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LM1572 Scheda tecnica(PDF) 3 Page - National Semiconductor (TI) |
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LM1572 Scheda tecnica(HTML) 3 Page - National Semiconductor (TI) |
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3 / 17 page  Pin Description (Continued) V IN (Pins 3,4) - This is the input supply to the power stage (connected to the Drain of the switching MOSFET). To aid thermal dissipation from the die, two pins are used for this function. Both these pins must be connected together, very close to the IC, onto a large PCB copper plane. SW (Pins 5,6) - This is the Source of the internal switching MOSFET and forms the ’switching node’ of the buck con- verter. These two pins should be connected together on the PCB close to the IC. The length of the trace from this node to the cathode of the catch diode, and from the anode of the diode to the IC ground must be kept very small. The maxi- mum inductance connected to the switching node (for any application) is recommended to be 15µH. See the Inductor Selection procedure for more details. GND (Pins 7,9,13,14) - This is the Ground for the IC and for the input and output rails of the buck converter. To aid thermal dissipation from the die, four pins are used for this function. Connect as many as possible of these ground pins together, close to the IC onto a large PCB copper plane. A two-sided PCB with one side serving as a ’ground plane’ is strongly recommended. The ground pins must then connect to the ground plane very close to the IC through several vias. The vias also serve to transfer heat to the other side of the board for better thermal management. SD (Pin 8) - Shutdown/Standby/UVLO Pin. This pin actually has two thresholds. If it is taken below 2.38V (typical), the switch turns off and the output of the converter falls to zero.This is the ’standby mode’. The internal circuitry of the IC remains active, continuing to draw about 2.3mA from the input. If the voltage on this pin is lowered below 1V (typical), the IC enters ’shutdown mode’ drawing only 26 µA from the input. Above 2.38V, the switching action resumes, and so this pin can also be used to set an undervoltage lockout threshold (UVLO) for the input rail. If this pin is not intended to be used actively, it can be left floating to allow continuous switching. The voltage on this pin should not exceed 7V to avoid damage. NC (Pins 10,11) - No Internal Connection. SS (Pin 12) - Softstart pin. A small capacitor connected from this pin to ground programs the amount of softstart. This capacitor charges up by means of an internal 4.5µA current source, during power-up, and also whenever the output of the converter is enabled. The allowed duty cycle increases slowly as the capacitor charges, reaching the maximum allowed when the voltage on this pin approaches 2V. The capacitor continues to charge, finally reaching 6V, at which level it is internally clamped. This pin is internally forced to ground (to discharge the softstart capacitor and to reset the softstart function) whenever the shutdown pin is taken below 2.38V. If the softstart feature is not required, the softstart pin can be left floating. FB (Pin 15) - This is the feedback pin for the IC and is used to set the output of the converter to regulate to the desired value. For the fixed voltage part this pin is normally con- nected directly to the output. For the adjustable part, a resistive divider is used between the output and ground, so that the voltage on this pin is 2.42V when the output is at the required level. For fixed voltage parts, the internal divider draws about 0.5mA, a consideration possibly affecting the choice of the bootstrap capacitor (see description of Pin 1 above). COMP (Pin 16) - This is the output of the error transconduc- tance amplifier and is used for frequency compensation of the feedback loop. A small capacitor from this pin to ground (about 3.3nF to 6.8nF) provides the simplest loop compen- sation, but a series resistor-capacitor combination (R be- tween 1k to 1.5k) may also be used to improve the phase margin/crossover frequency of the loop. The voltage on this pin is at about 1V at very light loads. Under very heavy loads or under output short-circuit, the voltage on this pin clamps to 2V, and the converter enters protective foldback. The IC automatically recovers from this mode when the load is reduced. www.national.com 3 |
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