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LT1934-1 Datasheet(PDF) 11 Page - Linear Technology |
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LT1934-1 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1934/LT1934-1 1934f APPLICATIO S I FOR ATIO Figure 4. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit from a Reversed Input. The LT1934 Runs Only When the Input is Present maximum duty cycle of the LT1934, requiring a higher input voltage to maintain regulation. Shorted Input Protection If the inductor is chosen so that it won’t saturate exces- sively, an LT1934 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT1934 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT1934’s output. If the VIN pin is allowed to float and the SHDN pin is held high (either by a logic signal or because it is tied to VIN), then the LT1934’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the SHDN pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT1934 can pull large currents from the output through the SW pin and the VIN pin. Figure 4 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. VIN BOOST GND FB SHDN SW 5 D4 VIN 4 1 6 23 1M 100k LT1934 1934 F07 VOUT BACKUP D4: MBR0530 Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit Minimum Input Voltage VOUT = 3.3V Minimum Input Voltage VOUT = 5V LOAD CURRENT (mA) 3.5 4.0 4.5 5.0 5.5 6.0 0.1 10 100 1934 G12 3.0 1 LT1934 VOUT = 3.3V TA = 25°C BOOST DIODE TIED TO OUTPUT VIN TO START VIN TO RUN LOAD CURRENT (mA) 5 6 7 8 0.1 10 100 1934 G13 4 1 LT1934 VOUT = 5V TA = 25°C BOOST DIODE TIED TO OUTPUT VIN TO START VIN TO RUN PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 5 shows the high current paths in the buck regulator circuit. Note that large, switched currents flow in the power switch, the catch diode (D1) and the input capacitor (C2). The loop formed by these components should be as small as possible. Furthermore, the system ground should be tied to the regulator ground in only one place; this prevents the switched current from injecting noise into the system ground. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C1. Additionally, the SW and BOOST nodes should be kept as small as possible. Finally, keep the FB node as small as possible so that the ground pin and |
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