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LT1107C Datasheet(PDF) 9 Page - Linear Technology |
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LT1107C Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT1107 S APPLICATI I FOR ATIO I V L t PEAK IN ON =( ) 20 The usual step-up configuration for the LT1107 is shown in Figure 1. The LT1107 first pulls SW1 low causing VIN – VCESAT to appear across L1. A current then builds up in L1. At the end of the switch ON time the current in L1 is1: Figure 2. Step-Down Mode Hookup + D1 L1 R3 ILIM VIN FB SW1 SW2 GND C1 LT1107 1107 F01 VIN VOUT R2 R1 + D1 1N5818 L1 R3 100 Ω ILIM VIN FB SW2 GND C1 LT1107 1107 F02 VIN VOUT R2 R1 SW1 + C2 Immediately after switch turn-off, the SW1 voltage pin starts to rise because current cannot instantaneously stop flowing in L1. When the voltage reaches VOUT + VD, the inductor current flows through D1 into C1, increasing VOUT. This action is repeated as needed by the LT1107 to keep VFB at the internal reference voltage of 1.25V. R1 and R2 set the output voltage according to the formula: Step-Down (Buck Mode) Operation A step-down DC/DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1107 based step-down converter is shown in Figure 2. When the switch turns on, SW2 pulls up to VIN – VSW. This puts a voltage across L1 equal to VIN – VSW – VOUT, causing a current to build up in L1. At the end of the switch ON time, the current in L1 is equal to: Figure 1. Step-Up Mode Hookup V R R V OUT =+ () 1 2 1 125 21 .( ) I V VV L t PEAK IN SW OUT ON = −− () 22 When the switch turns off, the SW2 pin falls rapidly and actually goes below ground. D1 turns on when SW2 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go below –0.5V. A silicon diode such as the 1N4933 will allow SW2 to go to –0.8V, causing potentially destructive power dissipation inside the LT1107. Output voltage is deter- mined by: R3 programs switch current limit. This is especially im- portant in applications where the input varies over a wide range. Without R3, the switch stays on for a fixed time each cycle. Under certain conditions the current in L1 can build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 100 Ω resistor pro- grams the switch to turn off when the current reaches approximately 700mA. When using the LT1107 in step- down mode, output voltage should be limited to 6.2V or less. Higher output voltages can be accommodated by inserting a 1N5818 diode in series with the SW2 pin (anode connected to SW2). Inverting Configurations The LT1107 can be configured as a positive-to-negative converter (Figure 3), or a negative-to-positive converter (Figure 4). In Figure 3, the arrangement is very similar to a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As in the step-down mode, D1 must be a Schottky diode, and V R R V OUT =+ () 1 2 1 125 23 .( ) Note 1: This simple expression neglects the effects of switch and coil resistance. This is taken into account in the “Inductor Selection” section. |
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