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LTC3452 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3452 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC3452 3452f duty cycle DC4SW. The input voltage VIN where the four switch region begins is given by: V V ns f IN OUT = () 1 150 –• and the input voltage VIN where the four switch region ends is given by: V V ns f IN OUT = () [] •– • 1 150 Boost Mode (VIN < VOUT) In boost mode, switch A is always on and switch B is always off. Referring to Figure 2, when the control voltage VC is above voltage V3, switches C and D will alternate conducting similar to a typical synchronous boost regula- tor. The maximum duty cycle of the converter is limited to 88% typical and is reached when VC is above V4. Forward Current Limit If the current delivered from VIN through PMOS switch A exceeds 1600mA (typical), switch A is shut off immedi- ately. Switches B and D are turned on for the remainder of the cycle in order to safely discharge the forward inductor current at the maximum rate possible. Reverse Current Limit If the current delivered from VOUT backwards through PMOS switch D exceeds 200mA (typical), switch D is shut off immediately. Switches A and C are turned on for the remainder of the cycle in order to safely discharge the reverse inductor current at the maximum rate possible. Undervoltage Lockout To prevent operation of the power switches at high RDS(ON), an undervoltage lockout is incorporated on the LTC3452. When the input supply voltage drops below approximately 1.9V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws only a few microamperes. Overtemperature Protection If the junction temperature of the LTC3452 exceeds 130 °C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hys- teresis of 11 °C. Soft-Start The LTC3452 includes an internally fixed soft-start which is active when powering up or coming out of shutdown. The soft-start works by clamping the voltage on the VC node and gradually releasing it such that it requires 650 µs to linearly slew from 0.9V to 2.1V. This has the effect of limiting the rate of duty cycle change as VC transitions from the buck region through the buck-boost region into the boost region. Once the soft-start times out, it can only be reset by entering shutdown, or by an undervoltage or overtemperature condition. Main Error Amp The main error amplifier is a transconductance amplifier with source and sink capability. The output of the main error amplifier drives a capacitor to GND at the VC pin. This capacitor sets the dominant pole for the regulation loop. (See the Applications Information section for selecting the capacitor value.) The error amp gets its feedback signal from a proprietary circuit which monitors all 7 LED current sources to determine which LED to close the regulation loop on. Safety Error Amp The safety error amplifier is a transconductance amplifier with sink only capability. In normal operation, it has no effect on the loop regulation. However, if any of the LED pins open-circuits, the output voltage will keep rising, and safety error amp will eventually take over control of the regulation loop to prevent VOUT runaway. The VOUT thresh- old at which this occurs is approximately 4.5V. OPERATIO |
Similar Part No. - LTC3452_15 |
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Similar Description - LTC3452_15 |
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