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IRU3037ACSPBF Datasheet(PDF) 4 Page - International Rectifier |
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IRU3037ACSPBF Datasheet(HTML) 4 Page - International Rectifier |
4 / 21 page 4 IRU3037/IRU3037A & (PbF) www.irf.com BLOCK DIAGRAM Figure 2 - Simplified block diagram of the IRU3037. THEORY OF OPERATION Introduction The IRU3037 is a fixed frequency, voltage mode syn- chronous controller and consists of a precision refer- ence voltage, an error amplifier, an internal oscillator, a PWM comparator, 0.5A peak gate driver, soft-start and shutdown circuits (see Block Diagram). The output voltage of the synchronous converter is set and controlled by the output of the error amplifier; this is the amplified error signal from the sensed output voltage and the reference voltage. This voltage is compared to a fixed frequency linear sawtooth ramp and generates fixed frequency pulses of variable duty-cycle, which drives the two N-channel ex- ternal MOSFETs.The timing of the IC is provided through an internal oscillator circuit which uses on-chip capaci- tor to set the oscillation frequency to 200 KHz (400 KHz for “A” version). Soft-Start The IRU3037 has a programmable soft-start to control the output voltage rise and limit the current surge at the start-up. To ensure correct start-up, the soft-start se- quence initiates when the Vc and Vcc rise above their threshold (3.3V and 4.2V respectively) and generates the Power On Reset (POR) signal. Soft-start function operates by sourcing an internal current to charge an external capacitor to about 3V. Initially, the soft-start func- tion clamps the E/A’s output of the PWM converter. As the charging voltage of the external capacitor ramps up, the PWM signals increase from zero to the point the feedback loop takes control. Short-Circuit Protection The outputs are protected against the short-circuit. The IRU3037 protects the circuit for shorted output by sens- ing the output voltage (through the external resistor di- vider). The IRU3037 shuts down the PWM signals, when the output voltage drops below 0.6V (0.4V for IRU3037A). The IRU3037 also protects the output from over-voltaging when the control FET is shorted. This is done by turning on the sync FET with the maximum duty cycle. Under-Voltage Lockout The under-voltage lockout circuit assures that the MOSFET driver outputs remain in the off state whenever the supply voltage drops below set parameters. Lockout occurs if Vc or Vcc fall below 3.3V and 4.2V respec- tively. Normal operation resumes once Vc and Vcc rise above the set values. 20uA 64uA Max POR Oscillator Error Amp Ct Error Comp Reset Dom POR 0.5V FbLo Comp 3 Vc HDrv Vcc LDrv Gnd Vcc 4.0V Vc 3.5V 0.2V 0.2V Bias Generator 3V 1.25V POR 8 SS/SD Fb 1 Comp 7 25K 25K 1.25V 3V 2 6 5 R S Q 4 |
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