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Details, datasheet, quote on part number:LM1572
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| Part: | LM1572 |
| Category: | Power Management => Regulators => Switching Regulators |
| Description: | LM1572 - 1.5A, 500kHz Step-down Voltage Regulator, Package: Evaluation Board, Pin Nb=- |
| Company: | National Semiconductor Corporation |
| Datasheet: | Download LM1572 datasheet File size : 598 kB |
| Request For quote: | Find where to buy LM1572
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Datasheet text preview:
LM1572 1.5A, 500kHz Step-down Voltage Regulator
July 2002
LM1572 1.5A, 500kHz Step-down Voltage Regulator
General Description
The LM1572 is a 500kHz step-down (buck) switching voltage regulator capable of driving up to 1.5A in to a load while occupying a very small PCB area. Current Mode Control results in superior transient response and regulation over a wider range of operating conditions. National's advanced analog bipolar, CMOS plus DMOS process enables high efficiency at high switching frequency, and the internal 150m MOSFET switch provides more power from a smaller package. The LM1572 has programmable soft-start and frequency foldback to limit the inrush current, and a TTL compatible shutdown for easy sequencing. It draws 2.3mA of supply current in standby mode, and only 26µA in shutdown mode. The LM1572 is available in a TSSOP-16 package with an adjustable output or fixed outputs of 5V and 3.3V. The adjustable version can be set between 2.42V and 5V.
Features
n n n n n n n n n n n n 500kHz clock allows small, surface mount components 150m MOSFET switch Guaranteed load current of 1.5A Current mode control Programmable soft-start Internally set slope compensation TTL compatible shutdown Fixed 5V, 3.3V or adjustable output Low shutdown supply current of 26µA Cycle-by-cycle current limit Short-circuit protection and thermal protection TSSOP-16 package
Applications
n n n n LCD Monitors and TVs Set-Top Boxes Cable Modems Down conversion from 12V in local/distributed systems
Typical Applications (Fixed/Adjustable Voltage Parts)
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© 2002 National Semiconductor Corporation
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LM1572
Typical Applications (Fixed/Adjustable Voltage Parts)
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Connection Diagram
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16-Lead TSSOP NS Package Number MTC16
Pin Description
BOOT (Pin 1) - Bootstrap pin. It provides the upper rail for the floating driver stage of the internal MOSFET switch, the lower rail being the switching node (Pins 5 and 6). A small decoupling capacitor (typically 0.1µF-0.22µF) is therefore connected between the Bootstrap pin and the switching node. This capacitor should be 0.18µF-0.22µF for applications with an output voltage greater than 3.3V, if the minimum load (including the current drawn by internal/external feedback resistor divider) is less than 1mA. Additional drive voltage is provided by connecting this pin directly to the 5V output rail via a diode as shown in the Typical Application Circuit for the fixed voltage part. The same method can be used for an adjustable part provided the part is adjusted for an output of 5V. For other output voltages (between 2.42V to 5V) a more general method of providing this external drive voltage is illustrated in the Typical Application Circuit for the
adjustable part. Note that the NPN signal transistor shown, must have a guaranteed hfe greater than 400. Damage can occur to Pin 1 if it is connected (via a diode) to any external voltage source greater than 6V. AVIN (Pin 2) - This is the Analog VIN and provides the supply to the internal control circuitry, the (Power) VIN pins (Pins 3,4) providing the supply to the internal power stage. In the simplest layout scheme, the Analog VIN pin can simply be connected to the VIN pins directly on the pads where the IC is mounted. But for better noise rejection the trace to Pin 2 can be routed separately from the (Power) VIN trace, starting from the positive terminal of the input capacitor. A simple RC filter solution can also be used for better results, particularly at low input voltages. This consists of a 10 resistor connected between Analog VIN and VIN, and a 0.47µF capacitor between Analog VIN and Ground. Note that if this RC filter is used, a 1M resistor between Pin 1 and Ground is also required.
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LM1572
Pin Description
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VIN (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 converter. 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 maximum 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 connected 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 transconductance 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 compensation, but a series resistor-capacitor combination (R between 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.
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