Datasheet LT1185 (Analog Devices) - 6

LT1185
U U W U APPLICATIO S I FOR ATIO
Amplifier A2 is used to generate an internal current through
Output Capacitor
Q4 when an external resistor is connected from the REF The LT1185 has a collector output NPN pass transistor, pin to ground. This current is equal to 2.37V divided by which makes the open-loop output impedance much RLIM. It generates a current limit sense voltage across R1. higher than an emitter follower. Open-loop gain is a direct The regulator will current limit via A4 when the voltage function of load impedance, and causes a main-loop across R2 is equal to the voltage across R1. These two “pole” to be created by the output capacitor, in addition to resistors essentially form a current “amplifier” with a gain an internal pole in the error amplifier. To ensure loop of 350/0.055 = 6,360. Good temperature drift is inherent stability, the output capacitor must have an ESR (effective because R1 and R2 are made from the same diffusions. series resistance) which has an upper limit of 2Ω, and a Their ratio, not absolute value, determines current limit. lower limit of 0.2 divided by the capacitance in µF. A 2µF Initial accuracy is enhanced by trimming R1 slightly at output capacitor, for instance, should have a maximum wafer level. Current limit is equal to 15kΩ/RLIM. ESR of 2Ω, and a minimum of 0.2/2 = 0.1Ω. These values D1 and I1 are used to guarantee regulator shutdown when are easily encompassed by standard solid tantalum REF pin current drops below 2µA. A current less than 2µA capacitors, but occasionally a solid tantalum unit will have through Q4 causes the +input of A5 to go low and shut abnormally high ESR, especially at very low tempera- down the regulator via D2. tures. The suggested 2µF value shown in the circuit A3 is an internal current limit amplifier which can override applications should be increased to 4.7µF for – 40°C and the external current limit. It provides “goof proof” protec- – 55°C designs if the 2µF units cannot be guaranteed to tion for the pass transistor. Although not shown, A3 has stay below 2Ω at these temperatures. a nonlinear foldback characteristic at input-output volt- Although solid tantalum capacitors are suggested, other ages above 12V to guarantee safe area protection for Q3. types can be used if they meet the ESR requirements. See the graph, Internal Current Limit in the Typical Perfor- Standard aluminum electrolytic capacitors need to be mance Characteristics of this data sheet. upward of 25µF in general to hold 2Ω maximum ESR, especially at low temperatures. Ceramic, plastic film, and
Setting Output Voltage
monolithic capacitors have a problem with ESR being too The LT1185 output voltage is set by two external resistors low. These types should have a 1Ω carbon resistor in (see Figure 2). Internal reference voltage is trimmed to series to guarantee loop stability. 2.37V so that a standard 1% 2.37k resistor (R1) can be The output capacitor should be located close to the regu- used to set divider current at 1mA. R2 is then selected lator (≤ 3") to avoid excessive impedance due to lead from: inductance. A six inch lead length (2 • 3") will generate an extra 0.8 (V Ω inductive reactance at 1MHz, and unity-gain R2 = OUT – 2.37) R1 frequency can be up to that value. VREF For remote sense applications, the capacitor should still be for R1 = 2.37k and VREF = 2.37V, this reduces to: located close to the regulator. Additional capacitance can R2 = V be added at the remote sense point, but the remote OUT – 2.37k capacitor must be at least 2µF solid tantalum. It cannot be suggested values of 1% resistors are shown. a low ESR type like ceramic or mylar unless a 0.5Ω to 1Ω carbon resistor is added in series with the capacitor. Logic
VOUT R2 WHEN R1 = 2.37k
boards with multiple low ESR bypass capacitors should 5V 2.67k have a solid tantalum unit added in parallel whose value is 5.2V 2.87k 6V 3.65k approximately five times the combined value of low ESR 12V 9.76k capacitors. 15V 12.7k 1185ff 6