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SP4446EK Datasheet(PDF) 3 Page - Sipex Corporation |
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SP4446EK Datasheet(HTML) 3 Page - Sipex Corporation |
3 / 10 page 3 Rev. 7/14/03 SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator © Copyright 2003 Sipex Corporation FUNCTIONAL DIAGRAM + - VI (EXTERNAL) (EXTERNAL) R1 R2 C1 SHDN 4 Q1 Q2 R3 R4 R6 R5 X1 Shutdown Logic 300ns ONE-SHOT CLEAR X2 + - SET DRIVER 52.5mV 0.35 GND 2 1 5 3 L1 SW C2 D1 VOUT POWER TRANSISTOR DISABLE VIN FB General Overview: Operation can be best understood by referring to the above block diagram. Q1 and Q2 along with R3 and R4 form a band gap reference. The input to this circuit completes a feedback path from the high voltage output through a voltage di- vider, and is used as the regulation control input. When the voltage at the FB pin is slightly above 1.22V, comparator X1 disables most of the internal circuitry. Current is then provided by capacitor C2, which slowly discharges until the voltage at the FB pin drops below the lower hysteresis point of X1, about 6mV. X1 then enables the internal circuitry, turns on chip power, and the current in the inductor begins to ramp up. When the current through the driver transistor reaches about 150mA, comparator X2 clears the latch, which turns off the driver tran- sistor for a preset 0.3 µs. At the instant of shutoff, inductor current is diverted to the output through diode D1. During this 0.3 µs time limit, inductor current decreases while its energy charges C2. THEORY OF OPERATION At the end of the 0.3 µs time period, driver transistor is again allowed to turn on which ramps the current back up to the 150mA level. Comparator X2 clears the latch, it’s output turns off the driver transistor, and this allows delivery of L1’s stored kinetic energy to C2. This switch- ing action continues until the output capacitor voltage is charged to the point where FB is at band gap (1.22V). When this condition is reached, X1 turns off the internal circuitry and the cycle repeats. The SP4446 contains circuitry to provide protection during start-up and while in short-circuit conditions. When FB pin volt- age is less than approximately 300mV, the switch off time is increased to about 1.2 µs and the current limit is reduced to about 70% of its normal value. While in this mode, the average inductor current is reduced and helps minimize power dissipation in the SP4446, the external inductor and diode. |
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