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IR3831M Datasheet(PDF) 11 Page - International Rectifier |
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IR3831M Datasheet(HTML) 11 Page - International Rectifier |
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11 / 31 page  11/05/10 11 IR3831MPbF Circuit Description THEORY OF OPERATION Introduction The IR3831 uses a PWM voltage mode control scheme with external compensation to provide good noise immunity and maximum flexibility in selecting inductor values and capacitor types. The switching frequency is programmable from 250kHz to 1.5MHz and provides the capability of optimizing the design in terms of size and performance. IR3831 provides precisely regulated output voltage programmed via two external resistors from 0.7V to 0.9*Vin. The IR3831 operates with an external bias supply from 4.5V to 5.5V, allowing an extended operating input voltage range from 1.0V to 16V. The device utilizes the on-resistance of the low side MOSFET as current sense element, this method enhances the converter’s efficiency and reduces cost by eliminating the need for external current sense resistor . IR3831 includes two low R ds(on) MOSFETs using IR’s HEXFET technology. These are specifically designed for high efficiency applications. Under-Voltage Lockout and POR The under-voltage lockout circuit monitors the input supply Vcc and the Enable input. It assures that the MOSFET driver outputs remain in the off state whenever either of these two signals drop below the set thresholds. Normal operation resumes once Vcc and Enable rise above their thresholds. The POR (Power On Ready) signal is generated when all these signals reach the valid logic level (see system block diagram). When the POR is asserted the soft start sequence starts (see soft start section). Enable The Enable features another level of flexibility for start up. The Enable has precise threshold which is internally monitored by Under-Voltage Lockout (UVLO) circuit. Therefore, the IR3831 will turn on only when the voltage at the Enable pin exceeds this threshold, typically, 1.2V. If the input to the Enable pin is derived from the bus voltage by a suitably programmed resistive divider, it can be ensured that the IR3831 does not turn on until the bus voltage reaches the desired level. Only after the bus voltage reaches or exceeds this level will the voltage at Enable pin exceed its threshold, thus enabling the IR3831. Therefore, in addition to being a logic input pin to enable the IR3831, the Enable feature, with its precise threshold, also allows the user to implement an Under-Voltage Lockout for the bus voltage V in. This is desirable particularly for high output voltage applications, where we might want the IR3831 to be disabled at least until V in exceeds the desired output voltage level. Figure 3b. shows the recommended start-up sequence for the non-tracking operation of IR3831, when Enable is used as a logic input. Fig. 3a. Normal Start up, Device turns on when the Bus voltage reaches 10.2V Fig. 3b. Recommended startup sequence, Non-Tracking operation |
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