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IR3801MTR1PbF Datasheet(PDF) 8 Page - International Rectifier |
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IR3801MTR1PbF Datasheet(HTML) 8 Page - International Rectifier |
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8 / 21 page  11/04/08 8 PD-60354 IR3801MPbF Circuit Description THEORY OF OPERATION The IR3801 is a voltage mode PWM synchronous regulator and operates with a fixed 600kHz switching frequency, allowing the use of small external components. The output voltage is set by feedback pin (Fb) and the internal reference voltage (0.6V). These are two inputs to error amplifier. The error signal between these two inputs is amplified and it is compared to a fixed frequency linear sawtooth ramp. A trailing edge modulation is used for generating fixed frequency pulses (PWM) which drives the internal N-channel MOSFETs. The internal oscillator circuit uses on-chip circuitry, eliminating the need for external components. The IR3801 operates with single input voltage from 4.5V to 14V allowing an extended operating input voltage range. The over-current protection is performed by sensing current through the R DS(on) of low side MOSFET. This method enhances the converter’s efficiency and reduces cost by eliminating a current sense resistor. The current limit is programmable by using an external resistor. Under-Voltage Lockout The under-voltage lockout circuit monitors the two input supplies (Vcc and Vc) and assures that the MOSFET driver outputs remain in the off state whenever the supply voltage drops below set thresholds. Lockout occurs if Vcc or Vc fall below 4.3V and 3.3V respectively. Normal operation resumes once Vcc and Vc rise above the set values. Thermal Shutdown Temperature sensing is provided inside the IR3801. The trip threshold is typically set to 140oC. When trip threshold is exceeded, thermal shutdown turns off both MOSFETs. Thermal shutdown is not latched and automatic restart is initiated when the sensed temperature drops within the operating range. There is a 20oC hysteresis in the thermal shutdown threshold. Pre-Bias Startup The IR3801 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 first gate signal for control MOSFET is generated. Figure 4 shows a typical Pre-Bias condition at start up. Depending on system configuration, a specific amount of output capacitors may be required to prevent discharging the output voltage. Fig. 4: Pre-Bias start up Vo Time V Pre-Bias Voltage Shutdown The output can be shutdown by pulling the soft- start pin below 0.3V. This can easily be done by using an external small signal transistor. During shutdown both MOSFET drivers will be turned off. Normal operation will resume by cycling soft start pin. |
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