Datasheet LT1073 (Analog Devices) - 9

LT1073
APPLICATIONS INFORMATION Step-Down (Buck Mode) Operation
saturating the inductor. The 220Ω resistor programs the A step-down DC/DC converter converts a higher voltage switch to turn off when the current reaches approximately to a lower voltage. It is short-circuit protected because the 400mA. When using the LT1073 in step-down mode, output switch is in series with the output. Step-down converters voltage should be limited to 6.2V or less. are characterized by low output voltage ripple but high in-
Inverting Configurations
put current ripple. The usual hookup for an LT1073-based step-down converter is shown in Figure 7. The LT1073 can be configured as a positive-to-negative converter (Figure 8), or a negative-to-positive converter VIN (Figure 9). In Figure 8, the arrangement is very similar to R3 220Ω a step-down, except that the high side of the feedback is ILIM VIN SW1 referred to ground. This level shifts the output negative. FB As in the step-down mode, D1 must be a Schottky diode, LT1073 L1 and V + OUT should be less than 6.2V. C2 SW2 VOUT In Figure 9, the input is negative while the output is positive. GND R2 + D1 In this configuration, the magnitude of the input voltage 1N5818 C1 R1 can be higher or lower than the output voltage. A level shift, provided by the PNP transistor, supplies proper polarity 1073 FO7 feedback information to the regulator.
Figure 7. Step-Down Mode Hookup
+VIN + When the switch turns on, SW2 pulls up to VIN – VSW. C2 R3 This puts a voltage across L1 equal to VIN – VSW – VOUT, ILIM VIN SW1 causing a current to build up in L1. At the end of the FB switch ON time, the current in L1 is equal to LT1073 L1 SW2 iPEAK = VIN – VSW – VOUT tON GND R2 L + When the switch turns off the SW2 pin falls rapidly and D1 1N5818 C1 R1 actually goes below ground. D1 turns on when SW2 –VOUT reaches 0.4V below ground. D1 MUST BE A SCHOTTKY 1073 FO8 DIODE. The voltage at SW2 must never be allowed to go
Figure 8. Positive-to-Negative Converter
below –0.5V. A silicon diode such as the 1N4933 will al- low SW2 to go to –0.8V, causing potentially destructive L1 D1 +V power dissipation inside the LT1073. Output voltage is OUT + determined by C1 R1 I   LIM VIN V + SW1 2N3906 OUT = 1+ R2 (212mV) C2  R1 LT1073 AO FB R3 programs switch current limit. This is especially im- GND SW2 portant in applications where the input varies over a wide R2 range. Without R3, the switch stays on for a fixed time each –VIN V R1 OUT = ( )212mV + 0.6V cycle. Under certain conditions the current in L1 can build R2 1073 F09 up to excessive levels, exceeding the switch rating and/or
Figure 9. Negative-to-Positive Converter
Rev B 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagrams Operation Applications Information Typical Applications Package Description Typical Application Related Parts