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FAN4603MM13X Datenblatt(PDF) 9 Page - Fairchild Semiconductor |
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FAN4603MM13X Datenblatt(HTML) 9 Page - Fairchild Semiconductor |
9 / 12 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN4603 • Rev. 1.0.1 9 Operation Description The FAN4603 is a 600mA, step-down, switching-voltage regulator that delivers a fixed output from an input voltage supply of 2.3V to 5.5V. Using a proprietary architecture with synchronous rectification, the FAN4603 is capable of delivering a peak efficiency of >90%, while maintaining efficiency over 80% at load currents as low as 1mA. The regulator operates at a nominal PWM frequency of 6MHz. Control Scheme The FAN4603 uses a proprietary, non-linear, fixed-frequency PWM modulator to deliver a fast load transient response, while maintaining a constant switching frequency over a wide range of operating conditions. Regulator stability is not dependent on output capacitor ESR, which allows the use of ceramic capacitors. Although this type of operation normally results in a switching frequency that varies with input voltage and load current, an internal frequency loop holds the switching frequency constant over a large range of input voltages and load currents. For very light loads, FAN4603 incorporates a discontinuous current (DCM) single-pulse PFM mode, which produces lower output ripple when compared with other PFM architectures. Transition between PWM and PFM is seamless, with a glitch of less than 20mV at VOUT during the transition between DCM and CCM modes. Combined with exceptional transient response characteristics, the very low quiescent current of the controller (35µA) maintains high efficiency; even at very light loads, while preserving fast transient response for applications requiring tight output regulation. Enable and Soft-Start When EN is LOW, all circuits in FAN4603 are off and the IC draws ~100nA of current. When EN is HIGH and VIN is above its UVLO threshold, the regulator begins a soft-start cycle. The output ramp during soft-start is a fixed slew rate of 50mV/ μs from 0 to 1 VOUT, then 12.5mV/μs until the output reaches its setpoint. PWM mode operation is prohibited during the soft-start cycle to prevent COUT from being discharged. This allows glitchless starting into a pre-charged output. Startup into Large COUT The IC may fail to start if heavy load is applied during startup and a large external COUT is present. This is due to the current-limit fault response, which protects the IC in an over- current condition during soft-start. The current required to charge COUT during soft-start, referred to as “displacement current,” is given as: dt dV C I OUT DISP • = (1) where dt dV refers to the soft-start slew rate. To prevent shutdown during soft-start, the following condition must be met: ) DC ( MAX LOAD DISP I I I < + (2) where IMAX(DC) is the maximum load current the IC is guaranteed to support (600mA). Table 1 shows combinations of external COUT that allow the IC to start successfully with the minimum RLOAD that can be supported at each. Table 1. Minimum RLOAD Values for Soft-Start with Various External COUT Values COUT Minimum RLOAD 2.2 μF, 0402 VOUT / 0.55 4.7 μF, 0402 VOUT / 0.50 10 μF, 0603 VOUT / 0.45 Multiple soft-start cycles may be required for COUT>10μf (15 μf with no load). The IC shuts down for 85μs when IDISP+ ILOAD exceeds ILIMIT for more than 21μs of current limit. The IC then begins a new soft-start cycle. Subsequent soft-start cycles begin with any charge retained by COUT while the IC is off, allowing VOUT to incrementally reach regulation over multiple soft-start attempts. Current Limit, Fault Shutdown, and Restart A heavy load or short circuit on the output causes the current to increase until a maximum current threshold is reached. Upon reaching this point, the high-side switch turns off, preventing high currents from causing damage. The regulator continues to limit the current cycle-by-cycle. After 21µs of current limit, the regulator triggers an over-current fault, causing the regulator to shut down for about 85 μs before attempting an automatic restart. If the fault is caused by short circuit, the soft-start circuit attempts to restart and produces an over-current fault after about 32 μs, which results in a duty cycle of less than 30%, limiting power dissipation. Under-Voltage Lockout (UVLO) When EN is HIGH, the under-voltage lockout keeps the part from operating until the input supply voltage rises high enough to properly operate. This ensures no misbehavior of the regulator during startup or shutdown. Thermal Shutdown When the die temperature increases, due to a high load condition and/or a high ambient temperature, the output switching is disabled until the temperature on the die has fallen sufficiently. The junction temperature at which the thermal shutdown activates is nominally 150°C with 15°C hysteresis. After cooling, the IC automatically restarts, with a soft-start cycle. |
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