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FAN5234QSCX データシート(PDF) 7 Page - Fairchild Semiconductor |
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FAN5234QSCX データシート(HTML) 7 Page - Fairchild Semiconductor |
7 / 15 page FAN5234 PRODUCT SPECIFICATION REV. 1.0.10 5/3/04 7 Figure 4. Current Limit / Summing Circuits LDRV PGND ISNS in + in – 2.5V ILIM det. R SENSE SS 1.5M C SS VSEN V to I Reference and Soft Start 17pf ISNS S/H TO PWM COMP 4.14K 300K ISNS 0.17pf I2 = ILIM*11.2 ILIM 0.9V R ILIM ILIM The hysteretic comparator causes HDRV turn-on when the output voltage (at VSEN) falls below the lower threshold (10mV below VREF) and terminates the PFM signal when VSEN rises over the higher threshold (5mV above VREF). The switching frequency is primarily a function of: 1. Spread between the two hysteretic thresholds 2. ILOAD 3. Output Inductor and Capacitor ESR A transition back to PWM (Continuous Conduction Mode or CCM) mode occurs when the inductor current rises suffi- ciently to stay positive for 8 consecutive cycles. This occurs when: where ∆VHYSTERESIS = 15mV and ESR is the equivalent series resistance of COUT. Because of the different control mechanisms, the value of the load current where transition into PWM operation takes place is typically higher compared to the load level at which transition into hysteretic mode occurs. Hysteretic mode can be disabled by setting the FPWM pin HIGH. Current Processing Section The following discussion refers to Figure 4. The current through RSENSE resistor (ISNS) is sampled shortly after Q2 is turned on. That current is held, and summed with the output of the error amplifier. This effec- tively creates a current mode control loop. The resistor con- nected to ISNS pin (RSENSE) sets the gain in the current feedback loop. For stable operation, the voltage induced by the current feedback at the PWM comparator input should be set to 30% of the ramp amplitude at maximum load currrent and line voltage. The following expression estimates the rec- ommended value of RSENSE as a function of the maximum load current (ILOAD(MAX)) and the value of the MOSFET’s RDS(ON): RSENSE must, however, be kept higher than: Setting the Current Limit A ratio of ISNS is also compared to the current established when a 0.9 V internal reference drives the ILIM pin: Since the tolerance on the current limit is largely dependent on the ratio of the external resistors it is fairly accurate if the voltage drop on the Switching Node side of RSENSE is an accurate representation of the load current. When using the MOSFET as the sensing element, the variation of RDS(ON) causes proportional variation in the ISNS. This value not only varies from device to device, but also has a typical junction temperature coefficient of about 0.4%/°C (consult the MOSFET datasheet for actual values), so the actual current limit set point will decrease propotional to increasing MOSFET die temperature. A factor of 1.6 in the current limit setpoint should compensate for all MOSFET RDS(ON) variations, assuming the MOSFET’s heat sinking will keep its operating die temperature below 125°C. I LOAD CCM () ∆V HYSTERESIS 2 ESR --------------------------------------- = (3) R SENSE I LOAD MAX () R DS ON () × 4.1K × 30% 0.125 × V IN MAX () × ------------------------------------------------------------------------------ 100 – = (4a) R SENSE I LOAD MAX () R DS ON () × 150 µA ------------------------------------------------------------ 100 – = (4b) (5) R ILIM 11.2 I LIMIT ---------------- 100 R SENSE + () R DS ON () ---------------------------------------- × = |
同様の部品番号 - FAN5234QSCX |
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同様の説明 - FAN5234QSCX |
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