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SC4612HMLTRT Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4612HMLTRT Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 21 page 11 © 2008 Semtech Corp. www.semtech.com SC4612H POWER MANAGEMENT Applications Information (Cont.) Under Voltage Lock Out Under Voltage Lock Out (UVLO) circuitry senses the VDD through a voltage divider. If this signal falls below 4.5V (typical) with a 400mV hysteresis (typical), the output driv- ers are disabled. During the thermal shutdown, the out- put drivers are disabled. OVERCURRENT PROTECTION The SC4612H features low side MOSFET R DS(ON) current sensing and hiccup mode over current protection. The voltage across the bottom FET is sampled approximately 150ns after it is turned on to prevent false tripping due to ringing of the phase node. The internally set over current threshold is 100mV typical. This can be adjusted up or down by connecting a resistor between ILIM and DRV or GND respectively. When programming with an external resistor, threshold set point accuracy will be degraded to 30%. The FET R DS(ON) at temperature will typically be 150% or more of the room temperature value. Allowance should be made for these sources of error when programming a threshold value. When an over current event occurs, the SC4612H immediately disables both gate drives. The SS ramp continues to its final value, if not already there. Once at final value, the SS capacitor is discharged at approximately 1uA until SS low value is reached (approx 0.8V). The SS/ Hiccup cycle will then repeat until the fault condition is removed and the SC4612H starts up normally on the next SS cycle. Gate Drive/Control The SC4612H provides integrated high current drivers for fast switching of large MOSFETs. The higher gate current will reduce switching losses of the larger MOSFETs. The low side gate drive is supplied directly from the DRV. The high side gate drive is bootstraped from the DRV pin. Cross conduction prevention circuitry ensures a non over- lapping (30ns typical) gate drive between the top and bot- tom MOSFETs. This prevents shoot through losses which provides higher efficiency. Typical total minimum off time for the SC4612H is about 30ns. ERROR AMPLIFIER DESIGN The SC4612H is a voltage mode buck controller that utilizes an externally compensated high bandwidth error amplifier to regulate the output voltage. The power stage of the synchronous rectified buck converter control-to-output transfer function is as shown below. + + + × = LC 2 s L R L s 1 C C sESR 1 S V IN V ) s ( VD G where, V IN – Input voltage L – Output inductance ESR C – Output capacitor ESR V S – Peak to peak ramp voltage R L – Load resistance C – Output capacitance The classical Type III compensation network can be built around the error amplifier as shown below: R3 C3 R2 R1 C1 Vref C2 - + Figure 1. Voltage mode buck converter compensation network. The transfer function of the compensation network is as follows: ) s 1 )( s 1 ( ) s 1 )( s 1 ( s ) s ( G 2 P 1 P 2 Z 1 Z I COMP ω + ω + ω + ω + ⋅ ω = where, Cout Lout 1 , C ) R R ( 1 , C R 1 o 2 3 1 2 Z 1 2 1 Z × = ω + = ω = ω 3 1 3 1 2 2 P 2 3 1 P 3 1 1 I C C C C R 1 , C R 1 , ) C C ( R 1 + = ω = ω + = ω |
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