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SC4609MLTRT Datasheet(PDF) 8 Page - Semtech Corporation |
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SC4609MLTRT Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 18 page  8 2006 Semtech Corp. www.semtech.com SC4609 POWER MANAGEMENT Application Information (Cont.) Over Current Protection The SC4609 detects over current conditions by sensing the voltage across the drain-to-source of the high side MOSFET. The SC4609 determines the high side MOS- FET current level by sensing the drain-to-source conduc- tion voltage across the high side MOSFET via the V in (see the Typical Application Circuit on page 1) and PHASE pin during the high side MOSFET’s conduction period. This voltage value is then compared internally to a user pro- grammed current limit threshold. Note that user should place Kelvin sensing connections directly from the high side MOSFET source to the PHASE pin. The current limit threshold is programmed by the user based on the RDS(on) of the high side MOSFET and the value of the external set resistor RSET (where RSET is represented by R3 in the applications schematics of this document). The SC4609 uses an internal current source to pull a 50µA current from the input voltage to the ISET pin through external resistor RSET. The current limit threshold resistor (RSET) value is calcu- lated using the following equation: A 50 R I R ) ON ( DS MAX SET µ ⋅ = The R DS(ON) sensing used in the SC4609 has an addi- tional feature that enhances the performance of the over current protection. Because the R DS(ON) has a positive temperature coefficient, the 50µA current source has a positive coefficient of about 0.28%/C° providing first or- der correction for current sensing vs temperature. This compensation depends on the high amount of thermal transferring that typically exists between the high side N- MOSFET and the SC4609 due to the compact layout of the power supply. When the converter detects an over current condition (I > I MAX) as shown in Figure 2, the first action the SC4609 takes is to enter the cycle by cycle protection mode (Point B to Point C), which responds to minor over current cases. Then the output voltage is monitored. If the over current and low output voltage (set at 70% of nominal output voltage) occur at the same time, the Hiccup mode op- eration (Point C to Point D) of the SC4609 is invoked and the internal soft start capacitor is discharged. This is like a typical soft start cycle: nom O V − ⋅ 6 . 0 nom O V − O V O I I MAX D C B A nom O V − ⋅ 6 . 0 nom O V − O V O I I MAX D C B A Figure 2. Over current protection characteristic of SC4609 Power MOSFET Drivers The SC4609 has two drivers which are optimized for driv- ing external power N-Channel MOSFETs.. The driver block consists two 1 Amp drivers. DRVH drives the high side N-MOSFET (main switch), and DRVL drives the low side N-MOSFET (synchronous rectifier transistor). The output drivers also have gate drive non-overlap mechanism that provides a dead time between DRVH and DRVL transitions to avoid potential shoot through problems in the external MOSFETs. By using the proper design and the appropriate MOSFETs, the SC4609 is capable of driving a converter with up to 12A of output current. As shown in Figure 3, td1 , the delay from the top MOSFET off to the bottom MOSFET on is adaptive by detecting the voltage of the phase node. td2, the delay from the bottom MOSFET off to the top MOSFET on is fixed, is 40ns for the SC4609. This control scheme guar- antees avoidance of cross conduction or shoot through between the upper and lower MOSFETs and also mini- mizes the conduction loss in the body diode of the bot- tom MOSFET for high efficiency applications. 0.7 |
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