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LT1308A Datasheet(PDF) 9 Page - Linear Technology |
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LT1308A Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT1308A/LT1308B APPLICATIONS INFORMATION SHDN PIN The LT1308A/LT1308B SHDN pin is improved over the LT1308. The pin does not require tying to VIN to enable the device, but needs only a logic level signal. The voltage on the SHDN pin can vary from 1V to 10V independent of VIN. Further, floating this pin has the same effect as grounding, which is to shut the device down, reducing current drain to 1 µA or less. LOW-BATTERY DETECTOR The low-battery detector on the LT1308A/LT1308B fea- tures improved accuracy and drive capability compared to the LT1308. The 200mV reference has an accuracy of ±2% and the open-collector output can sink 50 µA.TheLT1308A/ LT1308B low-battery detector is a simple PNP input gain stage with an open-collector NPN output. The negative input of the gain stage is tied internally to a 200mV reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward. Figure 7 details hookup. R1 and R2 need only be low enough in value so that the bias current of the LBI pin doesn’t cause large errors. For R2, 100k is adequate. The 200mV refer- ence can also be accessed as shown in Figure 8. A cross plot of the low-battery detector is shown in Figure 9. The LBI pin is swept with an input which varies from 195mV to 205mV, and LBO with a 100k pull-up resistor, is displayed. START-UP The LT1308A/LT1308B can start up into heavy loads, unlike many CMOS DC/DC converters that derive operat- ing voltage from the output (a technique known as “bootstrapping”). Figure 10 details start-up waveforms of Figure 1’s circuit with a 20 Ω load and VIN of 1.5V. Inductor current rises to 3.5A as the output capacitor is charged. After the output reaches 5V, inductor current is about 1A. In Figure 11, the load is 5 Ω and input voltage is 3V. Output voltage reaches 5V 500 µs after the device is enabled. Figure 12 shows start-up behavior of Figure 5’s SEPIC circuit, driven from a 9V input with a 10 Ω load. The output reaches 5V in about 1ms after the device is enabled. GSM AND CDMA PHONES The LT1308A/LT1308B are suitable for converting a single Li-Ion cell to 5V for powering RF power stages in GSM or CDMA phones. Improvements in the LT1308A/LT1308B error amplifiers allow external compensation values to be reduced, resulting in faster transient response compared to the LT1308. The circuit of Figure 13 (same as Figure 1, printed again for convenience) provides a 5V, 1A output from a Li-Ion cell. Figure 14 details transient response at the LT1308A operating at a VIN of 4.2V, 3.6V and 3V. Ripple voltage in Burst Mode operation can be seen at 10mA load. Figure 15 shows transient response of the LT1308B under the same conditions. Note the lack of Burst Mode ripple at 10mA load. Figure 7. Setting Low-Battery Detector Trip Point Figure 8. Accessing 200mV Reference LBO LBI TO PROCESSOR R1 100k R2 100k VIN VBAT LT1308A LT1308B 1308 F07 5V GND 200mV INTERNAL REFERENCE – + R1 = VLB – 200mV 2 µA VIN VBAT LT1308A LT1308B LBI LBO 200k 10 µF GND 10k 1308 F08 2N3906 VREF 200mV + |
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