Datasheet LTC3901 (Analog Devices) - 9

LTC3901
U U W U APPLICATIO S I FOR ATIO
output goes high; this is to avoid any ringing immediately denly increases when the MOSFETs are off, it creates a after the MOSFETs are switched on. large output voltage drop. To overcome this, add a resistor Under no/light load conditions, if the inductor average divider, RCSX1 and RCSX2 at the CSX+ pin to increase the current is less than half of its peak-to-peak ripple current, 10.5mV threshold so that the LTC3901 operates in con- the inductor current will reverse into MOSFETs during a tinuous mode at no load. portion of the free-wheeling period, forcing CSX+ to rise The LTC3901 CSX+ pin has an internal current sinking above CSX–. The current sense comparator input thresh- clamp circuit (ZCSX) that clamps the pin to around 11V. old is set at 10.5mV to prevent tripping under this normal The clamp circuit, together with the external series resis- no load condition. If at no load, the product of the inductor tor RCSX1, protects the CSX+ pins from the high MOSFET negative peak current and MOSFET RDS(ON) is higher than drain voltage in the power delivery cycle. During the power 10.5mV; this may trip the comparator and force the delivery cycle, one of the MOSFETs (ME or MF) is off. The LTC3901 to operate in discontinuous mode. Figure 7 drain voltage of the MOSFET that is off is determined by the shows the LTC3901 operating in discontinuous mode; the primary input voltage and the transformer turn ratio. This driver’s output goes low before the next SYNC transition voltage can be high and may damage the internal circuit if edge when the inductor current goes negative. In push- CSX+ is connected directly to the drain of its MOSFET. The pull topology, both MOSFETs conduct the same amount of current sinking capability of the clamp circuit is 5mA current during the free-wheeling period; this will trip both minimum. comparators at the same time. Discontinuous mode is The value of the resistorsR sometimes undesirable because if the load current sud- CSX1, RCSX2 and RCSX3 should be calculated using the following formulas to meet both the clamp and threshold voltage requirements: SDRA k = {48 • IRIPPLE • RDS(ON)} –1 SDRB RCSX2 = {200 • VIN(MAX) • NS/NP –2200 • (1 + k)} /k SYNC 0V RCSX1 = k • RCSX2 RCSX3 = {RCSX1 • RCSX2} / {RCSX1 + RCSX2} ME If k = 0 or less than zero, RCSX2 is not needed and RCSX1 = RCSX3 = {VIN(MAX) • (NS/NP) – 11V} / 5mA MF where: L1 CURRENT 0V IRIPPLE = Inductor peak-to-peak ripple current CURRENT SENSE COMPARATOR TRIP RDS(ON) = On-resistance of MOSFET at IRIPPLE/2
Figure 7a. Discontinuous Mode Operation at No Load
VIN(MAX) = Primary side main supply maximum input voltage SYNC 0V NS/NP = Power transformer T1, turn ratio If the LTC3901 still operates in discontinuous mode with ME the calculated resistance value, increase the value of RCSX1 to raise the threshold. The resistors RCSX1 and MF RCSX2 and the CSX+ pins input capacitance plus the PCB L1 trace capacitance forms an R-C delay; this slows down the CURRENT 0V response time of the comparators. The resistors and CSX+ ADJUSTED CURRENT SENSE THRESHOLD 3901 F06 input leakage currents also create an input offset error. To minimize this delay and error, do not use resistance
Figure 7b. Continuous Mode Operation with Adjusted Current Sense Threshold
value higher than required and make the PCB trace from 3901f 9