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SC1486 Datasheet(PDF) 11 Page - Semtech Corporation |
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SC1486 Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 17 page  11 2002 Semtech Corp. www.semtech.com SC1486 POWER MANAGEMENT Power Good Output Each controller has its own PGOOD. Power good is an open-drain output and requires a pull-up resistor. When the output voltage is 10% above or below its set voltage, PGOOD gets pulled low. It is held low until the output voltage returns to within 10% of the output set voltage. PGOOD is also held low during start-up and will not be allowed to transition high until soft start is over and the output reaches 90% of its set voltage. There is a 2us delay built into the PGOOD circuit to prevent false transi- tions. Output Overvoltage Protection When the output exceeds 10% of the its set voltage the low-side MOSFET is latched on. It stays latched and the SMPS is off until the enable input, REFIN or VCCA is toggled. There is a 2us delay built into the OV protection circuit to prevent false transitions. A OV fault in either controller will not cause the other one to shutdown. Note: to reset VDDQ from a fault, VCCA1 or EN/PSV must be togled. To reset VTT from a fault, VCCA2 or REFIN must be togled. Output Undervoltage Protection When the output is 30% below its set voltage the output is latched in a tristated condition, and the SMPS is off until the enable input is toggled. There is a 2us delay built into the UV protection circuit to prevent false transi- tions. An UV fault in either controller will not effect the other controller. POR, UVLO and Softstart An internal power-on reset (POR) occurs when VCCA1 and VCCA2 exceed 3V, resetting the fault latch and soft-start counter, and preparing the PWM for switching. VCCA undervoltage lockout (UVLO) circuitry inhibits switching and forces the DL gate driver high until VCCA rises above 4.2V. At this time the circuit will come out of UVLO and begin switching, and the softstart circuit being enabled, will progressively limit the output current over a prede- termined time period. The ramp occurs in four steps: 25%, 50%, 75% and 100%, thereby limiting the slew rate of the output voltage. There is 100mV of hysteresis built into the UVLO circuit and when the VCCA falls to 4.1V the output drivers are shutdown and tristated. MOSFET Gate Drivers The DH and DL drivers are optimized for driving moder- ate-sized high-side, and larger low-side power MOSFETs. An adaptive dead-time circuit monitors the DL output and prevents the high-side MOSFET from turning on, until DL is fully off, and conversely, monitors the DH output and prevents the low-side MOSFET from turning on until DH is fully off. Be sure there is low resistance and low induc- tance between the DH and DL outputs to the gate of each MOSFET. Design Procedure Prior to any design of a switch mode power supply (SMPS) for notebook computers, determination of input voltage, load current, switching frequency and inductor ripple cur- rent must be specified. Input Voltage Range The maximum input voltage (VIN MAX) is determined by the highest AC adaptor voltage. The minimum input voltage (VIN MIN) is determined by the lowest battery voltage after accounting for voltage drops due to connectors, fuses and battery selector switches. Maximum Load Current There are two values of load current to consider. Con- tinuous load current and peak load current. Continuous load current has more to do with thermal stresses and therefore drives the selection of input capacitors, MOSFETs and commutation diodes. Whereas, peak load current determines instantaneous component stresses and filtering requirements such as, inductor saturation, output capacitors and design of the current limit circuit. Switching Frequency Switching frequency determines the trade-off between size and efficiency. Increased frequency increases the switching losses in the MOSFETs, since losses are a func- tion of VIN2. Knowing the maximum input voltage and budget for MOSFET switches usually dictates where the design ends up. Inductor Ripple Current Low inductor values create higher ripple current, result- ing in smaller size, but are less efficient because of the high AC currents flowing through the inductor. Higher in- ductor values do reduce the ripple current and are more efficient, but are larger and more costly. The selection of the ripple current is based on the maximum output cur- rent and tends to be between 20% to 50% of the maxi- mum load current. Again, cost, size and efficiency all play a part in the selection process. Application Information (Cont.) |
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