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LTC3527EUD-TR Datasheet(PDF) 11 Page - Linear Technology |
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LTC3527EUD-TR Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC3527/LTC3527-1 11 35271f limit the minimum and maximum error amp output volt- ages for improved large-signal transient response. Power converter control loop compensation is provided internally. An external resistive voltage divider from VOUT1 (VOUT2) to ground programs the respective output voltage via FB1 (FB2) from 1.6V to 5.25V. VV R R VV R R OUT OUT 1 2 120 1 1 2 120 1 3 4 =+ ⎡ ⎣⎢ ⎤ ⎦⎥ =+ ⎡ .• .• ⎣⎣⎢ ⎤ ⎦⎥ () See Block Diagram Current Sensing: Lossless current sensing converts the peak current signal of each N-channel MOSFET switch into a voltage which is summed with its corresponding internal slope compensation. The summed signals are compared to their respective error amplifier outputs to provide individual peak current control commands for the PWM of each converter. Current Limit: The current limit comparators shut off the N-channel MOSFET switches once their threshold is reached. Each current limit comparator delay time to output is typically 60ns. Peak switch current is limited to approximately 900mA for converter 1 and 500mA for converter 2, independent of input or output voltage. If VOUT1 or VOUT2 falls below 1V, its respective current limit is cut in half. Zero Current Comparator: The zero current comparators monitor the inductor current to the outputs and shut off the synchronous rectifiers when the current reduces to ap- proximately 30mA. This prevents the inductor current from reversing in polarity, improving efficiency at light loads. Synchronous Rectifier: To control inrush current and to prevent the inductor currents from running away when VOUT1 or VOUT2 is close to VIN, the P-channel MOSFET synchronous rectifiers are only enabled when their respec- tive VOUT > (VIN + 0.24V). Anti-Ringing Control: The anti-ringing control connects a resistor across the inductor to prevent high frequency ring- ing on the SW1 (SW2) pins during discontinuous current mode operation. Although the ringing of the resonant circuit formed by the inductors and CSW (capacitance on SW1 or SW2 pins) is low energy, it can cause EMI radiation. Output Disconnect: The LTC3527/LTC3527-1 are designed to allow true output disconnect by eliminating body diode conduction of the internal P-channel MOSFET rectifiers. This allows VOUT1 and VOUT2 to go to zero volts during shutdown, drawing no current from the input source. It also allows for inrush current limiting at turn-on, mini- mizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, there must not be external Schottky diodes connected between the SW1 (SW2) pins and VOUT1 (VOUT2). The output discon- nect feature also allows VOUT1 or VOUT2 to be pulled high, without any reverse current into a battery on VIN. Thermal Shutdown: If the die temperature exceeds 160°C, the device will go into thermal shutdown. All switches will be turned off and the soft-start capacitors will be discharged. The device will be enabled again when the die temperature drops by about 15°C. Burst Mode Operation To realize the efficiency benefits of Burst Mode operation, both VOUT1 and VOUT2 must be under a light load current condition, if they are both enabled. If one channel is shut down, then Burst Mode operation is enabled on the other channel. With the MODE pin low, the LTC3527/LTC3527-1 will automatically enter Burst Mode operation at light load and return to fixed frequency PWM mode when the load increases. Refer to the Typical Performance Characteristics to see the Output Load Burst Mode Threshold Current vs VIN. The load current at which Burst Mode operation is entered can be changed by adjusting the inductor value. Raising the inductor value will lower the load current at which Burst Mode is operation entered. In Burst Mode operation, the LTC3527/LTC3527-1 still switches at a fixed frequency of 1.2MHz (FSEL = 0) or 2.2MHz (FSEL = 1), using the same error amplifier and loop compensation for peak current mode control. This control method eliminates the output transient when switching between modes. In Burst Mode operation, energy is delivered to the output until it reaches the nominal regulation value, then the LTC3527/LTC3527-1 transitions to sleep mode where the outputs are off and the LTC3527/LTC3527-1 consumes only 12µA of quies- cent current from the higher of VOUT1 or VOUT2. When OPERATION |
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