Datasheet LTC3568 (Analog Devices) - 10

LTC3568 applicaTions inForMaTion Ceramic capacitors have the lowest ESR and cost but also cycles are required to respond to a load step, but only in have the lowest capacitance density, a high voltage and the first cycle does the output drop linearly. The output temperature coefficient and exhibit audible piezoelectric droop, VDROOP, is usually about 2 to 3 times the linear effects. In addition, the high Q of ceramic capacitors along drop of the first cycle. Thus, a good place to start is with with trace inductance can lead to significant ringing. Other the output capacitor size of approximately: capacitor types include the Panasonic specialty polymer I (SP) capacitors. C OUT OUT ≈ Δ 2 5 . fO •VDROOP In most cases, 0.1µF to 1µF of ceramic capacitors should also be placed close to the LTC3568 in parallel with the More capacitance may be required depending on the duty main capacitors for high frequency decoupling. cycle and load step requirements.
Ceramic Input and Output Capacitors
In most applications, the input capacitor is merely required to supply high frequency bypassing, since the impedance Higher value, lower cost ceramic capacitors are now be- to the supply is very low. A 22µF ceramic capacitor is coming available in smaller case sizes. These are tempting usually enough for these conditions. for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop
Setting the Output Voltage
stability problems. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumental in giving The LTC3568 develops a 0.8V reference voltage between acceptable loop phase margin. Ceramic capacitors remain the feedback pin, VFB, and the signal ground as shown in capacitive to beyond 300kHz and ususally resonate with Figure 5. The output voltage is set by a resistive divider their ESL before ESR becomes effective. Also, ceramic according to the following formula: caps are prone to temperature effects which requires the  R2 designer to check loop stability over the operating tem- VOUT ≈ 0. V 8 1+  R  1 perature range. To minimize their large temperature and voltage coefficients, only X5R or X7R ceramic capacitors Keeping the current small (<5µA) in these resistors maxi- should be used. A good selection of ceramic capacitors mizes efficiency, but making them too small may allow is available from Taiyo Yuden, TDK and Murata. stray capacitance to cause noise problems and reduce the Great care must be taken when using only ceramic input phase margin of the error amp loop. and output capacitors. When a ceramic capacitor is used To improve the frequency response, a feed-forward capaci- at the input and the power is being supplied through long tor CF may also be used. Great care should be taken to wires, such as from a wall adapter, a load step at the output route the VFB line away from noise sources, such as the can induce ringing at the VIN pin. At best, this ringing can inductor or the SW line. couple to the output and be mistaken as loop instability. At worst, the ringing at the input can be large enough to
Shutdown and Soft-Start
damage the part. The SHDN/RT pin is a dual purpose pin that sets the oscil- Since the ESR of a ceramic capacitor is so low, the input lator frequency and provides a means to shut down the and output capacitor must instead fulfill a charge storage LTC3568. This pin can be interfaced with control logic in requirement. During a load step, the output capacitor must several ways, as shown in Figure 3(a) and Figure 3(b). instantaneously supply the current to support the load The ITH pin is primarily for loop compensation, but it can until the feedback loop raises the switch current enough also be used to increase the soft-start time. Soft start to support the load. The time required for the feedback reduces surge currents from VIN by gradually increasing loop to respond is dependent on the compensation com- the peak inductor current. Power supply sequencing can ponents and the output capacitor size. Typically, 3 to 4 also be accomplished using this pin. The LTC3568 has an 3568fa 0 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts