Datasheet LT1107 (Analog Devices) - 10
| Hersteller | Analog Devices |
| Beschreibung | Micropower DC/DC Converter Adjustable and Fixed 5V, 12V |
| Seiten / Seite | 16 / 10 — APPLICATI. S I FOR ATIO. Using the ILIM Pin. Figure 3. … |
| Dateiformat / Größe | PDF / 217 Kb |
| Dokumentensprache | Englisch |
APPLICATI. S I FOR ATIO. Using the ILIM Pin. Figure 3. Positive-to-Negative Converter. Figure 4. Negative-to-Positive Converter
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LT1107
O U U W U APPLICATI S I FOR ATIO
VOUTshould be less than 6.2V. More negative output
Using the ILIM Pin
voltages can be accommodated as in the prior section. The LT1107 switch can be programmed to turn off at a set In Figure 4, the input is negative while the output is switch current, a feature not found on competing devices. positive. In this configuration, the magnitude of the input This enables the input to vary over a wide range without voltage can be higher or lower than the output voltage. A exceeding the maximum switch rating or saturating the level shift, provided by the PNP transistor, supplies proper inductor. Consider the case where analysis shows the polarity feedback information to the regulator. LT1107 must operate at an 800mA peak switch current with a 2V input. If VIN rises to 4V, the peak switch current +VIN will rise to 1.6A, exceeding the maximum switch current + C2 R3 rating. With the proper resistor selected (see the “Maxi- mum Switch Current vs RLIM” characteristic), the switch ILIM VIN SW1 FB current will be limited to 800mA, even if the input voltage LT1107 increases. L1 SW2 Another situation where the I GND LIM feature is useful occurs R1 + D1 when the device goes into continuous mode operation. C1 1N5818 This occurs in step-up mode when: R2 –VOUT 1107 F03 V + V OUT DIODE < 1 24 ( )
Figure 3. Positive-to-Negative Converter
V − V 1− DC I N SW When the input and output voltages satisfy this relation- D1 L1 +V ship, inductor current does not go to zero during the OUT + R1 switch OFF time. When the switch turns on again, the R3 C1 current ramp starts from the non-zero current level in the 2N3906 ILIM VIN inductor just prior to switch turn-on. As shown in Figure SW1 + 5, the inductor current increases to a high level before the LT1107 C2 comparator turns off the oscillator. This high current can FB cause excessive output ripple and requires oversizing the GND SW2 R2 R1 VOUT = 1.25V + 0.6V (R2) output capacitor and inductor. With the ILIM feature, the –V 1107 F04 IN switch turns off at the programmed current as shown in
Figure 4. Negative-to-Positive Converter
Figure 6, keeping output ripple to a minimum. 1107fa 10
O U U W U APPLICATI S I FOR ATIO
VOUTshould be less than 6.2V. More negative output
Using the ILIM Pin
voltages can be accommodated as in the prior section. The LT1107 switch can be programmed to turn off at a set In Figure 4, the input is negative while the output is switch current, a feature not found on competing devices. positive. In this configuration, the magnitude of the input This enables the input to vary over a wide range without voltage can be higher or lower than the output voltage. A exceeding the maximum switch rating or saturating the level shift, provided by the PNP transistor, supplies proper inductor. Consider the case where analysis shows the polarity feedback information to the regulator. LT1107 must operate at an 800mA peak switch current with a 2V input. If VIN rises to 4V, the peak switch current +VIN will rise to 1.6A, exceeding the maximum switch current + C2 R3 rating. With the proper resistor selected (see the “Maxi- mum Switch Current vs RLIM” characteristic), the switch ILIM VIN SW1 FB current will be limited to 800mA, even if the input voltage LT1107 increases. L1 SW2 Another situation where the I GND LIM feature is useful occurs R1 + D1 when the device goes into continuous mode operation. C1 1N5818 This occurs in step-up mode when: R2 –VOUT 1107 F03 V + V OUT DIODE < 1 24 ( )
Figure 3. Positive-to-Negative Converter
V − V 1− DC I N SW When the input and output voltages satisfy this relation- D1 L1 +V ship, inductor current does not go to zero during the OUT + R1 switch OFF time. When the switch turns on again, the R3 C1 current ramp starts from the non-zero current level in the 2N3906 ILIM VIN inductor just prior to switch turn-on. As shown in Figure SW1 + 5, the inductor current increases to a high level before the LT1107 C2 comparator turns off the oscillator. This high current can FB cause excessive output ripple and requires oversizing the GND SW2 R2 R1 VOUT = 1.25V + 0.6V (R2) output capacitor and inductor. With the ILIM feature, the –V 1107 F04 IN switch turns off at the programmed current as shown in
Figure 4. Negative-to-Positive Converter
Figure 6, keeping output ripple to a minimum. 1107fa 10