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FAN5353MPX Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FAN5353MPX Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 13 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5353 • Rev. 1.0.2 9 Operation Description The FAN5353 is a step-down switching voltage regulator that delivers an adjustable output from an input voltage supply of 2.7V to 5.5V. Using a proprietary architecture with synchronous rectification, the FAN5353 is capable of delivering 3A at over 80% efficiency. The regulator operates at a nominal frequency of 3MHz at full load, which reduces the value of the external components to 470nH for the output inductor and 20µF for the output capacitor. Control Scheme The FAN5353 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. The regulator performance is independent of the output capacitor ESR, allowing for the use of ceramic output 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. Setting the Output Voltage The output voltage is set by the R1, R2, and VREF (0.8V): REF REF OUT V V V 2 R 1 R − = (1) R1 must be set at or below 100KΩ. Therefore: () 8 . 0 V 8 . 0 1 R 2 R OUT − • = (2) For example, for VOUT = 1.2V, R1 = 100KΩ, R2 = 200KΩ. Enable and Soft Start When the EN pin is LOW, the IC is shut down, all internal circuits are off, and the part draws very little current. Raising EN above its threshold voltage activates the part and starts the soft-start cycle. During soft-start, the modulator’s internal reference is ramped slowly to minimize any large surge currents on the input and prevents any overshoot of the output voltage. If large values of output capacitance are used, the regulator may fail to start. If VOUT fails to achieve regulation within 320 μs from the beginning of soft-start, the regulator shuts down and waits 1200 μs before attempting a restart. If the regulator is at its current limit for more than about 60 μs, the regulator shuts down before restarting 1200 μs later. This limits the COUT capacitance when a heavy load is applied during the startup. For a typical FAN5353 starting with a resistive load: ) A ( I 100 400 ) F ( COUT LOAD MAX ∗ − ≈ μ where LOAD OUT LOAD R V I = (3) Synchronous rectification is inhibited during soft-start, allowing the IC to start into a pre-charged load. PGOOD Pin The PGOOD pin is an open drain output that indicates the IC is in regulation when its state is open. PGOOD requires an external pull-up resistor. PGOOD pulls LOW under the following conditions: 1. The IC has operated in cycle-by-cycle current limit for eight or more consecutive PWM cycles. 2. The circuit is disabled; either after a fault occurs, or when EN is LOW. 3. The IC is performing a soft-start. Under-Voltage Lockout 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. Input Over-Voltage Protection (OVP) When VIN exceeds VSDWN (about 6.2V) the IC stops switching, to protect the circuitry from internal spikes above 6.5V. An internal 40 μs filter prevents the circuit from shutting down due to noise spikes. For the circuit to fully protect the internal circuitry, the VIN slew rate above 6.2V must be limited to no more than 15V/ms when the IC is switching. The IC protects itself if VIN overshoots to 7V during initial power-up as long as the VIN transition from 0 to 7V occurs in less than 10 μs (10% to 90%). Current Limiting A heavy load or short circuit on the output causes the current in the inductor to increase until a maximum current threshold is reached in the high-side switch. Upon reaching this point, the high-side switch turns off, preventing high currents from causing damage. 16 consecutive PWM cycles in current limit cause the regulator to shut down and stay off for about 1200 μs before attempting a restart. In the event of a short circuit, the soft-start circuit attempts to restart and produces an over-current fault after about 50 μs, which results in a duty cycle of less than 10%, providing current into a short circuit. 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 a 20°C hysteresis. |
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