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MSK5045-5.0HG Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK5045-5.0HG Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 9 page APPLICATION NOTES SELECTING RS: The MSK 5045 monitors the inductor current and the aver- age load current by sensing the voltage across RS. Cycle-by- cycle current limiting is controlled with an upper threshold of 100mV ±20mV; the high side MOSFET switch is gated off whenever the upper threshold is exceeded. Pulse skipping oc- curs in power save mode when the signal falls below the 30% current threshold of 30mV. The sychronous rectifier is disabled when the signal falls below 0V indicating discontinuous induc- tor current. Selection of RS must take all of these features into consideration. When operated in the continuous conduction mode peak to peak inductor current is approximated by the equation (VIN-VOUT) VOUT f L VIN where f=300KHz and L=6.4µH. (If optional output inductance is used L=6.4uH + optional L). The device will operate in continuous conduction as long as IOUT ≥ ½ Ip-p. The maxi- mum and minimum current peaks are equal to IOUT±½ Ip-p. RS translates the current levels into the control signal. Once the current levels are established the designer can size RS for specific applications. Care must be taken when selecting RS because under a short circuit condition the output current will approach the cycle-by-cycle current limit. For most applications, it may be useful to wire the sense inputs with a twisted pair instead of PCB traces. Low induc- tance current sense resistors, such as metal film surface mount styles are best. Rev. G 2/06 3 . . . POWER DISSIPATION: In high current applications, it is very important to ensure that both MOSFETS are within their maximum junction tem- perature at high ambient temperatures. Temperature rise can be calculated based on package thermal resistance and worst case dissipation for each MOSFET. These worst case dissipa- tions occur at minimum voltage for the high side MOSFET and at maximum voltage for the low side MOSFET. Calculate power dissipation using the following formulas: Pd (upper FET)=ILOAD² x 0.090 Ω x DUTY + VIN x ILOAD x f x VIN x CRSS+25ns IGATE Pd (lower FET)=ILOAD² x 0.090 Ω x (1-DUTY) DUTY= (VOUT+VQ2) (VIN-VQ1) Where: VQ1 or VQ2 (on state voltage drop)=ILOAD x 0.090 Ω CRSS=65pF IGATE=2A During output short circuit, Q2, the synchronous-rectifier MOSFET, will have an increased duty factor and will see addi- tional stress. This can be calculated by: Q2 DUTY=1- VQ2 VIN(MAX)-VQ1 Where: VQ1 or VQ2=(120MV/RSENSE) x 0.090 INPUT CAPACITOR SELECTION: The MSK 5045 should have an external high frequency ceramic capacitor (0.1uF) between VIN and GND. Connect a low-ESR bulk capacitor directly to the input pin of the MSK 5045. Select the bulk input filter capacitor according to input ripple-current requirements and voltage rating, rather than ca- pacitor value. Electrolytic capacitors that have low enough ESR to meet the ripple-current requirement invariably have more than adequate capacitance values. Aluminum-electrolytic ca- pacitors are preferred over tantalum types, which could cause power-up surge-current failure when connecting to robust AC adapters or low-impedance batteries. RMS input ripple current is determined by the input voltage and load current, with the worst possible case occuring at VIN = 2 x VOUT: IRMS = ILOAD x √VOUT(VIN-VOUT) VIN INPUT BIAS AND UVLO: Pin 11 of the MSK 5045 provides bias to an internal linear regulator that powers the control circuitry. The Vbias pin can be connected directly to the input bus for 12V to 80V opera- tion or it can be biased separately with a 12V to 15V source to extend the input range of the device and improve efficiency at high line; refer to the paragraph titled "INPUT VOLTAGE RANGE". Vbias must be applied simultaneous with or prior to the input voltage. The MSK 5045's built in under voltage lock- out feature prevents damage to downstream devices in the event of a drop in bias voltage. Under voltage lockout occurs at bias voltages of approximately 10V rising and 9.7V falling. When separating the bias voltage from Vin to extend the input range below the Vbias UVLO set point, a simple open collector circuit can disable the device at any desired set point for Vin if UVLO is required. The internal bias draws approximately 30mA under normal operation and less than 10mA in Power Save mode with a light load on the output. INPUT VOLTAGE RANGE The MSK 5045's wide input range of 12V to 80V can be further extended down to VOUT + VDROPOUT by using a sepa- rate bias supply; refer to the paragraph titled "LOW VOLTAGE OPERATION". In this configuration very efficient low V to low V conversion can be achieved. At high line voltages the inter- nal linear regulator dissipates more power than at low line. This loss in efficiency can be eliminated with a separate bias supply pushing the high line efficiency up close to the low line perfor- mance. Output ripple changes with line voltage; refer to the paragraph titled "OUTPUT INDUCTOR (OPTIONAL)" for more information. SOFT START/Cton: The internal soft-start circuitry allows a gradual increase of the internal current-limit level at start-up for the purpose of reducing input surge currents, and possibly for power-supply sequencing. In Disable mode, the soft-start circuit holds the Cton capacitor discharged to ground. When Enable goes high, a 4µA current source charges the Cton capacitor up to 3.2V. The resulting linear ramp causes the internal current-limit thresh- old to increase proportionally from 20mV to 100mV. The out- put capacitors charge up relatively slowly, depending on the Cton capacitor value. The exact time of the output rise de- pends on output capacitance and load current and is typically 1mS per nanofarad of soft-start capacitance. With no capaci- tor connected, maximum current limit is reached typically within 10µS. |
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