Datasheet LT1073 (Analog Devices) - 8

LT1073
APPLICATIONS INFORMATION
50mV/DIV 50mV/DIV 50mV/DIV 20µs/DIV 20µs/DIV 20µs/DIV
Figure 3. Aluminum Figure 4. Tantalum Figure 5. OS-CON
microampere range, a leaky capacitor will noticeably not short-circuit protected since there is a DC path from decrease efficiency. In this type application tantalum ca- input to output. pacitors are the best choice, with typical leakage currents The usual step-up configuration for the LT1073 is shown in the 1µA to 5µA range. in Figure 6. The LT1073 first pulls SW1 low causing VIN-
Diode Selection
VCESAT to appear across L1. A current then builds up in L1. At the end of the switch ON time the current in L1 is2: Speed, forward drop and leakage current are the three main considerations in selecting a catch diode for LT1073 i t converters. “General-purpose” rectifiers such as the PEAK = VIN ON L 1N4001 are unsuitable for use in any switching regulator
NOTE 2:
This simple expression neglects the effect of switch and coil resistance. These are application. Although they are rated at 1A, the switching taken into account in the “Inductor Selection” section. time of a 1N4001 is in the 10µs to 50µs range. At best, efficiency will be severely compromised when these L1 D1 diodes are used and at worst, the circuit may not work at VIN VOUT all. Most LT1073 circuits will be well served by a 1N5818 R3* Schottky diode. The combination of 500mV forward drop ILIM VIN R2 at 1A current, fast turn-on and turn-off time and 4µA to SW1 + 10µA leakage current fit nicely with LT1073 requirements. C1 LT1073 At peak switch currents of 100mA or less, a 1N4148 signal FB diode may be used. This diode has leakage current in the GND SW2 R1 1nA to 5nA range at 25°C and lower cost than a 1N5818. 1073 F06 (You can also use them to get your circuit up and running, *= OPTIONAL but beware of destroying the diode at 1A switch currents.)
Figure 6. Step-Up Mode Hookup.
In situations where the load is intermittent and the LT1073
(Refer to Table 1 for Component Values)
is idling most of the time, battery life can sometimes be extended by using a silicon diode such as the 1N4933, Immediately after switch turn-off, the SW1 voltage pin which can handle 1A but has leakage current of less than starts to rise because current cannot instantaneously 1µA. Efficiency will decrease somewhat compared to a stop flowing in L1. When the voltage reaches VOUT + VD, 1N5818 while delivering power, but the lower idle current the inductor current flows through D1 into C1, increasing may be more important. VOUT. This action is repeated as needed by the LT1073 to keep VFB at the internal reference voltage of 212mV. R1
Step-Up (Boost Mode) Operation
and R2 set the output voltage according to the formula: A step-up DC/DC converter delivers an output voltage   V   (212mV) higher than the input voltage. Step-up converters are OUT = 1+ R2  R1 Rev B 8 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