基于lt1301和lt1300的微功率DC/DC转换器应用

　　Introduction

　　The new LT1300 and LT1301 micropower DC/DC converters provide improvements in both electrical and physical efficiency， two key areas of battery-based power supply design. Housed in 8-lead DIP or SOIC packages， the devices feature a 1A on-chip switch with a VCESAT of just 170mV. The internal oscillator frequency is set at 155kHz， allowing the use of tiny， 5mm diameter surface mount inductors along with standard D-case size tantalum capacitors. A complete 2-cell to 12V， 5V， or 3.3V converter can fit in less than 0.4 square inches of PC board area.

　　The devices use Burst ModeTM operation to maintain high efficiency across the full load range. The fully operating quiescent current is only 120µA. It can be further reduced to 10µA by taking the SHUTDOWN pin high， which also disables the device. The output voltage of the LT1300 can be set at either 5V or 3.3V via the logic-controlled SELECT pin， and the LT1301 output can be set at either 5V or 12V using the same pin. The ILIM pin allows the reduction of peak switch current and allows the use of even smaller components. The switch current is nominally set at 1A and can be reduced via the ILIM pin to approximately 400mA， further improving efficiency in systems requiring lower peak powers.

　　Theory of Operation Figure 1 is a block diagram of the LT1300/LT1301. Refer also to Figure 2 for associated component hookup. When A1’s negative input， related to the SENSE pin voltage by the appropriate resistor-divider ratio， is higher than the 1.25V reference voltage， A1’s output is low. A2， A3 and the oscillator are turned off， drawing no current. Only the reference and A1 consume current， typically 120µA. When the voltage at A1’s negative input decreases below 1.25V， overcoming A1’s 6mV hysteresis， A1’s output goes high， enabling the oscillator， current comparator A2， and driver A3. Quiescent current increases to 2mA as the device prepares for high current switching. Q1 then turns on in a controlled saturation for （nominally） 5.3µs or until current comparator A2 trips， whichever comes first. After a fixed off-time of （nominally） 1.2µs， Q1 turns on again. Refering to Figure 2， the LT1300’s switching causes current to alternately build up in L1 and dump into output capacitor C1via D1， increasing the output voltage. When the output is high enough to cause A1’s output to go low （Figure 1）， switching action ceases. C1 is left to supply current to the