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LTC3872 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3872 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 32 page  LT3759 9 3759fb APPLICATIONS INFORMATION Main Control Loop The LT3759 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 1. The start of each oscillator cycle sets the SR latch (SR1) and turns on the external power MOSFET switch M1 through driver G2. The switch current flows through the external current sensing resistor RSENSE and generates a voltage proportional to the switch current. This current sense voltage VISENSE (amplified by A5) is added to a stabilizing slope compensation ramp and the resulting sum (SLOPE) is fed into the positive terminal of the PWM comparator A7. When SLOPE exceeds the level at the negative input of A7 (VC pin), SR1 is reset, turning off the power switch. The level at the negative input of A7 is set by the error amplifier A1 (or A2) and is an amplified version of the difference between the feedback voltage (FBX pin) and the reference voltage (1.6V or –0.8V, depending on the configuration). In this manner, the error amplifier sets the correct peak switch current level to keep the output in regulation. The LT3759 has a switch current limit function. The current sense voltage is input to the current limit comparator A6. If the SENSE pin voltage is higher than the sense current limit threshold VSENSE(MAX) (50mV, typical), A6 will reset SR1 and turn off M1 immediately. The LT3759 is capable of generating either positive or nega- tive output voltage with a single FBX pin. It can be configured as a boost or SEPIC converter to generate positive output voltage, or as an inverting converter to generate negative output voltage. When configured as a SEPIC converter, as shown in Figure 1, the FBX pin is pulled up to the internal bias voltage of 1.6V by a voltage divider (R1 and R2) connected from VOUT to GND. Comparator A2 becomes inactive and comparator A1 performs the inverting amplification from FBX to VC. When the LT3759 is in an inverting configuration, the FBX pin is pulled down to –0.8V by a voltage divider connected from VOUT to GND. Comparator A1 becomes inactive and comparator A2 performs the noninverting amplification from FBX to VC. The LT3759 has overvoltage protection functions to protect the converter from excessive output voltage overshoot during start-up or recovery from a short-circuit condition. An overvoltage comparator A11 (with 40mV hysteresis) senses when the FBX pin voltage exceeds the positive regulated voltage (1.6V) by 7.5% and turns off M1. Similarly, an overvoltage comparator A12 (with 20mV hysteresis) senses when the FBX pin voltage exceeds the negative regulated voltage (–0.8V) by 7.5% and turns off M1. Both reset pulses are sent to the main RS latch (SR1) through G6 and G5. The external power MOSFET switch M1 is actively held off for the duration of an output overvoltage condition. Programming Turn-On and Turn-Off Thresholds with EN/UVLO Pin The EN/UVLO pin controls whether the LT3759 is enabled or is in shutdown state. A micropower 1.22V reference, a comparator A10 and controllable current source IS1 allow the user to accurately program the supply voltage at which the IC turns on and off. The falling value can be accurately set by the resistor dividers R3 and R4. When EN/UVLO is above 0.7V, and below the 1.22V threshold, the small pull-down current source IS1 (typical 2μA) is active. The purpose of this current is to allow the user to program the rising hysteresis. The Block Diagram of the comparator and the external resistors is shown in Figure 1. The typical falling threshold voltage and rising threshold voltage can be calculated by the following equations: VVIN(FALLING) = 1.22 • (R3 +R4) R4 VVIN(RISING) = 2μA • R3+ VIN(FALLING) For applications where the EN/UVLO pin is only used as a logic input, the EN/UVLO pin can be connected directly to the input voltage VIN for always-on operation. INTVCC Low Dropout Voltage Regulators The LT3759 features two internal low dropout (LDO) volt- age regulators (VIN LDO and DRIVE LDO) powered from different supplies (VINandDRIVErespectively).BothLDO’s regulate the internal INTVCC supply which powers the gate driver and the internal loads, as shown in Figure 1. Both regulators are designed so that current does not flow from INTVCC to the LDO input under a reverse bias condition. DRIVE LDO regulates the INTVCC to 4.75V, while VIN LDO |
Similar Part No. - LTC3872 |
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Similar Description - LTC3872 |
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