Datasheet LT1614 (Analog Devices) - 6

LT1614
U OPERATIO
The LT1614 combines a current mode, fixed frequency The LT1614 can work in either of two topologies. The PWM architecture with a –1.23V reference to directly simpler topology appends a capacitive level shift to a regulate negative outputs. Operation can be best under- boost converter, generating a negative output voltage, stood by referring to the block diagram of Figure 2. Q1 and which is directly regulated. The circuit schematic is de- Q2 form a bandgap reference core whose loop is closed tailed in Figure 3. Only one inductor is required, and the around the output of the converter. The driven reference two diodes can be in a single SOT-23 package. Output point is the lower end of resistor R4, which normally sits noise is the same as in a boost converter, because current at a voltage of –1.23V. As the load current changes, the is delivered to the output only during the time when the NFB pin voltage also changes slightly, driving the output LT1614’s internal switch is on. of gm amplifier A1. Switch current is regulated directly on If D2 is replaced by an inductor, as shown in Figure 4, a a cycle-to-cycle basis by A1’s output. The flip-flop is set at higher performance solution results. This converter topol- the beginning of each cycle, turning on the switch. When ogy was developed by Professor S. Cuk of the California the summation of a signal representing switch current and Institute of Technology in the 1970s. A low ripple voltage a ramp generator (introduced to avoid subharmonic oscil- results with this topology due to inductor L2 in series with lations at duty factors greater than 50%) exceeds the VC the output. Abrupt changes in output capacitor current are signal, comparator A2 changes stage, resetting the flip- eliminated because the output inductor delivers current to flop and turning off the switch. Output voltage decreases the output during both the off-time and the on-time of the (the magnitude increases) as switch current is increased. LT1614 switch. With proper layout and high quality output The output, attenuated by external resistor divider R1 and capacitors, output ripple can be as low as 1mV R2, appears at the NFB pin, closing the overall loop. P–P. Frequency compensation is provided externally by a series The operation of Cuk’s topology is shown in Figures 5 RC connected from the V and␣ 6. During the first switching phase, the LT1614’s C pin to ground. Typical values are 100k and 1nF. Transient response can be tailored by switch, represented by Q1, is on. There are two current adjustment of these values. loops in operation. The first loop begins at input capacitor C1, flows through L1, Q1 and back to C1. The second loop As load current is decreased, the switch turns on for a flows from output capacitor C3, through L2, C2, Q1 and shorter period each cycle. If the load current is further back to C3. The output current from R decreased, the converter will skip cycles to maintain LOAD is supplied by L2 and C3. The voltage at node SW is V output voltage regulation. CESAT and at node SWX the voltage is –(VIN + |VOUT|). Q1 must conduct both L1 and L2 current. C2 functions as a voltage level shifter, with an approximately constant voltage of (VIN + |VOUT|) across it. C2 C2 1µF 1µF L1 D2 L1 L2 VIN VIN + D1 D1 V C1 IN SW V + IN SW LT1614 –VOUT –V C1 LT1614 OUT SHUTDOWN SHDN R1 SHUTDOWN SHDN R1 V NFB C C3 + V NFB C C3 + GND R2 GND 10Ok R2 10Ok 10k 10k 1nF 1nF 1614 F03 1614 F04
Figure 3. Direct Regulation of Negative Output Figure 4. L2 Replaces D2 to Make Low Output Ripple Using Boost Converter with Charge Pump Inverting Topology. Coupled or Uncoupled Inductors Can Be Used. Follow Phasing If Coupled for Best Results
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