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LT1175CS8-5-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LT1175CS8-5-PBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 18 page  LT1175 0 1175fe applicaTions inForMaTion LT1175 A1 3.8V R1 ESR OUTPUT 1175 F03 COUT OUTPUT GND SENSE R2 RC 0.5 RLIM RN CF 20pF + PARASITIC COLLECTOR RESISTANCE POWER TRANSISTOR NEGATIVE DC FEEDBACK AT LIGHT LOADS AC FEEDFORWARD PATH CURRENT LIMIT SENSE RESISTOR Q2 Q1 VIN LOAD Q3 This condition can occur with ceramic or film capacitors which often have an ESR under 0.1Ω. With previous de- signs, the user was forced to add a real resistor in series with the capacitor to guarantee loop stability. The LT1175 uses a unique AC feedforward technique to eliminate this problem. CF is a conventional feedforward capacitor often used in regulators to cancel the pole formed by the output capacitor. It would normally be connected from the regulated output node to the feedback node at the R1/R2 junction or to an internal node on the amplifier as shown. In this case, however, the capacitor is connected to the internalstructureofthepowertransistor.RCistheunavoid- able parasitic collector resistance of the power transistor. Access to the node at the bottom of RC is available only in monolithic structures where Kelvin connections can be made to the NPN buried collector layer. The loop now responds as if RC were in series with the output capacitor and good loop stability is achieved even with extremely low ESR in the output capacitor. The end result of all this attention to loop stability is that the output capacitor used with the LT1175 can range in value from 0.1µF to hundreds of microfarads, with an ESR from 0Ω to 10Ω. This range allows the use of ceramic, solid tantalum, aluminum and film capacitors over a wide range of values. The optimum output capacitor type for the LT1175 is still solid tantalum, but there is considerable leeway in selecting the exact unit. If large load current transients are expected, larger capacitors with lower ESR may be needed to control worst-case output variation during transients. If transients are not an issue, the capacitor can be chosen for small physical size, low price, etc. Concerns about surge currents in tantalum capacitors are not an issue for the output capacitor because the LT1175 limits inrush current to well below the level which can cause capacitor damage. Surges caused by shorting the regulator output are also not a problem because tantalum Figure 3 |
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