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LTC3402 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3402 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LTC3402 3402fb Input Capacitor Selection The input filter capacitor reduces peak currents drawn from the input source and reduces input switching noise. Since the IC can operate at voltages below 0.5V once the output is regulated, then demand on the input capacitor is much less and in most applications a 4.7 μF is recommended. Output Diode For applications with output voltages over 4.3V, a Schottky diode is required to ensure that the SW pin voltage does not exceed its absolute maximum rating. The Schottky diode across the synchronous PMOS switch provides a lower drop during the break-before-make time (typically 20ns) of the NMOS to PMOS transition. The Schottky diode improves peak efficiency (see graph “Efficiency Loss Without Schottky vs Frequency). Use of a Schottky diode such as a MBR0520L, 1N5817 or equivalent. Since slow recovery times will compromise efficiency, do not use ordinary rectifier diodes. Operating Frequency Selection There are several considerations in selecting the operating frequency of the converter. The first is determining the sensitive frequency bands that cannot tolerate any spec- tral noise. For example, in products incorporating RF communications, the 455kHz IF frequency is sensitive to any noise, therefore switching above 600kHz is desired. Some communications have sensitivity to 1.1MHz. In this case, a 2MHz converter frequency may be employed. The second consideration is the physical size of the converter. As the operating frequency goes up, the induc- tor and filter caps go down in value and size. The trade off is in efficiency since the switching losses due to gate charge are going up proportional with frequency. For example in Figure 2, for a 2.4V to 3.3V converter, the efficiency at 100mA is 5% less at 2MHz compared to 300kHz. Another operating frequency consideration is whether the application can allow “pulse skipping.” In this mode, the minimum on time of the converter cannot support the duty cycle, so the converter ripple will go up and there will be a low frequency component of the output ripple. In many APPLICATIO S I FOR ATIO applications where physical size is the main criterion then running the converter in this mode is acceptable. In applications where it is preferred not to enter this mode, then the maximum operating frequency is given by: f VV Vt Hz MAX NOSKIP OUT IN OUT ON MIN _ () – • = where tON(MIN) = minimum on time = 120ns. Figure 2. Converter Efficiency 2.4V to 3.3V OUTPUT CURRENT (mA) 30 90 100 20 10 80 50 70 60 40 0.1 10 100 1000 3402 G08 0 1 300kHz 3MHz 1MHz Burst Mode OPERATION Reducing Output Capacitance with a Load Feed Forward Signal In many applications the output filter capacitance can be reduced for the desired transient response by having the device commanding the change in load current, (i.e. system microcontroller), inform the power converter of the changes as they occur. Specifically, a “load feed forward” signal coupled into the VC pin gives the inner current loop a head start in providing the change in output current. The transconductance of the LTC3402 converter at the VC pin with respect to the inductor current is typically 170mA/100mV, so the amount of signal injected is pro- portional to the anticipated change of inductor current with load. The outer voltage loop performs the remainder of the correction, but because of the load feed forward signal, the range over which it must slew is greatly reduced. This results in an improved transient response. A logic level feed forward signal, VFF, is coupled through components C5 and R6. The amount of feed forward |
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