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LTC3407EMSE Datasheet(PDF) 9 Page - Linear Technology |
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LTC3407EMSE Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3407 9 sn3407 3407fs capacitors, such as Sanyo POSCAP, offer very low ESR, but have a lower capacitance density than other types. Tantalum capacitors have the highest capacitance density, but it has a larger ESR and it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Aluminum electrolytic capacitors have a signifi- cantly larger ESR, and are often used in extremely cost- sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have the lowest ESR and cost, but also have the lowest capacitance density, a high voltage and tempera- ture coefficient, and exhibit audible piezoelectric effects. In addition, the high Q of ceramic capacitors along with trace inductance can lead to significant ringing. Other capacitor types include the Panasonic Special Polymer (SP) capacitors. In most cases, 0.1µF to 1µF of ceramic capacitors should also be placed close to the LTC3407 in parallel with the main capacitors for high frequency decoupling. Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempt- ing for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumen- tal in giving acceptable loop phase margin. Ceramic ca- pacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before ESR becomes effective. APPLICATIO S I FOR ATIO Figure 2. LTC3407 General Schematic RUN2 VIN VIN = 2.5V TO 5.5V VOUT2 VOUT1 RUN1 POR SW1 VFB1 GND VFB2 SW2 MODE/SYNC LTC3407 CIN R5 POWER-ON RESET C4 C5 L1 L2 R4 R2 R1 R3 COUT2 COUT1 3407 F02 PULSESKIP* BURST* *MODE/SYNC = 0V: PULSE SKIP MODE/SYNC = VIN: Burst Mode Also, ceramic caps are prone to temperature effects which requires the designer to check loop stability over the operating temperature range. To minimize their large temperature and voltage coefficients, only X5R or X7R ceramic capacitors should be used. A good selection of ceramic capacitors is available from Taiyo Yuden, TDK, and Murata. Great care must be taken when using only ceramic input and output capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a load step at the output can induce ringing at the VIN pin. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, the ringing at the input can be large enough to damage the part. Since the ESR of a ceramic capacitor is so low, the input and output capacitor must instead fulfill a charge storage requirement. During a load step, the output capacitor must instantaneously supply the current to support the load until the feedback loop raises the switch current enough to support the load. The time required for the feedback loop to respond is dependent on the compensation and the output capacitor size. Typically, 3-4 cycles are required to respond to a load step, but only in the first cycle does the output drop linearly. The output droop, VDROOP, is usually about 3 times the linear drop of the first cycle. Thus, a good place to start is with the output capacitor size of approxi- mately: C I fV OUT OUT O DROOP ≈ ∆ 3 • More capacitance may be required depending on the duty cycle and load step requirements. In most applications, the input capacitor is merely re- quired to supply high frequency bypassing, since the impedance to the supply is very low. A 10µF ceramic capacitor is usually enough for these conditions. Setting the Output Voltage The LTC3407 develops a 0.6V reference voltage between the feedback pin, VFB, and the ground as shown in Figure 2. The output voltage is set by a resistive divider according to the following formula: |
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