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LTC3423 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3423 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC3423/LTC3424 34234f Operating Frequency Selection There are several considerations in selecting the operat- ing 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, converter frequencies up to 3MHz may be em- ployed. 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. 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 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 = 140ns 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 LTC3423 converter at the VC pin with respect to the inductor current is typically APPLICATIO S I FOR ATIO 130mA/100mV, and the LTC3424 is typically 170mA/ 100mV, so the amount of signal injected is proportional to the anticipated change of inductor current with load. The outer voltage loop performs the remainder of the correc- tion, 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 signal is attenuated with resistor R6 and is given by the following relationship: R VR V VI R FF IN OUT OUT 6 51 5 5 ≈ •• • . • – ∆ where ∆IOUT = load current change. Figure 2 6 10 3 2 1 3423/24 F02 LTC3423/LTC3424 VDD SHDN VIN MODE/SYNC Rt SW VOUT FB VC GND 4 7 8 9 5 R5 C3 LOAD FEED FORWARD SIGNAL VFF R6 C5 3.3nF VIN VDD IN VOUT Closing the Feedback Loop The LTC3423/LTC3424 uses current mode control with internal adaptive slope compensation. Current mode con- trol eliminates the 2nd order filter due to the inductor and output capacitor exhibited in voltage mode controllers, and simplifies it to a single-pole filter response. The product of the modulator control to output DC gain plus the error amp open-loop gain equals the DC gain of the system. |
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