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LT1766EFE-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LT1766EFE-PBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 30 page LT1766/LT1766-5 11 1766fc APPLICATIONS INFORMATION divider impedance is not critical, caution should be used if resistors are increased beyond the suggested values and short-circuit conditions occur with high input voltage. High frequency pickup will increase and the protection accorded by frequency and current foldback will decrease. CHOOSING THE INDUCTOR For most applications, the output inductor will fall into the range of 15μH to 100μH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1766 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage. When choosing an inductor you will need to consider output ripple voltage, maximum load current, peak induc- tor current and fault current in the inductor. In addition, other factors such as core and copper losses, allowable component height, EMI, saturation and cost should also be considered. The following procedure is suggested as a way of handling these somewhat complicated and conflicting requirements. Output Ripple Voltage Figure 3 shows a typical output ripple voltage wave- form for the LT1766. Ripple voltage is determined by ripple current (ILP-P) through the inductor and the high frequency impedance of the output capacitor. The fol- lowing equations will help in choosing the required inductor value to achieve a desirable output ripple volt- age level. If output ripple voltage is of less importance, the subsequent suggestions in Peak Inductor and Fault Current and EMI will additionally help in the selection of the inductor value. Peak-to-peak output ripple voltage is the sum of a triwave (created by peak-to-peak ripple current (ILP-P) times ESR) and a square wave (created by parasitic inductance (ESL) and ripple current slew rate). Capacitive reactance is as- sumed to be small compared to ESR or ESL. V I ESR ESL dI dt RIPPLE LP P = ()( )+() - Figure 2. Frequency and Current Limit Foldback – + 1.2V BUFFER VSW L1 VC GND TO SYNC CIRCUIT 1766 F02 TO FREQUENCY SHIFTING R3 1k R4 2k R1 C1 R2 5k OUTPUT 5V ERROR AMPLIFIER FB 1.4V Q1 LT1766 Q2 + Figure 3. LT1766 Ripple Voltage Waveform 2.5μs/DIV 40mV/DIV VOUT AT IOUT = 1A VOUT AT IOUT = 0.1A INDUCTOR CURRENT AT IOUT = 1A INDUCTOR CURRENT AT IOUT = 0.1A 0.5A/DIV VIN = 40V VOUT = 5V L = 47μH C = 100μF, 10V, 0.1Ω 1766 F03 |
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