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LTC3454 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3454 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LTC3454 8 3454fa APPLICATIONS INFORMATION 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 conduct- ing similar to a typical synchronous boost regulator. 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 3.4A (typical), switch A is shut off immediately. 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 275mA (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 LTC3454. When the input supply voltage drops below approximately 1.90V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws a few microamperes. Overtemperature Protection If the junction temperature of the LTC3454 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 LTC3454 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 200μs 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. Autozero Error Amp The error amplifier is an autozeroing transconductance amp with source and sink capability. The output of this 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 feedback signal to the error amp is developed across a resistor through which LED current flows. 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 the LED pin open- circuits, the output voltage will keep rising, and the safety error amp will eventually take over control of the regulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is approximately 5.15V. LED Current Programming and Enable Circuit Two enable pins work in conjunction with dual external resistors to program LED current to one of three nonzero settings. The table below explains how the current can be set. EN1 EN2 ILOAD (A) GND GND 0 (SHUTDOWN) VIN GND 3850 • 0.8V/RISET1 GND VIN 3850 • 0.8V/RISET2 VIN VIN 3850 • (0.8V/RISET1 + 0.8V/RISET2) With either enable pin pulled high, the buck-boost will regulate the output voltage at the current programmed by RISET1 and/or RISET2. With both enable pins pulled to GND, the LTC3454 is in shutdown and draws zero current. The enable pins are high impedance inputs and should not be floated. |
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