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LT1580CT Datasheet(PDF) 9 Page - Linear Technology |
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LT1580CT Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1580/LT1580-2.5 APPLICATIONS INFORMATION load current changes. Output capacitance can increase without limit and larger values of output capacitance further improve the stability and transient response of the LT1580. Modern microprocessors generate large high frequency current transients. The load current step contains higher order frequency components that the output coupling network must handle until the regulator throttles to the load current level. Capacitors are not ideal elements and contain parasitic resistance and inductance. These para- sitic elements dominate the change in output voltage at the beginning of a transient load step change. The ESR of the output capacitors produces an instantaneous step in out- put voltage ( ∆V = ∆I)(ESR). The ESL of the output capaci- tors produces a droop proportional to the rate of change of the output current (V = L)( ∆I/∆t). The output capaci- tance produces a change in output voltage proportional to the time until the regulator can respond ( ∆V = ∆t)(∆I/ C). These transient effects are illustrated in Figure 4 . ESR EFFECTS 1580 F04 ESL EFFECTS CAPACITANCE EFFECTS POINT AT WHICH REGULATOR TAKES CONTROL SLOPE, = V t ∆I C The use of capacitors with low ESR, low ESL and good high frequency characteristics is critical in meeting the output voltage tolerances of these high speed micropro- cessors. These requirements dictate a combination of high quality, surface mount, tantalum and ceramic capaci- tors. The location of the decoupling network is critical to transient performance. Place the decoupling network as close to the processor pins as possible because trace runs from the decoupling capacitors to the processor pins are inductive. The ideal location for the decoupling network is actually inside the microprocessor socket cavity. In addi- tion, use large power and ground plane areas to minimize distribution drops. Output Voltage The adjustable version of the LT1580 develops a 1.25V reference voltage between the SENSE pin and the ADJ pin (see Figure 5). Placing a resistor R1 between these two terminals causes a constant current to flow through R1 and down through R2 to set the overall output voltage. Normally R1 is chosen so that this current is the specified minimum load current of 10mA. The current out of the ADJ pin adds to the current from R1. The ADJ pin current is small, typically 50 µA. The output voltage contribution of the ADJ pin current is small and only needs to be consid- ered when very precise output voltage setting is required. Note that the top of the resistor divider should be con- nected directly to the SENSE pin for best regulation. See the section on grounding and Kelvin sensing above. VOUT SENSE ADJ R2 IADJ = 50µA 1580 F05 LT1580 VPOWER VPOWER VCONTROL VREF VCONTROL R1 VOUT VOUT = VREF 1 + + IADJ (R2) R2 R1 ( ) + + + Figure 4 Figure 5. Setting Output Voltage Protection Diodes In normal operation the LT1580 does not require protec- tion diodes. Older 3-terminal regulators require protection diodes between the VOUT pin and the Input pin or between the ADJ pin and the VOUT pin to prevent die overstress. On the LT1580, internal resistors limit internal current paths on the ADJ pin. Therefore even with bypass capaci- tors on the ADJ pin, no protection diode is needed to ensure device safety under short-circuit conditions. The ADJ pin can be driven on a transient basis ±7V with respect to the output without any device degradation. |
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