Datasheet LT1110 (Linear Technology) - 10

LT1110
O U U W U APPLICATI S I FOR ATIO Diode Selection
Immediately after switch turn off, the SW1 voltage pin starts to rise because current cannot instantaneously stop Speed, forward drop, and leakage current are the three flowing in L1. When the voltage reaches V main considerations in selecting a catch diode for LT1110 OUT + VD, the inductor current flows through D1 into C1, increasing converters. General purpose rectifiers such as the 1N4001 V are unsuitable for use in any switching regulator applica- OUT. This action is repeated as needed by the LT1110 to keep V tion. Although they are rated at 1A, the switching time of FB at the internal reference voltage of 220mV. R1 and R2 set the output voltage according to the formula a 1N4001 is in the 10µs-50µs range. At best, efficiency will be severely compromised when these diodes are used; at  R2 worst, the circuit may not work at all. Most LT1110 circuits V 1 mV OUT = + 220 . 21 ( )  R1 ( ) will be well served by a 1N5818 Schottky diode, or its surface mount equivalent, the MBRS130T3. The combina-
Step-Down (Buck Mode) Operation
tion of 500mV forward drop at 1A current, fast turn ON and turn OFF time, and 4µA to 10µA leakage current fit nicely A step-down DC-DC converter converts a higher voltage with LT1110 requirements. At peak switch currents of to a lower voltage. The usual hookup for an LT1110 based 100mA or less, a 1N4148 signal diode may be used. This step-down converter is shown in Figure 5. diode has leakage current in the 1nA-5nA range at 25°C VIN and lower cost than a 1N5818. (You can also use them to R3 get your circuit up and running, but beware of destroying 220Ω the diode at 1A switch currents.) + ILIM VIN SW1 C2 FB
Step-Up (Boost Mode) Operation
LT1110 L1 A step-up DC-DC converter delivers an output voltage SW2 VOUT GND R2 higher than the input voltage. Step-up converters are not D1 + short circuit protected since there is a DC path from input 1N5818 C1 R1 to output. The usual step-up configuration for the LT1110 is shown LT1110 • TA15 in Figure 4. The LT1110 first pulls SW1 low causing V
Figure 5. Step-Down Mode Hookup
IN – VCESAT to appear across L1. A current then builds up in L1. When the switch turns on, SW2 pulls up to VIN – VSW. This At the end of the switch ON time the current in L1 is1: puts a voltage across L1 equal to VIN – VSW – VOUT, V causing a current to build up in L1. At the end of the switch I I N = t PEAK ( ) 20 L ON ON time, the current in L1 is equal to L1 D1 V V V IN OUT I N − V − V I SW OUT = t PEAK . ( ) 22 R3* L ON ILIM VIN When the switch turns off, the SW2 pin falls rapidly and R2 SW1 + actually goes below ground. D1 turns on when SW2 LT1110 FB C1 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go GND SW2 R1 below –0.5V. A silicon diode such as the 1N4933 will allow SW2 to go to –0.8V, causing potentially destructive power * = OPTIONAL LT1110 • TA14
Note 1
: This simple expression neglects the effects of switch and coil
Figure 4. Step-Up Mode Hookup.
resistance. This is taken into account in the “Inductor Selection” section. 10