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MAX1854 Datasheet(PDF) 22 Page - Maxim Integrated Products |
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MAX1854 Datasheet(HTML) 22 Page - Maxim Integrated Products |
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22 / 33 page  High-Speed, Adjustable, Synchronous Step-Down Controllers with Integrated Voltage Positioning 22 ______________________________________________________________________________________ mal frequency is largely a function of maximum input voltage, due to MOSFET switching losses that are pro- portional to frequency and V+2. The optimum frequency is also a moving target, due to rapid improvements in MOSFET technology that are making higher frequen- cies more practical. Inductor operating point: This choice provides trade- offs between size vs. efficiency. Low inductor values cause large ripple currents, resulting in the smallest size, but poor efficiency and high output noise. The minimum practical inductor value is one that causes the circuit to operate at the edge of critical conduction (where the inductor current just touches zero with every cycle at maximum load). Inductor values lower than this grant no further size-reduction benefit. The MAX1716/MAX1854/MAX1855’s pulse-skipping algorithm initiates skip mode at the critical-conduction point. Thus, the inductor operating point also deter- mines the load-current value at which PFM/PWM switchover occurs. The optimum point is usually found between 20% and 50% ripple current. The inductor ripple current impacts transient-response performance, especially at low VIN - VOUT differentials. Low inductor values allow the inductor current to slew faster, replenishing charge removed from the output fil- ter capacitors by a sudden load step. The amount of output sag is also a function of the maximum duty fac- tor, which can be calculated from the on-time and mini- mum off-time: where tOFF(MIN) is the minimum off-time (see Electrical Characteristics), and K is from Table 3. Inductor Selection The switching frequency and operating point (% ripple or LIR) determine the inductor value as follows: Example: ILOAD(MAX) = 18A, VIN = 7V, VOUT = 1.6V, fSW = 300kHz, 30% ripple current or LIR = 0.3. Find a low-loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Ferrite cores are often the best choice, although powdered iron is inexpensive and can work well at 200kHz. The core must be large enough not to saturate at the peak inductor current (IPEAK). IPEAK = ILOAD(MAX) + (ILOAD(MAX) × LIR / 2) Setting the Current Limit The minimum current-limit threshold must be great enough to support the maximum load current when the current limit is at the minimum tolerance value. The val- ley of the inductor current occurs at ILOAD(MAX) minus half of the ripple current; therefore: ILIMIT(LOW) > ILOAD(MAX) - (ILOAD(MAX) × LIR / 2) where ILIMIT(LOW) equals the minimum current-limit threshold voltage divided by RSENSE. For the 120mV default setting, the minimum current-limit threshold is 110mV. Connect ILIM to VCC for a default 120mV current-limit threshold. In the adjustable mode, the current-limit threshold is precisely 1/10th the voltage seen at ILIM. For an adjustable threshold, connect a resistive divider from REF to GND, with ILIM connected to the center tap. The external 0.5V to 2.0V adjustment range corre- sponds to a current-limit threshold of 50mV to 200mV. When adjusting the current limit, use 1% tolerance resistors and a 10µA divider current to prevent a signifi- cant increase of errors in the current-limit value. Output Capacitor Selection The output filter capacitor must have low enough effec- tive series resistance (ESR) to meet output ripple and load-transient requirements, yet have high enough ESR to satisfy stability requirements. Also, the capacitance value must be high enough to absorb the inductor energy going from a full-load to no-load condition with- out tripping the overvoltage protection circuit. In CPU VCORE converters and other applications where the output is subject to violent load transients, the out- put capacitor’s size typically depends on how much ESR is needed to prevent the output from dipping too low under a load transient. Ignoring the sag due to finite capacitance: RESR = VSTEP(MAX) / ILOAD(MAX) The actual µF capacitance value required relates to the physical size needed to achieve low ESR, as well as to the chemistry of the capacitor technology. Thus, the capacitor is usually selected by ESR and voltage rating rather than by capacitance value (this is true of tanta- lums, OS-CONs, and other electrolytics). L VV V V kHz A H = ×− ×× × = 16 7 1 6 7 300 0 30 18 076 .( . ) . . µ L VV V V LIR I OUT OUT SW LOAD MAX = ×+ − () +× × × ƒ () V II L K V V t CV K VV V t SAG LOAD LOAD OUT OFF MIN OUT OUT OUT OFF MIN = −× × × + − ×× × × +− + − () () () 12 2 2 |
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