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IR3638SPBF Datasheet(PDF) 7 Page - International Rectifier |
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IR3638SPBF Datasheet(HTML) 7 Page - International Rectifier |
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7 / 15 page  IR3638SPbF 3/19/07 Circuit Description 7 THEORY OF OPEARTION Introduction The IR3638 is a voltage mode PWM synchronous controller and operates with a fixed 400kHz switching frequency, allowing the use of small external components. The output voltage is set by feedback pin (Fb) and the external reference voltage (Vp). These are two inputs to error amplifier. The error signal between these two inputs is compared to a fixed frequency linear sawtooth ramp and generates fixed frequency pulses of variable duty-cycle (D) which drivers N-channel external MOSFETs. The timing of the IC is controlled by an internal oscillator circuit that uses on-chip capacitor to set the switching frequency. Under-Voltage Lockout The under-voltage lockout circuit monitors the Vcc and Enable input and assures that the MOSFET driver outputs remain in the off state whenever the Vcc or Enable voltages drop below set thresholds. Pre-Bias Startup IR3638 is able to start up into pre-charged output, which prevents oscillation and disturbances of the output voltage. The output starts in asynchronous fashion and keeps the synchronous MOSFET off until the soft start reaches about 2.0V. Figure 4 shows a typical Pre-Bias condition at start up. Depends on system configuration, specific amount of output capacitors may be required to prevent discharging the output voltage. Error Amplifier The IR3638 is a voltage mode controller. The error amplifier is of transconductance type and it is capable of operating with Type III compensation control scheme using low ESR output capacitance. Short Circuit Protection The output is protected against the short-circuit. The IR3638 protects the circuit for shorted output by sensing the output voltage (through the external resistor divider). The device shuts down the PWM signals and latches off when the output voltage drops below 0.4V. Start Up / Down Considerations The “Typical Application” on page1 shows a standard configuration where multiple input supplies are available. This configuration minimizes the required extra components compared with a single input supply configuration. As it is shown the converter voltage (Bus Voltage) is set to 5V while the IC including the gate driver voltage are biased with 12V supply. In order to maintain a proper start up, it is required that the Bus Voltage ramps up first followed by Vcc, Vc supply. The system will be enabled when the Vp voltage passes the under voltage lock out threshold. Figure 5 shows start up / down sequencing. Vo Time Pre-Bias Voltage (Output Voltage before startup) Fig. 4: Pre-Bias start up V 5V Bus Voltage 12V (Vcc, Vc) V_CPUCore (Vp of IR3638) POR of IR3638 SS (IR3638) Vout 0.65V 0.60V Fig. 5: Start Up / Down Sequence (Refer to Figure1) |
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