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LT1934ES6-1 Datasheet(PDF) 7 Page - Linear Technology |
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LT1934ES6-1 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  7 LT1934/LT1934-1 1934f APPLICATIO S I FOR ATIO Which One to Use: LT1934 or LT1934-1? The only difference between the LT1934 and LT1934-1 is the peak current through the internal switch and the inductor. If your maximum load current is less than 60mA, use the LT1934-1. If your maximum load is higher, use the LT1934; it can supply up to ~300mA. While the LT1934-1 can’t deliver as much output current, it has other advantages. The lower peak switch current allows the use of smaller components (input capacitor, inductor and output capacitor). The ripple current at the input of the LT1934-1 circuit will be smaller and may be an important consideration if the input supply is current limited or has high impedance. The LT1934-1’s current draw during faults (output overload or short) and start-up is lower. The maximum load current that the LT1934 or LT1934-1 can deliver depends on the value of the inductor used. Table 1 lists inductor value, minimum output capacitor and maximum load for 3.3V and 5V circuits. Increasing the value of the capacitor will lower the output voltage ripple. Component selection is covered in more detail in the following sections. Minimum Input Voltage The minimum input voltage required to generate a particu- lar output voltage is determined by either the LT1934’s undervoltage lockout of ~3V or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC = (VOUT + VD)/(VIN – VSW + VD) where VD is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.3V at maximum load for the LT1934, ~0.1V for the LT1934-1). This leads to a minimum input voltage of: VIN(MIN) = (VOUT + VD)/DCMAX – VD + VSW with DCMAX = 0.85. Inductor Selection A good first choice for the inductor value is: L = 2.5 • (VOUT + VD) • 1.8µs/ILIM where ILIM is the switch current limit (400mA for the LT1934 and 120mA for the LT1934-1). This choice pro- vides a worst-case maximum load current of 250mA (60mA for the LT1934-1). The inductor’s RMS current rating must be greater than the load current and its saturation current should be greater than ILIM. To keep efficiency high, the series resistance (DCR) should be less than 0.3 Ω (1Ω for the LT1934-1). Table 2 lists several vendors and types that are suitable. This simple rule may not provide the optimum value for your application. If the load current is less, 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. The following provides more details to guide inductor selection. First, the value must be chosen so that the LT1934 can supply the maximum load current drawn from the output. Second, the inductor must be rated appropriately so that the LT1934 will function reliably and the inductor itself will not be overly stressed. Detailed Inductor Selection and Maximum Load Current The square wave that the LT1934 produces at its switch pin results in a triangle wave of current in the inductor. The LT1934 limits the peak inductor current to ILIM. Because Table 1 MINIMUM MAXIMUM PART VOUT LCOUT LOAD LT1934 3.3V 100 µH 100 µF 300mA 47 µH47µF 250mA 33 µH33µF 200mA 5V 150 µH47µF 300mA 68 µH33µF 250mA 47 µH22µF 200mA LT1934-1 3.3V 150 µH15µF 60mA 100 µH10µF 45mA 68 µH10µF 20mA 5V 220 µH10µF 60mA 150 µH4.7µF 45mA 100 µH4.7µF 20mA |
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