Datasheet LT1611 (Analog Devices) - 5

LT1611
U OPERATIO
boost converter, generating a negative output voltage, When Q1 turns off during the second phase of switching, which is directly regulated. The circuit schematic is de- the SW node voltage abruptly increases to (VIN + |VOUT|). tailed in Figure 3. Only one inductor is required, and the The SWX node voltage increases to VD (about 350mV). two diodes can be in a single SOT-23 package. Output Now current in the first loop, begining at C1, flows through noise is the same as in a boost converter, because current L1, C2, D1 and back to C1. Current in the second loop flows is delivered to the output only during the time when the from C3 through L2, D1 and back to C3. Load current LT1611’s internal switch is off. continues to be supplied by L2 and C3. If D2 is replaced by an inductor, as shown in Figure 4, a An important layout issue arises due to the chopped higher performance solution results. This converter topol- nature of the currents flowing in Q1 and D1. If they are both ogy was developed by Professor S. Cuk of the California tied directly to the ground plane before being combined, Institute of Technology in the 1970s. A low ripple voltage switching noise will be introduced into the ground plane. results with this topology due to inductor L2 in series with It is almost impossible to get rid of this noise, once present the output. Abrupt changes in output capacitor current are in the ground plane. The solution is to tie D1’s cathode to eliminated because the output inductor delivers current to the ground pin of the LT1611 before the combined cur- the output during both the off-time and the on-time of the rents are dumped into the ground plane as drawn in LT1611 switch. With proper layout and high quality output Figures 4, 5 and 6. This single layout technique can capacitors, output ripple can be as low as 1mVP–P. virtually eliminate high frequency “spike” noise so often present on switching regulator outputs. The operation of Cuk’s topology is shown in Figures 5 and␣ 6. During the first switching phase, the LT1611’s Output ripple voltage appears as a triangular waveform switch, represented by Q1, is on. There are two current riding on VOUT. Ripple magnitude equals the ripple current loops in operation. The first loop begins at input capacitor of L2 multiplied by the equivalent series resistance (ESR) C1, flows through L1, Q1 and back to C1. The second loop of output capacitor C3. Increasing the inductance of L1 flows from output capacitor C3, through L2, C2, Q1 and and L2 lowers the ripple current, which leads to lower back to C3. The output current from RLOAD is supplied by output voltage ripple. Decreasing the ESR of C3, by using L2 and C3. The voltage at node SW is VCESAT and at node ceramic or other low ESR type capacitors, lowers output SWX the voltage is –(VIN + |VOUT|). Q1 must conduct both ripple voltage. Output ripple voltage can be reduced to L1 and L2 current. C2 functions as a voltage level shifter, arbitrarily low levels by using large value inductors and with an approximately constant voltage of (VIN + |VOUT|) low ESR, high value capacitors. across it. C2 C2 1µF 1µF L1 D2 L1 L2 VIN VIN D1 D1 V + IN SW V + IN SW –VOUT C1 –VOUT C1 LT1611 R1 LT1611 R1 SHUTDOWN SHDN NFB C3 + NFB C3 + GND R2 GND R2 10k 10k 1611 F03 1611 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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