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MIC24420 Datasheet(PDF) 17 Page  Micrel Semiconductor 

MIC24420 Datasheet(HTML) 17 Page  Micrel Semiconductor 
17 / 34 page Micrel, Inc. MIC24420/MIC24421 June 2012 17 M9999062012C C S SS OUT OUT I I V C / ⋅ ⋅ > SS C Where IS/C is the short circuit, foldback current limit. CSS is the capacitor connected to EN/DLY pin ISS is the EN/DLY pull up current. Current Limit Resistor The current limit circuit responds to the peak inductor current flowing through the lowside FET. Calculating the current setting resistor RCS should take into account the peak inductor current and the blanking delay of approximately 100ns. Figure 9. Overcurrent waveform Figure 9 shows the lowside MOSFET current waveform. Peak current is measured after a small delay. The equations used to calculate the current limit resistor value are shown below: 2 OUT PK I I I PP ⋅ = L t V I I DLY OUT PK OC ⋅ − = CS ON OC CS I RDS I R ⋅ = Where: IOC is the current limit set point L = inductor value tDLY = Current limit blanking time ~ 100ns ICS is the overcurrent pin sense current (200µA nominal) RDS(ON) is the on resistance of the lowside MOSFET Short Circuit Protection It is recommended that a foldback current characteristic be implemented to protect both external and internal MOSFETs during short circuit (S/C) events. This can be achieved by the addition of one additional resistor RFBK (R14 & R19 on the evaluation board) from VOUT to the CS pin. Figure 9a. Short Circuit Protection Current limit will occur at: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − ⋅ + ⋅ ⋅ = OFF CS FBK CS OUT CS CS ON DS OC V R R V R I R I _ ) ( 1 Where VCS_OFF is the CS comparator offset voltage. For simplicity, assuming VCS_OFF is 0V, we can set IS/C (current limit when VOUT = 0V) to be half IOC (current limit when VOUT = nominal): CS OUT FBK CS ON DS OC CS I V R I R I R = ⋅ ⋅ = 2 ) ( To determine worst case values, one must take into account VCS offset voltage, ICS range and the range of values for RDS(ON) over the operating temperature range. Some typical example values for a 30m Ω MOSFET: VOUT IOC IS/C RCS RFBK 3.3A 1.7 249 24.9k 5 4.3A 1.7 249 16k 3.3A 1.7 249 16.5k 3.3 4.3A 1.7 249 10.5k 3.3A 1.7 249 5.1k 1.2 4.3A 1.7 249 3.83k Due to the leading edge blanking, a 100ns slew rate for the CS pin can be applied without interfering with current limit operation. Limiting the CS pin’s slew rate will help to prevent false triggering. A C·R product of at least 20ns should be used. 
