LTC3401 UUWUAPPLICATIO S I FOR ATIO In some layouts it may be required to place a 1μF low ESR In this case, converter frequencies up to 3MHz may be capacitor as close to the VOUT and GND pins as possible. employed. The second consideration is the physical size of the Input Capacitor Selection converter. As the operating frequency goes up, the induc- The input filter capacitor reduces peak currents drawn from tor and filter caps go down in value and size. The trade off the input source and reduces input switching noise. Since is in efficiency since the switching losses due to gate the IC can operate at voltages below 0.5V once the output charge are going up proportional with frequency. For is regulated, demand on the input capacitor is much less example in Figure 2, for a 2.4V to 3.3V converter, the and in most applications a 3.3μF is sufficient. efficiency at 100mA is 5% less at 2MHz compared to 300kHz. Output Diode Another operating frequency consideration is whether the For applications with output voltages over 4.3V, a Schottky application can allow “pulse skipping.” In this mode, the diode is required to ensure that the SW pin voltage does minimum on time of the converter cannot support the duty not exceed its absolute maximum rating. The Schottky cycle, so the converter ripple will go up and there will be diode across the synchronous PMOS switch provides a a low frequency component of the output ripple. In many lower drop during the break-before-make time (typically applications where physical size is the main criterion then 20ns) of the NMOS to PMOS transition. The Schottky running the converter in this mode is acceptable. In diode improves peak efficiency (see graph “Efficiency applications where it is preferred not to enter this mode, Loss Without Schottky vs Frequency”). Use of a Schottky then the maximum operating frequency is given by: diode such as a MBRM120T3, 1N5817 or equivalent. Since slow recovery times will compromise efficiency, do V – V OUT IN not use ordinary rectifier diodes. f = Hz MAX _NOSKIP V • t OUT ON MIN ( ) Operating Frequency Selection where tON(MIN) = minimum on time = 120ns There are several considerations in selecting the operat- ing frequency of the converter. The first is determining Reducing Output Capacitance with a Load Feed the sensitive frequency bands that cannot tolerate any Forward Signal spectral noise. For example, in products incorporating In many applications the output filter capacitance can be RF communications, the 455kHz IF frequency is sensitive reduced for the desired transient response by having the to any noise, therefore switching above 600kHz is de- device commanding the change in load current, (i.e. sired. Some communications have sensitivity to 1.1MHz. system microcontroller), inform the power converter of 100 the changes as they occur. Specifically, a “load feed 90 Burst Mode forward” signal coupled into the V OPERATION C pin gives the inner 80 current loop a head start in providing the change in output 3MHz 70 current. The transconductance of the LTC3401 converter 300kHz 1MHz 60 at the VC pin with respect to the inductor current is typically 50 130mA/100mV, so the amount of signal injected is pro- 40 EFFICIENCY (%) portional to the anticipated change of inductor current 30 with load. The outer voltage loop performs the remainder 20 of the correction, but because of the load feed forward 10 signal, the range over which it must slew is greatly 0 0.1 1 10 100 1000 reduced. This results in an improved transient response. OUTPUT CURRENT (mA) 3401 G08 Figure 2. Converter Efficiency 2.4V to 3.3V 3401fb 10
