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FAN5236MTC Fiches technique(PDF) 11 Page - Fairchild Semiconductor |
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FAN5236MTC Fiches technique(HTML) 11 Page - Fairchild Semiconductor |
11 / 20 page FAN5236 PRODUCT SPECIFICATION REV. 1.1.9 7/12/04 11 Hysteretic Mode Conversely, the transition from Hysteretic mode to PWM mode occurs when the SW node is negative for 8 consecutive cycles. A sudden increase in the output current will also cause a change from hysteretic to PWM mode. This load increase causes an instantaneous decrease in the output voltage due to the voltage drop on the output capacitor ESR. If the load causes the output voltage (as presented at VSNS) to drop below the hysteretic regulation level (20mV below VREF), the mode is changed to PWM on the next clock cycle. In hysteretic mode, the PWM comparator and the error amplifier that provide control in PWM mode are inhibited and the hysteretic comparator is activated. In hysteretic mode the low side MOSFET is operated as a synchronous rectifier, where the voltage across ( VDS(ON) ) it is monitored, and it is switched off when VDS(ON) goes positive (current flowing back from the load) allowing the diode to block reverse conduction. The hysteretic comparator initiates a PFM signal to turn on HDRV at the rising edge of the next oscillator clock, when the output voltage (at VSNS) falls below the lower threshold (10mV below VREF) and terminates the PFM signal when VSNS 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 CCM operation takes place is typically higher compared to the load level at which transi- tion into hysteretic mode occurs. Hysteretic mode can be disabled by setting the FPWM pin low. I LOAD CCM () ∆V HYSTERESIS 2 ESR --------------------------------------- = (3) Figure 11. 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 |
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Description similaire - FAN5236MTC |
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