поискавой системы для электроныых деталей |
|
FAN5182QSCX_NL датащи(PDF) 9 Page - Fairchild Semiconductor |
|
FAN5182QSCX_NL датащи(HTML) 9 Page - Fairchild Semiconductor |
9 / 18 page 9 www.fairchildsemi.com FAN5182 Rev. 1.0.1 Theory of Operation The FAN5182 combines a multi-loop, fixed frequency PWM con- trol with multi-phase logic outputs for use in 1-, 2-, and 3-phase synchronous buck point-of-load power supplies. Multi-phase operation is important for producing the high current and low voltage demanded by auxiliary supplies in desktop computers, workstations, and servers. Handling high current in a single- phase converter places high thermal stress on components such as inductors and MOSFETs, therefore is not preferred. The multi-loop control of the FAN5182 ensures a stable, high performance topology for: • Balancing current and thermal between/among phases •Fast response at the lowest possible switching frequency and output decoupling • Reducing switching losses due to low frequency operation • Tight line and load regulation • Reducing output ripple due to multiphase cancellation • Better noise immunity to facilitate PCB layout Start-up Sequence During start-up, the number of operational phases and their phase relationship are determined by the internal circuitry that monitors the PWM outputs. Normally, the FAN5182 operates as a 3-phase PWM controller. Grounding the PWM3 pin programs for 1- or 2-phase operation. When the FAN5182 is enabled, the controller outputs a voltage on PWM3 which is approximately 675mV. An internal compara- tor checks this pin's voltage versus a threshold of 300mV. If the PWM3 pin is grounded, it is below the threshold and the phase 3 is disabled. The output resistance of the PWM pin is approxi- mately 5k Ω during this detection period. Any external pull-down resistance connected to the PWM pin should not be less than 25k Ω to ensure proper operation. PWM1 and PWM2 are dis- abled during the phase detection interval, which occurs during the first two clock cycles of the internal oscillator. After this time, if the PWM3 output is not grounded, the 5k Ω resistance is dis- connected and PWM3 switches between 0V and 5V. If the PWM3 output is grounded, the controller will operate in 1 and/or 2-phase. The PWMs output logic-level signals in order to interface with external gate drivers such as the FAN5009. Since each phase is able to operate close to 100% duty cycle, more than one PWM output can be on at the same time. Master Clock Frequency The clock frequency of the FAN5182 is set by an external resis- tor connected from the RT pin to ground. The frequency setting follows the graph shown in Figure 2. To determine the frequency per phase, divide the clock frequency by the number of phases in use. One exception is single phase operation, in which the clock frequency is set to be twice the single phase frequency. Output Voltage Differential Sensing The FAN5182 uses a differential low offset voltage error ampli- fier to maintain ±2% differential sensing accuracy over tempera- ture. The output voltage is sensed between the FB and FBRTN pins. The power supply output connects to the FB pin through a resistor divider, and the FBRTN pin should be connected directly to the remote sense ground. The internal precision ref- erence is referenced to FBRTN, which has a typical current of 100µA to allow accurate remote sensing. The internal error amplifier compares the precision reference to the FB pin to reg- ulate the output voltage. Output Current Sensing The FAN5182 uses a current sense amplifier (CSA) to monitor the total output current for current limit detection. Sensing the load current at the output gives the total average current being delivered to the load, which is an inherently more accurate method than peak current detection or sampling the current across a sense element, such as the low-side MOSFET. This amplifier can be configured several ways depending on the objectives of the system design: • Output inductor DCR sensing without a thermistor for lowest cost • Output inductor DCR sensing with a thermistor for improved accuracy and moderate cost • Discrete resistor sensing for the best accuracy The positive input of the CSA is connected to the CSREF pin, and the CSREF is tied to the power supply output. The inverting input of the CSA, CSSUM, is the summing node of load current sense through sensing elements (such as the switch node side of the output inductors). The feedback resistor between CSCOMP and CSSUM sets the gain of the amplifier, and a filter capacitor is placed in parallel with this resistor. The gain of the amplifier is programmable by adjusting the feedback resistor. The current information is then given as the difference between CSREF and CSCOMP. This difference signal is then used as a differential input for the current limit comparator. To provide the best accuracy for sensing current, the CSA is designed to have low input offset voltage. The CSA gain is determined by external resistors, so that it can be set very accu- rately. Current Control Loop and Thermal Balance The FAN5182 adopts low side MOSFET RDSON sensing for phase current balance. The sensed individual phase current is combined with a fixed internal ramp, then compared with the common voltage error amplifier output to balance phase cur- rent. This current balance information is independent of the average output current information used for current limit described previously. The magnitude of the internal ramp can be set to optimize tran- sient response of the system. It also tracks the supply voltage for better line regulation and transient response. A resistor con- nected from the power supply input to the RAMPADJ pin deter- mines the slope of the internal PWM ramp. Resistors RSW1 through RSW3 (see Figure 5) can be used for adjusting phase current balance. It's recommended to put placeholders for these resistors during the initial PCB layout, so that phase current bal- ance fine adjustments can be made on bench if necessary. To increase the current in any given phase, make RSW for that phase larger (make RSW = 0Ω for the hottest phase as the start- ing point). Increasing RSW to 500Ω could typically make a sub- stantial increase in this particular phase current. Increase each RSW value by small amounts to optimize phase current balance, starting with the coolest phase first. |
Аналогичный номер детали - FAN5182QSCX_NL |
|
Аналогичное описание - FAN5182QSCX_NL |
|
|
ссылки URL |
Конфиденциальность |
ALLDATASHEETRU.COM |
Вашему бизинису помогли Аллдатащит? [ DONATE ] |
Что такое Аллдатащит | реклама | контакт | Конфиденциальность | обмен ссыками | поиск по производителю All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |