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RC4391 Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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RC4391 Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 22 page  RC4391 PRODUCT SPECIFICATION 11 Device Shutdown The entire device may be shut down to an extremely low cur- rent non-operating condition by disconnecting the ground (pin 4). This can be easily done by putting an NPN transistor in series with ground pin and switching it with an external signal. This switch will not affect the efficiency of operation, but will add to and increase the reference voltage by an amount equal to the saturation voltage of the transistor used. A mechanical switch can also be used in series between circuit ground and pin 4, without introducing any reference offset. Power Transistor Interfaces The most important consideration in selecting an external power transistor is the saturation voltage at IC = IMAX. The lower the saturation voltage is, the better the efficiency will be. Also, a higher beta transistor requires less base drive and therefore less power will be. Also, a higher beta transistor requires less base drive and therefore less power will be consumed in driving it, improv- ing efficiency losses in the interface. The part numbers given in the following applications are recommended, but other types may be more appropriate depending on voltage and power levels. When troubleshooting external power transistor circuits, ensure that clean, sharp-edged waveforms are driving the interface and power transistors. Monitor these waveforms with an oscilloscop—disconnect the inductor, and tie the VFB input (pin 8) high through a 10K resistor. This will cause the regulator to pulse at maximum duty cycle without drawing excessive inductor currents. Check for expected on time and off time, and look for slow rise times that might cause the power transistor to enter its linear operating region. The following external power transistor circuits may demand some adjustment to resistor values to satisfy various power levels and input/output voltages. CX and LX values must be selected according to the design equations (pages 2-213 and 2-214). Inverting Medium Power Application Figure 8 is a schematic of an inverting medium power supply (250mW to 1W) using an external PNP switch transistor. Supply voltage is applied to the IC via R3: when the internal switch transistor is turned on current through R4 is also drawn through R3; creating a voltage drop from base to emitter of the external switch transistor. This drop turns on the external transistor. Voltage pulses on the supply lead (pin 6) do not affect circuit operation because the internal reference and bias circuitry have good supply rejection capabilities. A power Schottky diode is used for higher efficiency. Inverting High Power Application For higher power applications (500mW to 5W), refer to Figure 9. This circuit uses an extra external transistor to pro- vide well controlled drive current in the correct phase to the power switch transistor. The value of R3 sets the drive current to the switch by making the interface transistor act as a current source. R4 and R5 must be selected such that the RC time constant of R4 and the base capacitance of Q2 do not slow the response time (and affect duty cycle), but not so low in value that excess power is consumed and efficiency suffers. The resistor values chosen should be proportional to the supply voltage (values shown are for +5V). Step-Down Power Applications Figures 16 and 17 show medium and high power interfaces modified to perform step-down functioning. The design Figure 14. Inverting Medium Power Application 65-2476 +V 4391 s Cx GND 3 4 L x 5 5 0.1 µF R2 62 k ½ 7 6 VREF FB V 150 pF +5V R3 1k ½ 220 µH Q1 2N3635 R4 50 ½ Motorola MBR030 R1 1.2 M ½ 100 µF -24V C1 C x CF |
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Similar Description - RC4391 |
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