Electronic Components Datasheet Search |
|
LT1185M Datasheet(PDF) 6 Page - Linear Technology |
|
LT1185M Datasheet(HTML) 6 Page - Linear Technology |
6 / 16 page 6 LT1185 APPLICATIO S I FOR ATIO Amplifier A2 is used to generate an internal current through Q4 when an external resistor is connected from the REF pin to ground. This current is equal to 2.37V divided by RLIM. It generates a current limit sense voltage across R1. The regulator will current limit via A4 when the voltage across R2 is equal to the voltage across R1. These two resistors essentially form a current “amplifier” with a gain of 350/0.055 = 6,360. Good temperature drift is inherent because R1 and R2 are made from the same diffusions. Their ratio, not absolute value, determines current limit. Initial accuracy is enhanced by trimming R1 slightly at wafer level. Current limit is equal to 15k Ω/RLIM. D1 and I1 are used to guarantee regulator shutdown when REF pin current drops below 2 µA. A current less than 2µA through Q4 causes the +input of A5 to go low and shut down the regulator via D2. A3 is an internal current limit amplifier which can override the external current limit. It provides “goof proof” protec- tion for the pass transistor. Although not shown, A3 has a nonlinear foldback characteristic at input-output volt- ages above 12V to guarantee safe area protection for Q3. See the graph, Internal Current Limit in the Typical Perfor- mance Characteristics of this data sheet. Setting Output Voltage The LT1185 output voltage is set by two external resistors (see Figure 2). Internal reference voltage is trimmed to 2.37V so that a standard 1% 2.37k resistor (R1) can be used to set divider current at 1mA. R2 is then selected from: for R1 = 2.37k and VREF = 2.37V, this reduces to: R2 = VOUT – 2.37k suggested values of 1% resistors are shown. VOUT R2 WHEN R1 = 2.37k 5V 2.67k 5.2V 2.87k 6V 3.65k 12V 9.76k 15V 12.7k Output Capacitor The LT1185 has a collector output NPN pass transistor, which makes the open-loop output impedance much higher than an emitter follower. Open-loop gain is a direct function of load impedance, and causes a main-loop “pole” to be created by the output capacitor, in addition to an internal pole in the error amplifier. To ensure loop stability, the output capacitor must have an ESR (effective series resistance) which has an upper limit of 2 Ω, and a lower limit of 0.2 divided by the capacitance in µF. A 2µF output capacitor, for instance, should have a maximum ESR of 2 Ω, and a minimum of 0.2/2 = 0.1Ω. These values are easily encompassed by standard solid tantalum capacitors, but occasionally a solid tantalum unit will have abnormally high ESR, especially at very low tempera- tures. The suggested 2 µF value shown in the circuit applications should be increased to 4.7 µF for –40°C and –55 °C designs if the 2µF units cannot be guaranteed to stay below 2 Ω at these temperatures. Although solid tantalum capacitors are suggested, other types can be used if they meet the ESR requirements. Standard aluminum electrolytic capacitors need to be upward of 25 µF in general to hold 2Ω maximum ESR, especially at low temperatures. Ceramic, plastic film, and monolithic capacitors have a problem with ESR being too low. These types should have a 1 Ω carbon resistor in series to guarantee loop stability. The output capacitor should be located close to the regu- lator ( ≤3") to avoid excessive impedance due to lead inductance. A six inch lead length (2 • 3") will generate an extra 0.8 Ω inductive reactance at 1MHz, and unity-gain frequency can be up to that value. For remote sense applications, the capacitor should still be located close to the regulator. Additional capacitance can be added at the remote sense point, but the remote capacitor must be at least 2 µF solid tantalum. It cannot be a low ESR type like ceramic or mylar unless a 0.5 Ω to 1Ω carbon resistor is added in series with the capacitor. Logic boards with multiple low ESR bypass capacitors should have a solid tantalum unit added in parallel whose value is approximately five times the combined value of low ESR capacitors. R2 = (VOUT – 2.37) R1 VREF |
Similar Part No. - LT1185M |
|
Similar Description - LT1185M |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |