Datasheet LT1585-1.5, LT1585A-1.5 (Analog Devices) - 5

LT1585-1.5/LT1585A-1.5
U U W U APPLICATIONS INFORMATION
stability and transient response of the LT1585-1.5/ range of input supply voltage. The lower current limit rating LT1585A-1.5. and 7V maximum supply voltage rating for this device Large load current changes are exactly the situation pre- permit this characteristic. Current limit oscillations are sented by modern microprocessors and their peripheral typically nonexistent unless the input and output decou- circuitry. The load current step contains higher order pling capacitors for the regulators are mounted several frequency components that the output decoupling network inches from the terminals. must handle until the regulator throttles to the load current
Protection Diodes
level. Capacitors are not ideal elements and contain para- sitic resistance and inductance. These parasitic elements In normal operation, the LT1585-1.5/LT1585A-1.5 do not dominate the change in output voltage at the beginning of require any protection diodes. Older 3-terminal regulators a transient load step change. The ESR of the output require protection diodes between the output pin and the capacitors produces an instantaneous step in output volt- input pin to prevent die overstress. age [∆V = ∆I(ESR)]. The ESL of the output capacitors A protection diode between the input and output pins is produces a droop proportional to the rate of change of usually not needed. An internal diode between the input output current [V = L(∆I/∆t)]. The output capacitance and output pins on the LT1585-1.5/LT1585A-1.5 can produces a change in output voltage proportional to the handle microsecond surge currents of 50A to 100A. Even time until the regulator can respond [∆V = ∆t(∆I/C)]. These with large value output capacitors it is difficult to obtain transient effects are illustrated in Figure 1. those values of surge currents in normal operation. Only with large values of output capacitance, such as 1000µF to 5000 ESR µF, and with the input pin instantaneously shorted to EFFECTS ground can damage occur. A crowbar circuit at the input of ESL the LT1585-1.5/LT1585A-1.5 can generate those levels of CAPACITANCE EFFECTS EFFECTS current and a diode from output to input is then recom- 1585-1.5 F01 mended. This is shown in Figure 2. Usually, normal power V ∆I SLOPE, = POINT AT WHICH REGULATOR t C supply cycling or system “hot plugging and unplugging” TAKES CONTROL will not generate current large enough to do any damage.
Figure 1
D1 1N4002 The use of capacitors with low ESR, low ESL and good high (OPTIONAL) frequency characteristics is critical in meeting the output voltage tolerances of these high speed microprocessor LT1585-1.5 VIN IN OUT VOUT applications. These requirements dictate a combination of + + C1 GND C2 10 high quality surface mount tantalum capacitors and ce- µF 10µF ramic capacitors. The location of the decoupling network is 1585-1.5 F02 critical to transient response performance. Place the
Figure 2
decoupling network as close as possible to the micropro- cessor control circuitry because a trace run from the
Ripple Rejection
decoupling capacitors to the actual circuitry is inductive. In The typical curve for ripple rejection reflects values for the addition, use large power and ground plane areas to LT1585-1.5/LT1585A-1.5 as a function of frequency. In minimize distribution drops. applications that require improved ripple rejection, use the A possible stability problem that occurs in monolithic linear adjustable LT1585/LT1585A. A bypass capacitor from the regulators is current limit oscillations. The LT1585-1.5/ adjust pin to ground reduces the output ripple by the ratio LT1585A-1.5 essentially have a flat current limit over the of VOUT/1.25V. 5