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FAN5307MPX 数据表(PDF) 7 Page - Fairchild Semiconductor |
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FAN5307MPX 数据表(HTML) 7 Page - Fairchild Semiconductor |
7 / 13 page 7 www.fairchildsemi.com FAN5307 Rev. 1.0.4 Block Diagram Detailed Operation Description The FAN5307 is a step-down converter operating in a current-mode PFM/PWM architecture with a typical switching frequency of 1MHz. At moderate to heavy loads, the converter operates in pulse-width-modulation (PWM) mode. At light loads the converter enters a power-save mode (PFM pulse skipping) to keep the effi- ciency high. PWM Mode In PWM mode, the device operates at a fixed frequency of 1MHz. At the beginning of each clock cycle, the P-channel transistor is turned on. The inductor current ramps up and is monitored via an internal circuit. The P-channel switch is turned off when the sensed current causes the PWM comparator to trip when the output volt- age is in regulation or when the inductor current reaches the current limit (set internally to typically 520mA). After a minimum dead time the N-channel transistor is turned on and the inductor current ramps down. As the clock cycle is completed, the N-channel switch is turned off and the next clock cycle starts. PFM (Power Save) Mode As the load current decreases and the peak inductor cur- rent no longer reaches the typical threshold of 80mA, the converter enters pulse-frequency-modulation (PFM) mode. In PFM mode the device operates with a variable frequency and constant peak current thus reducing the quiescent current to minimum. Consequently, the high efficiency is maintained at light loads. As soon as the output voltage falls below a threshold, set at 0.8% above the nominal value, the P-channel transistor is turned on and the inductor current ramps up. The P-channel switch turns off and the N-channel turns on as the peak induc- tor current is reached (typical 140mA). The N-channel transistor is turned off before the inductor current becomes negative. At this time the P-channel is switched on again starting the next pulse. The converter continues these pulses until the high threshold (typical 1.6% above nominal value) is reached. A higher output voltage in PFM mode gives additional headroom for the voltage drop during a load transient from light to full load. The voltage overshoot during this load transient is also minimized due to active regulation during turning on the FB ERROR AMPLIFIER REF FB REF IS IS CURRENT SENSE NEG. LIMIT SENSE DIGITAL SOFT START UNDER VOLTAGE LOCKOUT LOGIC CONTROL MOSFET DRIVER OSC SLOPE COMPENSATION PFM COMPARATOR COMPARATOR OVER VOLTAGE COMPARATOR NEG. LIMIT COMPARATOR IS GND LX VIN EN GND 0.5V |
类似零件编号 - FAN5307MPX |
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类似说明 - FAN5307MPX |
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