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Further, either the breakdown volt- age or the temperature coefficient can be adjusted to optimize circuit performance. Since the voltage across Q2 is simply the diode drop of a base-emitter junction: In Figure 6 a TTL input signal causes Q1 to ground the adjustment terminal decreasing the output to 1. Resistor R2 sets the output voltage while R1 provides the 5 mA programming current. Safe- area protection decreases the available output current at high input voltages to insure that the internal power transistor operates within its safe area.
At higher output voltages, they become more important since the energy stored in the capacitors is larger. The adjustment terminal can be bypassed to ground on the LMHV to improve ripple rejection.
With the LM as shown in Figure 8, a high performance current source useful from 10 mA to 1. However, in this design, the current contribution from the internal resistor is greater due to the 2 V BE drop across the Darlington pair.
Band-gap references are generally chosen for the higher current devices 0. The LM can be used in place of conventional zeners with improved performance. Both result in at least order of mag- nitude reductions in capacitance.
At higher values of capacitance, where the ratio of capacitor outside surface area to volume is significantly lower, internal heating becomes a problem. Figure 2 shows the discharge path for different capacitors used with a posi- tive regulator. On all other tests, even though power dissipation is internally limited, electrical specifica- tions apply only up to Pp.
Figure 1 shows an LM with a 10k potentiometer for adjusting the reverse breakdown voltage. I, to a reasonable value. An optional output capacitor can be added to improve transient response. Line regulation is 0. This list is only representative. Although designed primarily as a fixed-voltage regulator, the output of the LM series can be set to voltages above 5V, as shown below.
These regulators can provide local on card regulation, eliminating the distri- bution problems associated with single point regulation. With a heat sink, the effective thermal resistance can only approach the values specified, depending on the efficiency of the sink.
Further, only one regulator type need be stocked for a wide range of applications.
Discharge occurs when either the input or output is shorted. A 1 uF solid tanta- lum or 25 aiF aluminum electrolytic on the output swamps this effect and insures stability. Current limiting does not occur until the voltage across the sense resistor is higher than this opposing voltage by the amount shown in Figure 7. For thermal limiting to operate properly, the electrical circuitry on the Datasheeet must function and the IC chip must be well die-attached to the package so there are no hot spots.
Even though many heat sinks are commercially available, it is sometimes more practical, more convenient, or more economical to mount the regulator to chassis, to an aluminum or copper fin, to an aluminum extrusion, or to a custom heat sink. Computer datashee were used to optimize the electrical and thermal perfor- mance of the packaged IC which results in outstanding ripple rejection, superior line and load regulation in high power applications over l,305h.
Component Cross Reference List
Output current is sensed across R6 and compared to a fraction of the 1. Further, it is extremely well regulated against line voltage or load current changes so that it contributes virtually no error to dynamic regulation.
With a 10 i F bypass capacitor, 80 dB ripple rejection is obtainable at any output level. Resistor bypassing will not degrade regulation if, under worst-case conditions of maximum input voltage and minimum load current, the regulator is still delivering output current rather than absorbing current from the resistor. Usually the 5 mA programming current is sufficient; however, worst case minimum load for commercial grade parts requires a minimum load of 10 mA. In operation, the output of the regulator is the voltage of the adjustment terminal plus 1.
A 1 jiF aluminum electrolytic may be substituted. This can cause operating speed differences in digital circuitry, interfacing problems or decrease noise margins.
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Full-Wave Bridge four diodes 4. When the output is grounded, the opposing voltage is no longer present so current limiting occurs at a lower level. Since l 3 has no path to ground except through the load, a fixed load is provided by Q5, which is turned on by the variable current source Q4. Unfortunately, the impedance of this line and associated connectors caused voltage drops which varied throughout the system.
Transistors Q2 and Q3 are matched so a current identical to l 3 will flow through Q3. Thermal regulation is the effect of these temperature gradients on output voltage in percentage output change per Watt of power change in a specified time. Obviously either a larger capacitor is required or ambient tem- perature must be reduced.
They should be rated at the regulator short circuit current.
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This can be checked by: To illustrate, suppose a high current step load is placed on a regulator and the output voltage is observed on a storage oscilloscope as shown in Figure A3. The LM series alone of all the group requires an output capacitor to insure stable operation.
Exceptional effort has been made to make the LM immune to overload conditions. Refer to the data sheets on these parts and to the National Semiconductor Linear Applications Handbook.