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LTC3388-3 Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LTC3388-3 Datasheet(HTML) 11 Page - Linear Integrated Systems |
11 / 24 page LTC4420 11 4420f For more information www.linear.com/LTC4420 applicaTions inForMaTion In order to limit output rising slew rate dVOUT/dt, size: COUT ≥ ILIM dVOUT dt (9) as the LTC4420 limits OUT charging current to ILIM until OUT approaches the input supply to within ILIM • RON, where RON is the channel switch resistance. Refer to the Thermal Protection and Maximum COUT section to deter- mine maximum allowed COUT. Inductive Effects Parasitic inductance and resistance can impact circuit performancebycausingovershootandundershootofinput and output voltages when the LTC4420 turns off. Parasitic inductance in the power path causes positive-going overshoot on the input and a negative-going undershoot on the output. Another cause of positive input overshoot is R-L-Ctankringingduringhotplugofaninputsupply.Input overshoot is most pronounced when the total resistance of the input tank is low. Care must be taken to ensure over voltage transients do not exceed the Absolute Maximum ratings of the LTC4420. Additionally, parasitic resistance and inductance can cause input undershoot (droop) during power path turn on. If severe enough, undershoot can temporarily invalidate a supply and cause repeated power up cycles (motorboating) or unwanted switchover between sources. The first step to avoid these issues is to minimize parasitic inductance and resistance in the power path. Guidelines are given in the layout section for minimizing parasitic inductance on the printed circuit board (PCB). External to the PCB, twist the power and ground wires together to minimize inductance. Second, use a bypass capacitor at the input to limit input voltage overshoot during LTC4420 power path turn off. A few micro farads is sufficient for most applications. When hot plugging supplies with large parasitic inductances, it is possible for the R-L-C tank to ring to more than twice the nominal supply voltage. Wall adapters and batteries typicallyhaveenoughloss(i.e.seriesresistance)toprevent ringing of this magnitude. However, if this is a problem, snub input capacitor CSN1 with resistor RSN1, typically 0.5Ω. Place this network close to the supply pin. Third, if an input capacitor is not permissible, use a TVS (such as SMAJ16CA) in applications when supply pin transients can exceed 24V. Use a bidirectional TVS in applications requiring reverse input protection. Note that a TVS does not address droop and motorboating, which are solved only by input bypassing. During normal operation, the LTC4420 limits power path current to < 1.6A and internal circuitry prevents OUT from ringing below ground during power path turn off. This is also true for output shorts when the short is close to the LTC4420’s OUT pin. However, if the output is shorted through a long wire, current in the wire inductance (LPAR2 in Figure 3) builds up due to the discharge of COUT1 and can be much higher than 1.6A. This current causes the OUT pin to ring below its −0.3V absolute maximum rating once COUT1 has been fully discharged. For this special case, split the output capacitor between COUT1 and COUT2 and make COUT1 small. Snub COUT1 with resister RSN2 to damp R-L-C ringing if required. Size COUT2 to obtain the requiredtotaloutputcapacitance.Alsoaddadiodebetween OUT and ground close to the LTC4420 to clamp negative ringing if the OUT pin rings below –0.3V. Figure 3. Recommended Inductive Transient Suppression Circuitry V1 OUT 4420 F03 COUT1 1µF D1 1N5818 CSN1 5µF RSN1 0.5 LPAR1 OPTIONAL LPAR2 OUT V1 LTC4420 RSN2 1 OPTIONAL COUT2 10µF |
Similar Part No. - LTC3388-3 |
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Similar Description - LTC3388-3 |
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