Datasheet LTC3423, LTC3424 (Analog Devices) - 9

LTC3423/LTC3424
U U W U APPLICATIO S I FOR ATIO Operating Frequency Selection
130mA/100mV, and the LTC3424 is typically 170mA/ 100mV, so the amount of signal injected is proportional to There are several considerations in selecting the operat- the anticipated change of inductor current with load. The ing frequency of the converter. The first is determining the outer voltage loop performs the remainder of the correc- sensitive frequency bands that cannot tolerate any spec- tion, but because of the load feed forward signal, the range tral noise. For example, in products incorporating RF over which it must slew is greatly reduced. This results in communications, the 455kHz IF frequency is sensitive to an improved transient response. A logic level feed forward any noise, therefore switching above 600kHz is desired. signal, V Some communications have sensitivity to 1.1MHz. In this FF, is coupled through components C5 and R6. The amount of feed forward signal is attenuated with case, converter frequencies up to 3MHz may be em- resistor R6 and is given by the following relationship: ployed. The second consideration is the physical size of the  V •R5•V • .  FF IN 1 5 ≈ converter. As the operating frequency goes up, the induc- R6 – R5  V • ∆I OUT OUT  tor and filter caps go down in value and size. The trade off is in efficiency since the switching losses due to gate where ∆IOUT = load current change. charge are going up proportional with frequency. Another operating frequency consideration is whether the VIN VOUT application can allow “pulse skipping.” In this mode, the minimum on time of the converter cannot support the duty LTC3423/LTC3424 V cycle, so the converter ripple will go up and there will be DD 6 4 V IN DD SW a low frequency component of the output ripple. In many 10 7 SHDN VOUT applications where physical size is the main criterion then 3 8 V FB running the converter in this mode is acceptable. In IN 2 9 applications where it is preferred not to enter this mode, MODE/SYNC VC then the maximum operating frequency is given by: 1 5 C3 Rt GND R5 V – V f OUT IN = Hz MAX _NOSKIP V • t C5 OUT ON MIN ( ) LOAD FEED 3.3nF R6 FORWARD V where t FF ON(MIN) = minimum on time = 140ns SIGNAL 3423/24 F02
Reducing Output Capacitance with a Load Feed Figure 2 Forward Signal Closing the Feedback Loop
In many applications the output filter capacitance can be reduced for the desired transient response by having the The LTC3423/LTC3424 uses current mode control with device commanding the change in load current, (i.e. internal adaptive slope compensation. Current mode con- system microcontroller), inform the power converter of trol eliminates the 2nd order filter due to the inductor and the changes as they occur. Specifically, a “load feed output capacitor exhibited in voltage mode controllers, forward” signal coupled into the VC pin gives the inner and simplifies it to a single-pole filter response. The current loop a head start in providing the change in output product of the modulator control to output DC gain plus current. The transconductance of the LTC3423 converter the error amp open-loop gain equals the DC gain of the at the VC pin with respect to the inductor current is typically system. 34234f 9