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LT1173CN8-5 Datasheet(PDF) 10 Page - Linear Technology |
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LT1173CN8-5 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LT1173 10 L1 LT1173 • TA13 GND SW2 FB SW1 LIM I IN V D1 AO +VOUT R2 V = 1.245V + 0.6V OUT R1 R2 ( ) R1 2N3906 –VIN + C1 LT1173 + C2 Figure 8. Negative-to-Positive Converter Using the ILIM Pin The LT1173 switch can be programmed to turn off at a set switch current, a feature not found on competing devices. This enables the input to vary over a wide range without exceeding the maximum switch rating or saturating the inductor. Consider the case where analysis shows the LT1173 must operate at an 800mA peak switch current with a 2.0V input. If VIN rises to 4V, the peak switch current will rise to 1.6A, exceeding the maximum switch current rating. With the proper resistor selected (see the “Maxi- mum Switch Current vs RLIM” characteristic), the switch current will be limited to 800mA, even if the input voltage increases. Another situation where the ILIM feature is useful occurs when the device goes into continuous mode operation. This occurs in step-up mode when V V VV DC OUT DIODE IN SW + − < − () 1 1 17 . When the input and output voltages satisfy this relation- ship, inductor current does not go to zero during the switch OFF time. When the switch turns on again, the current ramp starts from the non-zero current level in the inductor just prior to switch turn on. As shown in Figure 9, the inductor current increases to a high level before the comparator turns off the oscillator. This high current can cause excessive output ripple and requires oversizing the output capacitor and inductor. With the ILIM feature, however, the switch current turns off at a programmed level as shown in Figure 10, keeping output ripple to a minimum. S APPLICATI I FOR ATIO LT1173 • TA14 I OFF L ON SWITCH Figure 9. No Current Limit Causes Large Inductor Current Build-Up LT1173 • TA15 I ON L OFF SWITCH PROGRAMMED CURRENT LIMIT Figure 10. Current Limit Keeps Inductor Current Under Control Figure 11 details current limit circuitry. Sense transistor Q1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2’s collector current flows in Q1’s collector. This current is passed through internal 80 Ω resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN sets the current limit. When suffi- cient switch current flows to develop a VBE across R1 + RLIM, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 2 µs. The current trip point becomes less accurate for switch ON times less than 4 µs. Resistor values programming switch ON time for 2 µs or less will cause spurious response in the switch circuitry although the device will still maintain output regulation. LT1173 • TA28 SW2 SW1 Q2 DRIVER OSCILLATOR VIN ILIM R1 80 Ω (INTERNAL) RLIM (EXTERNAL) Q1 Q3 Figure 11. LT1173 Current Limit Circuitry |
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Similar Description - LT1173CN8-5 |
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