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NCV8505D2T25R4 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCV8505D2T25R4 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 12 page NCV8505 Series http://onsemi.com 10 APPLICATION NOTES SETTING THE DELAY TIME The delay time is controlled by the Reset Delay Low Voltage, Delay Switching Threshold, and the Delay Charge Current. The delay follows the equation: tDELAY + [CDELAY(Vdt * Reset Delay Low Voltage)] Delay Charge Current Example: Using CDELAY = 33 nF. Assume reset Delay Low Voltage = 0. Use the typical value for Vdt = 1.8 V (2.5 V, 3.3 V, and 5.0 V options). Use the typical value for Delay Charge Current = 4.2 mA. tDELAY + [33 nF(1.8 * 0)] 4.2 mA + 14 ms STABILITY CONSIDERATIONS The output or compensation capacitor helps determine three main characteristics of a linear regulator: start−up delay, load transient response and loop stability. The capacitor value and type should be based on cost, availability, size and temperature constraints. A tantalum or aluminum electrolytic capacitor is best, since a film or ceramic capacitor with almost zero ESR can cause instability. The aluminum electrolytic capacitor is the least expensive solution, but, if the circuit operates at low temperatures (−25 °C to −40°C), both the value and ESR of the capacitor will vary considerably. The capacitor manufacturers data sheet usually provides this information. The value for the output capacitor COUT shown in Figure 18 should work for most applications, however it is not necessarily the optimized solution. Figure 18. Test and Application Circuit Showing Output Compensation VIN VOUT COUT** 33 mF RRST RESET CIN* 0.1 mF NCV8505 *CIN required if regulator is located far from the power supply filter. **COUT required for stability. Capacitor must operate at minimum temperature expected. CALCULATING POWER DISSIPATION IN A SINGLE OUTPUT LINEAR REGULATOR The maximum power dissipation for a single output regulator (Figure 19) is: PD(max) + [VIN(max) * VOUT(min)]IOUT(max) (1) ) VIN(max)IQ where: VIN(max) is the maximum input voltage, VOUT(min) is the minimum output voltage, IOUT(max) is the maximum output current for the application, and IQ is the quiescent current the regulator consumes at IOUT(max). Once the value of PD(max) is known, the maximum permissible value of RqJA can be calculated: RqJA + 150°C * TA PD (2) The value of RqJA can then be compared with those in the package section of the data sheet. Those packages with RqJA’s less than the calculated value in equation 2 will keep the die temperature below 150 °C. In some cases, none of the packages will be sufficient to dissipate the heat generated by the IC, and an external heatsink will be required. SMART REGULATOR® IQ Control Features IOUT IIN Figure 19. Single Output Regulator with Key Performance Parameters Labeled VIN VOUT } HEAT SINKS A heat sink effectively increases the surface area of the package to improve the flow of heat away from the IC and into the surrounding air. Each material in the heat flow path between the IC and the outside environment will have a thermal resistance. Like series electrical resistances, these resistances are summed to determine the value of RqJA: RqJA + RqJC ) RqCS ) RqSA (3) where: RqJC = the junction−to−case thermal resistance, RqCS = the case−to−heatsink thermal resistance, and RqSA = the heatsink−to−ambient thermal resistance. RqJC appears in the package section of the data sheet. Like RqJA, it too is a function of package type. RqCS and RqSA are functions of the package type, heatsink and the interface between them. These values appear in heat sink data sheets of heat sink manufacturers. |
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