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IR3651SPBF Datasheet(PDF) 9 Page - International Rectifier |
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IR3651SPBF Datasheet(HTML) 9 Page - International Rectifier |
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9 / 19 page  IR3651SPbF 10/11/2006 Circuit Description 9 THEORY OF OPERATION Introduction The IR3651 is a voltage mode PWM synchronous controller. The output voltage is set by feedback pin (Fb) and the internal reference voltage (1.25V). 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 can be externally programmed up to 400kHz. The IR3651 operates with a wide input voltage from 36V to 75V allowing an extended operating input voltage range. The current limit is programmable and uses on- resistance of the low-side MOSFET, eliminating the need for an external current sense resistor . Under-Voltage Lockout The under-voltage lockout circuit monitors the Vcc supply and assures that the IC doesn’t starts until the Vcc reaches the set threshold. Lockout occurs if Vcc falls below 4.1V. Normal operation resumes once Vcc rises above the set value. Shutdown The output can be shutdown by pulling the soft- start pin below 0.3V. This can be easily 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. Error Amplifier The IR3651 is a voltage mode controller. The error amplifier is of transconductance type. The amplifier is capable of operating with Type III compensation control scheme using low ESR output capacitance. Operating Frequency Selection The switching frequency is determined by connecting an external resistor (Rt) to ground. Figure 3 provides a graph of oscillator frequency versus Rt. Frequency Synchronization The IR3651 is capable of accepting an external digital synchronization signal. Synchronization will be enabled by the rising edge at an external clock. Switching frequency is set by external resistor (Rt). During synchronization, Rt is selected such that the free running frequency is 20% below the synchronization frequency. When unused, the sync pin will remain floating and is noise immune. Pre-Bias Startup IR3651 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. Below figure 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 Fig. 3: Switching Frequency vs. Rt Switching Frequency vs. Rt 0 50 100 150 200 250 300 350 400 450 0 50 100 150 200 250 300 Rt (Kohm) Vo Time Volt Pre-Bias Voltage (Output Voltage before startup) |
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