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LT1307B Datasheet(PDF) 8 Page - Linear Technology |
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LT1307B Datasheet(HTML) 8 Page - Linear Technology |
8 / 20 page 8 LT1307/LT1307B 1307fa OPERATION The LT1307 combines a current mode, fixed frequency PWM architecture with Burst Mode micropower operation to maintain high efficiency at light loads. Operation can best be understood by referring to the block diagram in Figure 2. Q1 and Q2 form a bandgap reference core whose loop is closed around the output of the converter. When VIN is 1V, the feedback voltage of 1.22V, along with an 80mV drop across R5 and R6, forward biases Q1 and Q2’s base collector junctions to 300mV. Because this is not enough to saturate either transistor, FB can be at a higher voltage than VIN. When there is no load, FB rises slightly above 1.22V, causing VC (the error amplifier’s output) to decrease. When VC reaches the bias voltage on hysteretic comparator A1, A1’s output goes low, turning off all circuitry except the input stage, error amplifier and low- battery detector. Total current consumption in this state is 50 µA. As output loading causes the FB voltage to de- crease, A1’s output goes high, enabling the rest of the IC. Switch current is limited to approximately 100mA initially after A1’s output goes high. If the load is light, the output voltage (and FB voltage) will increase until A1’s output goes low, turning off the rest of the LT1307. Low fre- quency ripple voltage appears at the output. The ripple frequency is dependent on load current and output capaci- tance. This Burst Mode operation keeps the output regu- lated and reduces average current into the IC, resulting in high efficiency even at load currents of 100 µA or less. If the output load increases sufficiently, A1’s output re- mains high, resulting in continuous operation. When the LT1307 is running continuously, peak switch current is controlled by VC to regulate the output voltage. The switch is turned on at the beginning of each switch cycle. When the summation of a signal representing switch current and a ramp generator (introduced to avoid subharmonic oscil- lations at duty factors greater than 50%) exceeds the VC signal, comparator A2 changes state, resetting the flip- flop and turning off the switch. Output voltage increases as switch current is increased. The output, attenuated by a resistor divider, appears at the FB pin, closing the overall loop. Frequency compensation is provided by an external series RC network connected between the VC pin and ground. Low-battery detector A4’s open collector output (LBO) pulls low when the LBI pin voltage drops below 200mV. There is no hysteresis in A4, allowing it to be used as an amplifier in some applications. The entire device is disabled when the SHDN pin is brought low. To enable the converter, SHDN must be at VIN or at a higher voltage. The LT1307B differs from the LT1307 in that there is no hysteresis in comparator A1. Also, the bias point on A1 is set lower than on the LT1307 so that switching can occur at inductor current less than 100mA. Because A1 has no hysteresis, there is no Burst Mode operation at light loads and the device continues switching at constant frequency. This results in the absence of low frequency output voltage ripple at the expense of efficiency. The difference between the two devices is clearly illus- trated in Figures 3 and 4. The top two traces in Figure 3 show an LT1307/LT1307B circuit, using the components indicated in Figure 1, set to a 5V output. Input voltage is 1.25V. Load current is stepped from 1mA to 41mA for both circuits. Low frequency Burst Mode operation voltage ripple is observed on Trace A, while none is observed on TRACE A TRACE B LT1307 VOUT 500mV/DIV AC COUPLED 41mA 1mA IL LT1307B VOUT 500mV/DIV AC COUPLED VIN = 1.25V 1ms/DIV 1307 F03 VOUT = 5V Figure 3. LT1307 Exhibits Burst Mode Operation Ripple at 1mA Load, LT1307B Does Not LT1307 VOUT 200mV/DIV AC COUPLED TRACE A 45mA 5mA IL LT1307B VOUT 200mV/DIV AC COUPLED TRACE B VIN = 1.5V 500 µs/DIV 1307 F04 VOUT = 5V Figure 4. At Higher Loading and a 1.5V Supply, LT1307 Again Exhibits Burst Mode Operation Ripple at 5mA Load, LT1307B Does Not APPLICATIO S I FOR ATIO |
Similar Part No. - LT1307B_15 |
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