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FAN5602MP33X 데이터시트(PDF) 11 Page - Fairchild Semiconductor |
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FAN5602MP33X 데이터시트(HTML) 11 Page - Fairchild Semiconductor |
11 / 14 page © 2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5602 Rev. 1.5.1 11 Switch Array Modes Figure 24. Mode 1 (1:1) Figure 26. Mode 3 (1:2 or 2:1) All Switches Set for Phase 1 and Reverse State for Phase 2 Figure 25. Mode 2 (2:3 or 3:2) All Switches Set for Phase 1 and Reverse State for Phase 2 Figure 27. Mode 4 (1:3 or 3:1) All Switches Set for Phase 1 and Reverse State for Phase 2 Light-Load Operation The power transistors used in the charge pump are very large in size. The dynamic loss from the switching the power transistors is not small and increases its propor- tion of the total power consumption as the load gets light. To save power, the FAN5602 switches, when the load is less than 10mA, from constant frequency to pulse-skip- ping mode (PFM) for modes 2:3(3:2), 1:2(2:1) and 1:3(3:1), except mode 1:1. In PFM mode, the linear loop is disabled and the error amplifier is turned off. A PFM comparator is used to setup an upper threshold and a lower threshold for the output. When the output is lower than the lower threshold, the oscillator is turned on and the charge pump starts working and keeps delivering charges from the input to the output until the output is higher than the upper threshold. The oscillator shuts off power transistors and delivers the charge to the output from the output capacitor. PFM operation is not used for Mode 1:1, even if at light load. Mode 1:1 is designed as an LDO with the oscillator off. The power transistors at LDO mode are not switching and therefore do not have the dynamic loss. Switching from linear operation to PFM mode (ILOAD<10mA) and from PFM to linear mode (ILOAD>10mA) is automatic, based on the load current, which is monitored all the time. Short Circuit When the output voltage is lower than 150mV, the FAN5602 enters short-circuit condition. In this condition, all power transistors are turned off. A small transistor shorting the input and the output turns on and charges the output. This transistor stays on as long as the VOUT < 150mV. Since this transistor is very small, the current from the input to the output is limited. Once the short at the output is eliminated, this transistor is large enough to charge the output higher than 150mV and the FAN5602 enters soft-start period. Soft Start The FAN5602 uses a constant current, charging a low- pass filter to generate a ramp. The ramp is used as refer- ence voltage during the startup. Since the ramp starts at zero and goes up slowly, the output follows the ramp and inrush current is restricted. When the ramp is higher than bandgap voltage, the bandgap voltage supersedes ramp as reference and the soft start is over. The soft start takes about 500µs. Thermal Shutdown The FAN5602 goes to thermal shutdown if the junction temperature is over 150°C with 15°C hysteresis. GND S1A S2A TOP C1+ C1- C1 MID MID TOP C1 C2 C1+ C1- C2+ C2- S3A S4A S1A S2A S3B S4B S1B S2B MID TOP GND C1+ C1+ C1- C1- C1 C2 S1A S1A S2A S3A S4B S5 S3B MID TOP GND C1+ C2+ C1- C2- C1 C2 S1A S2A S2B S4A S4B S5 S3B |
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