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MAX774CSA Datasheet(PDF) 13 Page - Maxim Integrated Products |
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MAX774CSA Datasheet(HTML) 13 Page - Maxim Integrated Products |
13 / 16 page To choose the proper current-sense resistor, simply fol- low the two-step procedure outlined below: 1) Determine: • Input voltage range, V+ • Maximum (absolute) output voltage, VOUT • Maximum output current, ILOAD For example, let V+ range from 4V to 6V, and choose VOUT = -24V and IOUT = 150mA. 2) Next, referring to Figure 9, find the curve with the lowest current limit whose output current (with the lowest input voltage) meets your requirements. In our example, a curve where IOUT is >150mA with a 4V input and a -24V output is optimal. The RSENSE = 80m Ω (Figure 9) shows only approxi- mately 125mA of output current with a 4V input, so we look next at the RSENSE = 70m Ω line. It shows IOUT >150mA for V+ = 4V and VOUT = -24V. The current limit will be 0.210V / 0.070 Ω = 3A. These curves take into account worst-case inductor (±10%) and current- sense trip levels, but not sense-resistor tolerance. The switch on resistance is 70m Ω. Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Metal-film resistors are usually deposited on a ceramic rod in a spiral, making their inductances relatively high. Surface-mount (or chip) resistors have very little induc- tance and are well suited for use as current-sense resistors. To use through-hole resistors, IRC has a wire resistor that is simply a band of metal shaped as a “U” so that inductance is less than 10nH (an order of mag- nitude less than metal-film resistors). These are avail- able in resistance values between 5m Ω and 0.1Ω. External Switching Transistor The MAX774/MAX775/MAX776 are capable of driving P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is dictated by input and output voltage, peak current rating, on-resistance, gate- source threshold, and gate capacitance. The drain-to- source rating must be greater than the V+ - VOUT input-to-output voltage differential. The gate-to-source rating must be greater than V+ (the source voltage) plus the absolute value of the most negative swing of EXT. For bootstrapped operation, the most negative swing of EXT is VOUT. In nonbootstrapped operation, this may be ground or some other negative voltage. Gate capaci- tance is not normally a limiting factor, but values should be less than 1nF for best efficiency. For maximum effi- ciency, the MOSFET should have a very-low on-resis- tance at the peak current and be capable of handling that current. The transistor chosen for the typical operat- ing circuit has a 30V drain-source voltage limit and a 0.07 Ω drain-source on-resistance at VGS = -10V. Table 1 lists suppliers of switching transistors suitable for use with the MAX774/MAX775/MAX776. -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-to-DC Controllers ______________________________________________________________________________________ 13 Figure 8. MAX776 Maximum Output Current vs. Input Voltage (VOUT = -15V) Figure 9. MAX774/MAX775/MAX776 Maximum Output Current vs. Input Voltage (VOUT = -24V) 100 INPUT VOLTAGE (V) 200 300 400 500 600 700 34 5 6 7 RSENSE = 0.05 Ω RSENSE = 0.06 Ω RSENSE = 0.07 Ω RSENSE = 0.08 Ω RSENSE = 0.09 Ω VOUT = -15V INPUT VOLTAGE (V) 0 3 4 5 678 9 10 11 12 13 14 15 200 400 600 800 RSENSE = 0.05 Ω RSENSE = 0.06 Ω RSENSE = 0.07 Ω RSENSE = 0.08 Ω RSENSE = 0.09 Ω VOUT = -24V |
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