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IR3621MTRPBF Datasheet(PDF) 9 Page - International Rectifier |
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IR3621MTRPBF Datasheet(HTML) 9 Page - International Rectifier |
9 / 29 page IR3621&(PbF) 9 www.irf.com L1 RSET IR3621 OCSet IOCSET VOUT Hiccup Control Q1 Q2 Figure 9 - Diagram of the over current sensing. VOCSET = IOCSET ×RSET-RDS(ON)×iL ---(2) VOCSET = IOCSET ×RSET - RDS(ON)×IL = 0 The critical inductor current can be calculated by set- ting: ISET = IL(CRITICAL) = ---(3) RSET ×IOCSET RDS(ON) The internal current source develops a voltage across RSET. When the low side switch is turned on, the induc- tor current flows through the Q2 and results a voltage which is given by: The value of RSET should be checked in an actual circuit to ensure that the Over Current Protection circuit activates as expected. The IR3621 current limit is designed primarily as disaster preventing, "no blow up" circuit, and is not useful as a precision current regulator. In two independent mode, the output of each channel is protected independently which means if one output is under overload or short circuit condition, the other output will remain functional. The OCP set limit can be programmed to different levels by using the external resistors. This is valid for both hiccup mode and latch up mode. In 2-phase configuration, the OCP's output depends on any one channel, which means as soon as one channel goes to overload or short circuit condition the output will enter either hiccup or latch-up, dependes on status of Hiccup pin. For a given start up time, the soft-start capacitor can be calculated by: CSS ≅ 28µA×TSTART/0.8V Over-Current Protection The IR3621 can provide two different schemes for Over- Current Protection (OCP). When the Hiccup pin is pulled high, the OCP will operate in hiccup mode. In this mode, during overload or short circuit, the outputs enter hiccup mode and stay in that mode until the overload or short circuit is removed. The converter will automatically re- cover. When the Hiccup pin is pulled low, the OCP scheme will be changed to the latch up type, in this mode the converter will be turned off during Overcurrent or short circuit. The power needs to be recycled for normal operation. Each phase has its own independent OCP circuitry. The OCP is performed by sensing current through the RDS(ON) of low side MOSFET. As shown in Figure 9, an external resistor (RSET) is connected between OCSet pin and the drain of low side MOSFET (Q2) which sets the current limit set point. The soft-start is part of the Over Current Protection scheme, during the overload or short circuit condition the external soft start capacitors will be charged and discharged in certain slope rate to achieve the hiccup mode function. Out-of-Phase Operation The IR3621 drives its two output stages 180 ! out-of-phase. In 2-phase configuration, the two inductor ripple currents cancel each other and result in a reduction of the output current ripple and yield a smaller output capacitor for the same ripple voltage requirement. In single input voltage applications, the input ripple current reduces. This results in much smaller input capacitor's RMS current and reduces the input capacitor quantity. SS1 / SD 20 28uA 3uA Hiccup If using one soft start capacitor in dual configuration for a precise power up the OCP needs to be set to latch mode. Figure 8 - 3uA current source for discharging soft start-capacitor during Hiccup mode Pre-bias Startup The IR3621 allows pre-bias startup without discharging the output capacitors. The output starts in asynchro- nous fashion and keeps the synchronous MOSFET off until the first gate signal for control MOSFET is gener- ated. |
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