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FC9911 Folha de dados(PDF) 10 Page - First Silicon Co., Ltd |
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FC9911 Folha de dados(HTML) 10 Page - First Silicon Co., Ltd |
10 / 13 page Reference FC9911 includes a 2% accurate, 1.25V reference, which can be used as the reference for the output current as well as to set the switch current limit. This reference is also used internally to set the over voltage protection threshold. The reference is buffered so that it can deliver a maximum of 500µA external current to drive the external circuitry. The reference should be bypassed with at least a 10nF low ESR capacitor. Note: In order to avoid abnormal startup conditions, the bypass capacitor at the REF pin should not exceed 0.22µF. Oscillator The oscillator can be set in two ways. Connecting the oscillator resistor between the RT and GATE pins will program the off-time. Connecting the resistor between RT and GND will program the time period. In both cases, resistor R T sets the current, which charges an internal oscillator capacitor. The capacitor voltage ramps up linearly and when the voltage increases beyond the internal set voltage, a comparator triggers the SET input of . e l c y c g n i h c ti w s t x e n e h t s tr a t s s i h T . p o fl - p i fl R S l a n r e t n i e h t The time period of the oscillator can be computed as: T S ≈ RT • 11pF Slope Compensation For converters operating in the constant frequency mode, slope compensation becomes necessary to ensure stability of the peak current mode controller, if the operating duty cycle is greater than 0.5. Choosing a slope compensation which is one half of the down slope of the inductor current ensures that the converter will be stable for all duty cycles. Slope compensation can be programmed by two resistors R SLOPE and RSC. Assuming a down slope of DS (A/µs) for the inductor current, the slope compensation resistors can be computed as: R SLOPE = (10 • RSC) / (DS • 10 6 • T S • RCS) A typical value for R SC is 499Ω. Note: The maximum current that can be sourced out of the SC pin is 100µA. This limits the minimum value of the R SLOPE resistor to 25kΩ. If the equation for slope compensation produces a value of R SLOPE less than this value, then RSC would have to be increased accordingly. It is recommended that R SLOPE be chosen in the range of 25 - 50kΩ. Current Sense The current sense input of the HV9911 includes a built in 100ns (minimum) blanking time to prevent spurious turn off due to the initial current spike when the FET turns on. The FC9911 includes two high-speed comparators - one is used during normal operation and the other is used to limit the maximum input current during input under voltage or overload conditions. The IC includes an internal resistor divider network, which steps down the voltage at the COMP pin by a factor of 15. This stepped-down voltage is given to one of the comparators as the current reference. The reference to the other comparator, which acts to limit the maximum inductor current, is given externally. It is recommended that the sense resistor R CS be chosen so as to provide about 250mV current sense signal. Current Limit Current limit has to be set by a resistor divider from the 1.25V reference available on the IC. Assuming a maximum operating inductor current I pk (including the ripple current), the voltage at the CLIM pin can be set as: V CLIM ≥ 1.2 • IPK • RCS + ( 5 • RSC / RSLOPE) • 0.9 Note that this equation assumes a current limit at 120% of the maximum input current. Also, if V CLIM is greater than 450mV, the saturation of the internal opamp will determine the limit on the input current rather than the CLIM pin. In such a case, the sense resistor R CS should be reduced till V CLIM reduces below 450mV. It is recommended that no capacitor be connected between CLIM and GND. Fault Protection The FC9911 has built-in output over-voltage protection and output short circuit protection. Both protection features are latched, which means that the power to the IC must be recycled to reset the IC. The IC also includes a FAULT pin which goes low during any fault condition. At startup, a monoshot circuit, (triggered by the POR circuit), resets an internal flip-flop which causes FAULT to go high, and remains high during normal operation. This also allows the GATE drive to function normally. This pin can be used to drive an external disconnect switch (Q2 in the Typical Boost Application Circuit on pg.1), which will disconnect the load during a fault condition. This disconnect switch is very important in a boost converter, as turning off the switching FC9911 2012. 10. 31 10/13 Revision No : 0 |
Nº de peça semelhante - FC9911 |
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Descrição semelhante - FC9911 |
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