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LTC1174IN8 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1174IN8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1174 LTC1174-3.3/LTC1174-5 1174fe APPLICATIO S I FOR ATIO output voltage is now taken off the GND pin. Therefore, the maximum input voltage is now determined by the difference between the absolute maximum voltage rating and the output voltage. A maximum of 12V is specified in Figure 5, giving the circuit a 1.5V of headroom for VIN. Note that the circuit can operate from a minimum of 4V, making it ideal for a 4 NiCad cell application. For a higher output current circuit, please refer to the Typical Applications section. Absolute Maximum Ratings and Latchup Prevention The absolute maximum ratings specify that SW (Pin 5) can never exceed VIN (Pin 6) by more than 0.3V. Normally this situation should never occur. It could, however, if the output is held up while the supply is pulled down. A con- dition where this could potentially occur is when a battery is supplying power to an LTC1174/LTC1174-3.3/ LTC1174-5 regulator and also to one or more loads in parallel with the the regulator’s VIN. If the battery is dis- connected while the LTC1174/LTC1174-3.3/LTC1174-5 regulator is supplying a light load and one of the parallel circuits is a heavy load, the input capacitor of the LTC1174/ LTC1174-3.3/LTC1174-5 regulator could be pulled down faster than the output capacitor, causing the absolute maximum ratings to be exceeded. The result is often a latchup which can be destructive if VIN is reapplied. Bat- tery disconnect is possible as a result of mechanical stress, bad battery contacts or use of a lithium-ion battery with a built-in internal disconnect. The user needs to assess his/her application to determine whether this situation could occur. If so, additional protection is necessary. Prevention against latchup can be accomplished by sim- ply connecting a Schottky diode across the SW and VIN pins as shown in Figure 6. The diode will normally be reverse biased unless VIN is pulled below VOUT at which time the diode will clamp the (VOUT – VIN) potential to less than the 0.6V required for latchup. Note that a low leakage Schottky should be used to minimize the effect on no-load supply current. Schottky diodes such as MBR0530, BAS85 and BAT84 work well. Another more serious effect of the protection diode leakage is that at no load with nothing to provide a sink for this leakage current, the output voltage can potentially float above the maximum allowable toler- ance. To prevent this from occuring, a resistor must be connected between VOUT and ground with a value low enough to sink the maximum possible leakage current. Figure 4. LTC1174 Adjustable Configuration R2 R1 1 VOUT 1174 F04 100pF* 6.8nF** * ** ADJUSTABLE APPLICATIONS LOW NOISE APPLICATIONS LTC1174 VFB Figure 5. Positive-to-Negative 5V Converter 3 SHUTDOWN 2 7 6 8 1 5 4 50 µH** VOUT –5V 45mA MBRS140T3 1174 F05 * ** AVX TPSD476K016 COILTRONICS CTX50-4 INPUT VOLTAGE 4V TO 12V 0.1 µF 47 µF* 16V ×2 47 µF* 16V ×2 LTC1174HV-5 LBIN LBOUT IPGM GND VIN VOUT SW + + Figure 6. Preventing Absolute Maximum Ratings from Being Exceeded 1174 F06 VIN VOUT LATCHUP PROTECTION SCHOTTKY SW LTC1174 LTC1174-3.3 LTC1174-5 + |
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