 75 70 600 500 400 300 200 100 0 1 10 100 1144 F05 V+ = 15V, C1 = C2 = 10µF IL = 20mA, TA = 25°C POWER CONVERSION EFFICIENCY OUTPUT RESISTANCE OSC (7) SCHMITT TRIGGER BOOST (1) 1144 F06 9I 9I I I V+ GND (3) ≈20pF 1 2 3 4 8 7 6 5 + C1 OSC INPUT NC REQUIRED FOR TTL LOGIC C2 100k –(V+) V+ 1144 F07 LTC1144 |
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