Datasheet LT1308A, LT1308B (Analog Devices) - 8

LT1308A/LT1308B
APPLICATIONS INFORMATION OPERATION
Low-battery detector A4’s open-collector output (LBO) pulls low when the LBI pin voltage drops below 200mV. The LT1308A combines a current mode, fi xed frequency There is no hysteresis in A4, allowing it to be used as an PWM architecture with Burst Mode micropower opera- amplifi er in some applications. The entire device is disabled tion to maintain high effi ciency at light loads. Operation when the SHDN pin is brought low. To enable the converter, can be best understood by referring to the block diagram SHDN must be at 1V or greater. It need not be tied to V in Figure 2. Q1 and Q2 form a bandgap reference core IN as on the LT1308. whose loop is closed around the output of the converter. When VIN is 1V, the feedback voltage of 1.22V, along with The LT1308B differs from the LT1308A in that there is no an 80mV drop across R5 and R6, forward biases Q1 and hysteresis in comparator A1. Also, the bias point on A1 is Q2’s base collector junctions to 300mV. Because this is not set lower than on the LT1308B so that switching can occur enough to saturate either transistor, FB can be at a higher at inductor current less than 100mA. Because A1 has no voltage than VIN. When there is no load, FB rises slightly hysteresis, there is no Burst Mode operation at light loads above 1.22V, causing VC (the error amplifi er’s output) to and the device continues switching at constant frequency. decrease. When VC reaches the bias voltage on hyster- This results in the absence of low frequency output voltage etic comparator A1, A1’s output goes low, turning off ripple at the expense of effi ciency. all circuitry except the input stage, error amplifi er and The difference between the two devices is clearly illus- low-battery detector. Total current consumption in this trated in Figure 3. The top two traces in Figure 3 shows an state is 140μA. As output loading causes the FB voltage to LT1308A/LT1308B circuit, using the components indicated decrease, A1’s output goes high, enabling the rest of the IC. in Figure 1, set to a 5V output. Input voltage is 3V. Load Switch current is limited to approximately 400mA initially current is stepped from 50mA to 800mA for both circuits. after A1’s output goes high. If the load is light, the output Low frequency Burst Mode operation voltage ripple is voltage (and FB voltage) will increase until A1’s output goes observed on Trace A, while none is observed on Trace B. low, turning off the rest of the LT1308A. Low frequency ripple voltage appears at the output. The ripple frequency At light loads, the LT1308B will begin to skip alternate cycles. is dependent on load current and output capacitance. The load point at which this occurs can be decreased by This Burst Mode operation keeps the output regulated increasing the inductor value. However, output ripple will and reduces average current into the IC, resulting in high continue to be signifi cantly less than the LT1308A output effi ciency even at load currents of 1mA or less. ripple. Further, the LT1308B can be forced into micropower mode, where I If the output load increases suffi ciently, A1’s output Q falls from 3mA to 200μA by sinking 40μA or more out of the V remains high, resulting in continuous operation. When the C pin. This stops switching by causing A1’s output to go low. LT1308A 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 TRACE A: LT1308A the summation of a signal representing switch current VOUT, 100mV/DIV AC COUPLED and a ramp generator (introduced to avoid subharmonic TRACE B: LT1308B oscillations at duty factors greater than 50%) exceeds the VOUT, 100mV/DIV AC COUPLED VC signal, comparator A2 changes state, resetting the fl ip- 800mA fl op and turning off the switch. Output voltage increases ILOAD as switch current is increased. The output, attenuated 50mA 1308 F03 by a resistor divider, appears at the FB pin, closing the 200μs/DIV VIN = 3V overall loop. Frequency compensation is provided by an (CIRCUIT OF FIGURE 1) external series RC network connected between the VC pin
Figure 3. LT1308A Exhibits Burst Mode Operation Output
and ground.
Voltage Ripple at 50mA Load, LT1308B Does Not
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