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LT1941EFE Datasheet(PDF) 11 Page - Linear Technology |
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LT1941EFE Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  11 LT1941 1941f APPLICATIO S I FOR ATIO The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor or one with a lower DCR resulting in higher efficiency. Be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage. In addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology’s Application Note AN44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid subharmonic oscillations. See AN19. The current in the inductor is a triangle wave with an average value equal to the load current. The peak switch current is equal to the output current plus half the peak-to- peak inductor ripple current. The LT1941 limits its switch current in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT1941 will deliver depends on the switch current limit, the inductor value and the input and output voltages. When the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. This gives the peak-to-peak ripple current in the inductor: ∆IL = (1 – DC)(VOUT + VF)/(L • f) where f is the switching frequency of the LT1941 and L is the value of the inductor. The peak inductor and switch current is: ISWPK = ILPK = IOUT + ∆IL/2 To maintain output regulation, this peak current must be less than the LT1941’s switch current limit ILIM. For SW1, ILIM is at least 3A at low duty cycles and decreases linearly to 2.4A at DC = 0.8. For SW2, ILIM is at least 2A for at low duty cycles and decreases linearly to 1.6A at DC = 0.8. The maximum output current is a function of the chosen inductor value: IOUT(MAX) = ILIM – ∆IL/2 = 3 • (1 – 0.25 • DC) – ∆IL/2 for SW1 = 2 • (1 – 0.25 • DC) – ∆IL/2 for SW2 Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors and choose one to meet cost or space goals. Then use these equations to check that the LT1941 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continuous. Discontinuous operation occurs when IOUT is less than ∆IL/2. Output Capacitor Selection For 5V and 3.3V outputs, a 10 µF, 6.3V ceramic capacitor (X5R or X7R) at the output results in very low output voltage ripple and good transient response. For lower voltages, 10 µF is adequate for ripple requirements but increasing COUT will improve transient performance. Other types and values will also work; the following discusses tradeoffs in output ripple and transient performance. The output capacitor filters the inductor current to gener- ate an output with low voltage ripple. It also stores energy in order to satisfy transient loads and stabilize the LT1941’s control loop. Because the LT1941 operates at a high frequency, minimal output capacitance is necessary. In addition, the control loop operates well with or without the presence of output capacitor series resistance (ESR). Ceramic capacitors, which achieve very low output ripple and small circuit size, are therefore an option. |
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