Datasheet LTC3407A-2 (Analog Devices) - 9

LTC3407A-2
U U W U APPLICATIO S I FOR ATIO Output Capacitor (C
trace inductance can lead to significant ringing. Other
OUT) Selection
capacitor types include the Panasonic Special Polymer The selection of COUT is driven by the required ESR to (SP) capacitors. minimize voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the capacitance is In most cases, 0.1μF to 1μF of ceramic capacitors should adequate for filtering. The output ripple (ΔV also be placed close to the LTC3407A-2 in parallel with the OUT) is deter- mined by: main capacitors for high frequency decoupling. VIN = 2.5V TO 5.5V ⎛ 1 ⎞ C ΔVOUT ≈ ΔI ESR L + IN R7 VIN R6 BURST* R5 ⎝⎜ 8f C O OUT ⎠ ⎟ MODE/SYNC POR POWER-ON PULSESKIP* RESET LTC3407A-2 RUN/SS2 RUN/SS1 where fO = operating frequency, COUT = output capacitance V L2 L1 OUT2 VOUT1 and ΔI SW2 SW1 L = ripple current in the inductor. The output ripple C2 C1 is highest at maximum input voltage since ΔIL increases C4 C3 with input voltage. With ΔIL = 0.3 • ILIM the output ripple V V FB2 FB1 R4 R2 GND will be less than 100mV at maximum VIN and fO = 2.25MHz C R3 R1 OUT2 COUT1 with: *MODE/SYNC = 0V: PULSE SKIP 3407A2 F01 ESR MODE/SYNC = VIN: Burst Mode COUT < 150mΩ Once the ESR requirements for COUT have been met, the
Figure 1. LTC3407A-2 General Schematic
RMS current rating generally far exceeds the IRIPPLE(P-P)
Ceramic Input and Output Capacitors
requirement, except for an all ceramic solution. Higher value, lower cost ceramic capacitors are now In surface mount applications, multiple capacitors may becoming available in smaller case sizes. These are tempt- have to be paralleled to meet the capacitance, ESR or RMS ing for switching regulator use because of their very low current handling requirement of the application. Alumi- ESR. Unfortunately, the ESR is so low that it can cause num electrolytic, special polymer, ceramic and dry tantulum loop stability problems. Solid tantalum capacitor ESR capacitors are all available in surface mount packages. The generates a loop “zero” at 5kHz to 50kHz that is instrumen- OS-CON semiconductor dielectric capacitor available from tal in giving acceptable loop phase margin. Ceramic ca- Sanyo has the lowest ESR(size) product of any aluminum pacitors remain capacitive to beyond 300kHz and usually electrolytic at a somewhat higher price. Special polymer resonate with their ESL before ESR becomes effective. capacitors, such as Sanyo POSCAP, offer very low ESR, Also, ceramic caps are prone to temperature effects which but have a lower capacitance density than other types. requires the designer to check loop stability over the Tantalum capacitors have the highest capacitance density. operating temperature range. To minimize their large However, they also have a larger ESR and it is critical that temperature and voltage coefficients, only X5R or X7R they are surge tested for use in switching power supplies. ceramic capacitors should be used. A good selection of An excellent choice is the AVX TPS series of surface mount ceramic capacitors is available from Taiyo Yuden, TDK, tantalums, available in case heights ranging from 2mm to and Murata. 4mm. Aluminum electrolytic capacitors have a signifi- cantly larger ESR, and are often used in extremely cost- Great care must be taken when using only ceramic input sensitive applications provided that consideration is given and output capacitors. When a ceramic capacitor is used to ripple current ratings and long term reliability. Ceramic at the input and the power is being supplied through long capacitors have the lowest ESR and cost, but also have the wires, such as from a wall adapter, a load step at the output lowest capacitance density, a high voltage and tempera- can induce ringing at the VIN pin. At best, this ringing can ture coefficient, and exhibit audible piezoelectric effects. couple to the output and be mistaken as loop instability. In addition, the high Q of ceramic capacitors along with 3407a2f 9