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ISL6567IRZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6567IRZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 26 page  11 FN9243.2 March 20, 2007 the MOSFET on, the lower MOSFET drive turns the freewheeling element off. The upper MOSFET is kept on until the clock signals the beginning of the next switching cycle and the PWM pulse is terminated. CURRENT SENSING ISL6567 senses current by sampling the voltage across the lower MOSFET during its conduction interval. MOSFET rDS(ON) sensing is a no-added-cost method to sense current for load line regulation, channel current balance, module current sharing, and overcurrent protection. The ISEN pins are used as current inputs for each channel. Internally, a virtual ground is created at the ISEN pins. The RISEN resistors are used to size the current flow through the ISEN pins, proportional to the lower MOSFETs’ rDS(ON) voltage, during their conduction periods. The current thus developed through the ISEN pins is internally averaged, then the current error signals resulting from comparing the average to the individual current signals are used for channel current balancing. Select the value for the RISEN resistors based on the room temperature rDS(ON) of the lower MOSFETs and the full-load total converter output current, IFL. As this current sense path is also used for OC detection, ensure that at maximum power train temperature rise and maximum output current loading the OC protection is not inadvertently tripped. OC protection current level through the ISEN pins is listed in the Electrical Specifications table. CHANNEL-CURRENT BALANCE Another benefit of multi-phase operation is the thermal advantage gained by distributing the dissipated heat over multiple devices and greater area. By doing this, the designer avoids the complexity of driving multiple parallel MOSFETs and the expense of using expensive heat sinks and exotic magnetic materials. All things being equal, in order to fully realize the thermal advantage, it is important that each channel in a multi-phase converter be controlled to deliver about the same current at any load level. Intersil’s ISL6567 ensure current balance by comparing each channel’s current to the average current delivered by both channels and making appropriate adjustments to each channel’s pulse width based on the resultant error. The error signal modifies the pulse width to correct any unbalance and force the error toward zero. Conversely, should a channel-to-channel imbalance be desired, such imbalance can be created by adjusting the individual channel’s RISEN resistor. Asymmetrical layouts, where one phase of the converter is naturally carrying more current than the other, or where one of the two phases is subject to a more stringent thermal environment limiting its current-carrying capability, are instances where this adjustment is particularly useful, helping to cancel out the design-intrinsic thermal or current imbalances. SOFT-START The soft-start function allows the converter to bring up the output voltage in a controlled fashion, resulting in a linear ramp-up. As soon as the controller is fully enabled for operation, the SS pin starts to output a small current which charges the external capacitor, CSS, connected to this pin. An internal reference clamp controlled by the potential at the SS pin releases the reference to the input of the error amplifier with a 1:1 correspondence for SS potential exceeding 0.7V (typically). Figure 5 details a normal soft-start startup. The following equation helps determine the approximate time period during which the controlled output voltage is ramped from 0V to the desired DC-set level. Whenever the ISL6567’s power-on reset falling threshold is tripped, or it is disabled via the EN pin, the SS capacitor is quickly discharged via an internal pull-down device (represented as the 1mA, typical, current source). As the SS pin’s positive excursion is internally clamped to about 3.5V, insure that any external pull-up device does not force more than 3mA into this pin. Should OC protection be tripped while the ISL6567 is operating in internal-reference mode and the SS pin not be allowed to fully discharge the SS capacitor, the ISL6567 cannot continue the normal SS cycling. R ISEN r DS ON () 50 10 6 – × ----------------------- I FL 2 -------- ⋅ = t SS C SS VREF ⋅ I SS -------------------------------- = GND> FIGURE 5. NORMAL SOFT-START WAVEFORMS FOR ISL6567-BASED MULTI-PHASE CONVERTER EN (5V/DIV) VSS (1V/DIV) GND> GND> GND> VintREF (0.5V/DIV) VOUT (0.5V/DIV) ISL6567 ISL6567 |
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