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LTC1148HVIS-5 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1148HVIS-5 Datasheet(HTML) 7 Page - Linear Technology |
7 / 20 page 7 LTC1148 LTC1148-3.3/LTC1148-5 114835fd OPERATIO The LTC1148 series uses a current mode, constant off- time architecture to synchronously switch an external pair of complementary power MOSFETs. Operating fre- quency is set by an external capacitor at the timing capacitor Pin 4. The output voltage is sensed by an internal voltage divider connected to SENSE – Pin 7 (LTC1148-3.3 and LTC1148-5) or external divider returned to VFB Pin 9 (LTC1148). A voltage comparator V, and a gain block G, compare the divided output voltage with a reference voltage of 1.25V. To optimize efficiency, the LTC1148 series automatically switches between two modes of operation, burst and continuous. The voltage compara- tor is the primary control element when the device is in Burst Mode operation, while the gain block controls the output voltage in continuous mode. During the switch “ON” cycle in continuous mode, current comparator C monitors the voltage between Pins 7 and 8 connected across an external shunt in series with the inductor. When the voltage across the shunt reaches its threshold value, the P-drive output is switched to VIN, turning off the P-channel MOSFET. The timing capacitor connected to Pin 4 is now allowed to discharge at a rate determined by the off-time controller. The discharge cur- rent is made proportional to the output voltage (measured by Pin 7) to model the inductor current, which decays at a rate which is also proportional to the output voltage. While the timing capacitor is discharging, the N-drive output goes to VIN, turning on the N-channel MOSFET. When the voltage on the timing capacitor has discharged past VTH1, comparator T trips, setting the flip-flop. This causes the N-drive output to go low (turning off the N- channel MOSFET) and the P-drive output to also go low (turning the P-channel MOSFET back on). The cycle then repeats. As the load current increases, the output voltage de- creases slightly. This causes the output of the gain stage (Pin 6) to increase the current comparator threshold, thus tracking the load current. The sequence of events for Burst Mode operation is very similar to continuous operation with the cycle interrupted by the voltage comparator. When the output voltage is at or above the desired regulated value, the P-channel MOSFET is held off by comparator V and the timing capacitor continues to discharge below VTH1. When the timing capacitor discharges past VTH2, voltage comparator S trips, causing the internal sleep line to go low and the N- channel MOSFET to turn off. The circuit now enters sleep mode with both power MOSFETs turned off. In sleep mode, a majority of the circuitry is turned off, dropping the quiescent current from 1.6mA to 160µA. The load current is now being supplied from the output capacitor. When the output voltage has dropped by the amount of hysteresis in comparator V, the P-channel MOSFET is again turned on and the process repeats. To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset (VOS) is incorpo- rated in the gain stage. This prevents the current compara- tor threshold from increasing until the output voltage has dropped below a minimum threshold. To prevent both the external MOSFETs from ever being turned on at the same time, feedback is incorporated to sense the state of the driver output pins. Before the N-drive output can go high, the P-drive output must also be high. Likewise, the P-drive output is prevented from going low while the N-drive output is high. Using constant off-time architecture, the operating fre- quency is a function of the input voltage. To minimize the frequency variation as dropout is approached, the off-time controller increases the discharge current as VIN drops below VOUT + 1.5V. In dropout the P-channel MOSFET is turned on continuously (100% duty cycle), providing extremely low dropout operation. |
Similar Part No. - LTC1148HVIS-5 |
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Similar Description - LTC1148HVIS-5 |
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