DC DC Converter From 1 5V To 34V

Posted on Feb 6, 2014

An interesting DC/DC converter IC is available from Linear Technology. The LT1615 step-up switching voltage regulator can generate an output voltage of up to +34V from a +1. 2 to +15V supply, using only a few external components. The tiny 5-pin SOT23 package makes for very compact construction. This IC can for example be used to generate the high v

DC DC Converter From 1 5V To 34V
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oltage needed for an LCD screen, the tuning voltage for a varicap diode and so on. The internal circuit diagram of the LT1615 is shown in Figure 1. It contains a monostable with a pulse time of 400 ns, which determines the off time of the transistor switch. If the voltage sampled at the feedback input drops below the reference threshold level of 1. 23 V, the transistor switches on and the current in the coil starts to increase. This builds up energy in the magnetic ¬eld of the coil. When the current through the coil reaches 350 mA, the monostable is triggered and switches the transistor off for the following 400 ns. Since the energy stored in the coil must go somewhere, current continues to ¬‚ow through the coil, but it decreases linearly. This current charges the output capacitor via the Schottky diode (SS24, 40V/2A). As long as the voltage at FB remains higher than 1. 23V, nothing else happens. As soon as it drops below this level, however, the whole cycle is repeated. The hysteresis at the FB input is 8mV. The output voltage can be calculated using the formula Vout = 1. 23V (R1+R2) / R2 The value of R1 can be selected in the megohm range, since the current into the FB input is only a few tens of nano-amperes. When the supply voltage is switched on, or if the output is short-circuited, the IC enters the power-up mode. As long as the voltage at FB is less than 0. 6V, the LT1615 output current is limited to 250mA instead...

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