Datasheet LT3759 (Analog Devices) - 9

LT3759
APPLICATIONS INFORMATION Main Control Loop
An overvoltage comparator A11 (with 40mV hysteresis) The LT3759 uses a fixed frequency, current mode control senses when the FBX pin voltage exceeds the positive scheme to provide excellent line and load regulation. regulated voltage (1.6V) by 7.5% and turns off M1. Operation can be best understood by referring to the Block Similarly, an overvoltage comparator A12 (with 20mV Diagram in Figure 1. hysteresis) senses when the FBX pin voltage exceeds the negative regulated voltage (–0.8V) by 7.5% and turns The start of each oscillator cycle sets the SR latch (SR1) and off M1. Both reset pulses are sent to the main RS latch turns on the external power MOSFET switch M1 through (SR1) through G6 and G5. The external power MOSFET driver G2. The switch current flows through the external switch M1 is actively held off for the duration of an output current sensing resistor RSENSE and generates a voltage overvoltage condition. proportional to the switch current. This current sense voltage VISENSE (amplified by A5) is added to a stabilizing
Programming Turn-On and Turn-Off Thresholds with
slope compensation ramp and the resulting sum (SLOPE)
EN/UVLO Pin
is fed into the positive terminal of the PWM comparator A7. When SLOPE exceeds the level at the negative input of A7 The EN/UVLO pin controls whether the LT3759 is enabled (VC pin), SR1 is reset, turning off the power switch. The or is in shutdown state. A micropower 1.22V reference, a level at the negative input of A7 is set by the error amplifier comparator A10 and controllable current source IS1 allow A1 (or A2) and is an amplified version of the difference the user to accurately program the supply voltage at which between the feedback voltage (FBX pin) and the reference the IC turns on and off. The falling value can be accurately voltage (1.6V or –0.8V, depending on the configuration). set by the resistor dividers R3 and R4. When EN/UVLO In this manner, the error amplifier sets the correct peak is above 0.7V, and below the 1.22V threshold, the small switch current level to keep the output in regulation. pull-down current source IS1 (typical 2µA) is active. The LT3759 has a switch current limit function. The current The purpose of this current is to allow the user to program sense voltage is input to the current limit comparator A6. the rising hysteresis. The Block Diagram of the comparator If the SENSE pin voltage is higher than the sense current and the external resistors is shown in Figure 1. The typical limit threshold V falling threshold voltage and rising threshold voltage can SENSE(MAX) (50mV, typical), A6 will reset SR1 and turn off M1 immediately. be calculated by the following equations: The LT3759 is capable of generating either positive or nega- VVIN(FALLING) = 1.22 • (R3+R4) tive output voltage with a single FBX pin. It can be configured R4 as a boost or SEPIC converter to generate positive output V voltage, or as an inverting converter to generate negative VIN(RISING) = 2µA •R3+ VIN(FALLING) output voltage. When configured as a SEPIC converter, as For applications where the EN/UVLO pin is only used as shown in Figure 1, the FBX pin is pulled up to the internal bias a logic input, the EN/UVLO pin can be connected directly voltage of 1.6V by a voltage divider (R1 and R2) connected to the input voltage VIN for always-on operation. from VOUT to GND. Comparator A2 becomes inactive and comparator A1 performs the inverting amplification from
INTVCC Low Dropout Voltage Regulators
FBX to VC. When the LT3759 is in an inverting configuration, the FBX pin is pulled down to –0.8V by a voltage divider The LT3759 features two internal low dropout (LDO) volt- connected from V age regulators (VIN LDO and DRIVE LDO) powered from OUT to GND. Comparator A1 becomes inactive and comparator A2 performs the noninverting different supplies (VIN and DRIVE respectively). Both LDO’s amplification from FBX to VC. regulate the internal INTVCC supply which powers the gate driver and the internal loads, as shown in Figure 1. Both The LT3759 has overvoltage protection functions to protect regulators are designed so that current does not flow from the converter from excessive output voltage overshoot INTVCC to the LDO input under a reverse bias condition. during start-up or recovery from a short-circuit condition. DRIVE LDO regulates the INTVCC to 4.75V, while VIN LDO 3759fc For more information www.linear.com/3759 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Related Parts