Datasheet LTC3402 (Analog Devices) - 10

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