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L4938EPD Datasheet(PDF) 6 Page - STMicroelectronics |
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L4938EPD Datasheet(HTML) 6 Page - STMicroelectronics |
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6 / 12 page  Connecting a resistor divider R1E,R2E to the ADJ, OUT2 pin the output voltage 2 can be pro- grammed to the value of VOUT2 = VOUT1 1 + R1E (R2E + RADJ) R2E ⋅ RADJ with RADJ = 60K to 150K and VOUT1 = 4.95 to 5.05V. For an exact calculation the temperature coeffi- cient (Tc -2000pprm) of the internal resistor (RADJ) must be taken into account. Pin ADJ in this mode should not have a capacitive burden be- cause this would reduce the phase margin of the regulator loop. Reset circuit The reset circuit supervises the standby output voltage. The reset output (RES) is defined from VOUT ≥ 1V. Even if VS is lacking, the reset generator is sup- plied by the output voltage VOUT1. The reset threshold of 4.7V is defined with the in- ternal reference voltage (note 9) and standby out- put divider, when pin PR is left open. The reset threshold voltage can be programmed in the range from 3.8V to 4.7V by connecting an exter- nal resistor from pin PR to GND. The value of the programming resistor RPR can be calculated with: RPR = 22K 4.7K VRT −1 − 92.9K,3.8V ≤ VRT ≤ 4.7V The reset pulse delay time tRD, is defined with the charge time of an external capacitor CT: tRDmin = CT ⋅ 0.6V 1 µA (note 5) tRDnom = CT ⋅ 1.4V 1 µA (note 6) The reaction time of the reset circuit originates from the noise immunity. Standby output voltage drops below the reset threshold only a bit longer than the reaction time results in a shorter reset delay time. The nominal reset delay time will be generated for standby output voltage drops longer than approximately 50 µs. The minimum rset time is generated if reset con- dition only occures for a short time triggering a re- set pulse but not completely discharging CT. The reset can be related to output2 on request. If higher charge currents for the reset capacitor are required a resistors from Pin CT to OUT1, may be used to increase the current. We recommended the use of 10K Ω to 5V as an output pull up. Sense Comparator The sense comparator compares an input signal with an internal voltage reference of typical 1.23V. The use of an external voltage divider makes this comparator very flexible in the application. It can be used to supervise the input voltage either be- fore or after the protection diode and to give addi- tional information to the microprocessor like low voltage warnings. We recommended the use of 10K Ω to 5V as an output pull up. Note 9: The reference is alternatively supplied from VS or VOUT1. If one supply is present, the reference is operating. Thermal Protection Both outputs are provided with an overtempera- ture shut down regulation power dissipation down to uncritical values. Output 2 will shut down approximately 10K before output 1. Under normal conditions shut down of output 2 will allow the chip to cool down again. Thus out- put 1 will be unaffected. The thermal shut down reduces the output volt- ages until power dissipation and the flow of ther- mal energy out of the chip balance. Transient Sensitivity In proper operation (VOUT > 4.5V) the reference is supplied by VOUT1 thus reducing sensitivity to in- put transients. Precise Data will be issued as soon as samples are available. L4938E - L4938ED - L4938EPD 6/12 |
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