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LTM8047 Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LTM8047 Datasheet(HTML) 11 Page - Linear Integrated Systems |
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11 / 18 page  LTM8049 11 8049f For more information www.linear.com/LTM8049 Soft-Start The soft-start circuitry provides for a gradual ramp-up of the switch current in each channel. When the channel is enabled, the external SS capacitor is first discharged. This resets the state of the logic circuits in the channel. Then an integrated resistor pulls the channel’s SS pin to about 1.8V. The LTM8049 has a built-in soft-start characteristic, but a slower ramp rate may be implemented by adding capacitance to the SS pin. Typical values are between 0.1µF and 1µF. Configurable Undervoltage Lockout Figure 1 shows how to configure an undervoltage lock- out (UVLO) for the LTM8049. Typically, UVLO is used in situations where the input supply is current-limited, has a relatively high source resistance, or ramps up/down slowly. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current-limit or latch low under low source voltage conditions. UVLO prevents the regulator from operating at source voltages where these problems might occur. APPLICATIONS INFORMATION The RUN pin has a voltage hysteresis with typical thresh- olds of 1.31V (rising) and 1.27V (falling). Resistor RUVLO2 is optional. RUVLO2 can be included to reduce the overall UVLO voltage variation caused by variations in the RUN pin current (see the Electrical Characteristics). A good choice for RUVLO2 is ≤10k + 1%. After choosing a value for RUVLO2, RUVLO1 can be determined from either of the following: RUVLO1= VIN(RISING) –1.31V 1.32V RUVLO2 +12.1µA or RUVLO1= VIN(FALLING) –1.27V 1.29V RUVLO2 +11.6µA where VIN(RISING) and VIN(FALLING) are the VIN threshold voltages when rising or falling respectively. For example, to disable the LTM8049 for VIN voltages below3.5Vusingthesingleresistorconfiguration,choose: RUVLO1= 3.5V –1.27V 1.29V ∞ +11.6µA ≅191k To activate the LTM8049 for VIN greater than 4.5V using the two resistor configuration, choose RUVLO2 = 10k and: RUVLO1= 4.5V –1.31V 1.32V 10k +12.1µA ≅ 22.1k Internal Undervoltage Lockout The LTM8049 monitors the VIN supply voltage in case VIN drops below a minimum operating level (typically about 2.3V). When VIN is detected low, the power switch is deactivated, and while sufficient VIN voltage persists, the soft-start capacitor is discharged. After VIN is sufficiently high,theLTM8049willreactivateandthesoft-startcapaci- tor will begin charging. The RUN pin sinks 12.1µA at the 1.31V rising threshold voltage and about 11.6µA at the 1.27V falling threshold. This makes it easy to set up an input voltage UVLO thresh- old with just a single resistor. For a desired VIN threshold, choose RUVLO1 using the equation: RUVLO1= VIN –1.31V 12.1µA Figure 1. The RUN Pin May Be Used to Implement an Accurate UVLO LTM8049 ACTIVE/ LOCKOUT 8049 F01 RUVLO1 RUVLO2 (OPTIONAL) VIN VIN RUN 1.31V 12.3µA AT 1.31V GND + – |
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