Datasheet LT1580, LT1580-2.5 (Analog Devices) - 9

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