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TCL1585-3.3CEB Fiches technique(PDF) 5 Page - TelCom Semiconductor, Inc |
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TCL1585-3.3CEB Fiches technique(HTML) 5 Page - TelCom Semiconductor, Inc |
5 / 13 page 5 TCL1584 TCL1585 TCL1587 PRELIMINARY INFORMATION 7A / 5A / 4.6A / 3A, FAST RESPONSE, LOW DROPOUT POSITIVE LINEAR VOLTAGE REGULATORS TCL1584/1585/1587-04 6/6/97 APPLICATIONS General The TCL158x family of devices combine high current output (up to 7A) with low dropout voltage and built-in fault protection in a traditional three-terminal LDO format. All three device types are available in fixed output voltage and adjustable output versions. Fault protection includes short- circuit current limit, over-temperature limit, and safe-operat- ing-area (SOA) governing. These devices are pin-compatible upgrades for the 1083/1084/1085 family of LDO’s. However, the TCL158x family delivers lower dropout voltage, faster load transient response and improved internal frequency compensation. Maximum supply voltage rating is 7.0V. Modern processors cycle load current from near zero to several amps in a time period measured in tens of nanosec- onds. Load step response requirements are worsened by tighter output voltage tolerances. The TCL1584/85/87 fam- ily of regulators meets these stringent requirements without an obnoxious amount of output capacitance, saving both board space and cost. Stability and Transient Response Like most low dropout voltage regulators, the TCL158x devices require the use of output capacitors to maintain stability. Normally a 22 µF solid tantalum or a 100µF alumi- num electrolytic unit will ensure stability over all operating conditions. Keep in mind that commercially available ca- pacitors can have significant non-ideal effects such as capacitance value tolerance, temperature coefficient, ESR, ESL. The TCL158x devices are optimized for use with low ESR (<1 Ω) capacitors. On the adjustable voltage versions, bypassing the ADJ pin will improve ripple rejection and transient response. This is discussed in the Ripple Rejection section. This bypassing increases the required output capacitance value. The previ- ously suggested minimum values (22 µF and 100µF) take this into account. If no bypassing is used, lower values of output capacitance may be used. Transient regulation is directly related to output capaci- tance value. For applications which require large load cur- rent step changes, it is recommended that large output capacitors (>100 µF) be used. The value of the output capacitor can be increased without limit and will only im- prove transient regulation. In a typical high-performance microprocessor applica- tion, the sudden transients can be so fast that the output decoupling network must handle the sudden current de- mand until the internal voltage regulator is able to respond. In this case the non-ideal effects of the output capacitor are critical in determining the regulator’s response. Output volt- age response to step load current change is illustrated in Figure 1. The capacitor’s ESR and ESL cause immediate step changes in the output voltage. These are calculated as follows: ∆VESR = ∆I x ESR DVESL = DI/Dt x ESL To reduce the initial voltage droop, one should select low ESR and ESL capacitors. It should also be noted that the ESR effect is multiplied by absolute change in load current while the ESL effect is multiplied by the rate of change in load current. After the initial voltage drop, the capacitor value dominates the rate of change in voltage. This rate is calcu- lated as follows: ∆V = ∆t x ∆I/C Figure 1. Transient Load Voltage Response ESR EFFECTS ESL EFFECTS POINT AT WHICH REGULATOR TAKES CONTROL CAPACITANCE EFFECTS SLOPE, V = ∆I t C Typically high quality ceramic and tantalum capacitors must be used in combination to minimize ESR and maximize C. This decoupling network must also be placed close to the microprocessor to reduce ESL (parasitic board trace induc- tance). If possible, the capacitors should be placed inside the microprocessor socket cavity. Of course, robust power and ground planes will also improve performance by reduc- ing parasitic voltage drops. The TCL1584 has an adaptive current limiting scheme where to ensure SOA for the output transistor, the current limit is reduced for increasing input to output differential. This means that the TCL1584 exhibits a negative resistance characteristic under certain conditions. This is a common technique in LDO design to ensure SOA - especially LDO’s with high maximum input voltage ratings. This negative resistance can interact with the external capacitance and inductance and cause oscillations during current limit. This effect is highly dependent on system parameters and is difficult to predict. However this oscillation, if it occurs, will not damage the regulator and can be ignored if the system |
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