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MAX1642D-T Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1642D-T Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page High-Efficiency, Step-Up DC-DC Converters for 1V Inputs _______________________________________________________________________________________ 9 Capacitor Selection Choose input and output capacitors to service input and output peak currents with acceptable voltage rip- ple. A 22µF, 6V, low-ESR, surface-mount tantalum out- put filter capacitor typically provides 60mV output ripple when stepping up from 1.3V to 3.3V at 20mA. The input filter capacitor (CIN) also reduces peak cur- rents drawn from the battery and improves efficiency. Low equivalent series resistance (ESR) capacitors are recommended. Capacitor ESR is a major contributor to output ripple (usually more than 60%). Ceramic capaci- tors have the lowest ESR, but low-ESR tantalums repre- sent a good balance between cost and performance. Low-ESR aluminum electrolytic capacitors are tolerable, and standard aluminum electrolytic capacitors should be avoided. Do not exceed tantalum capacitors’ ripple- current ratings; select capacitors with a rating exceed- ing the peak inductor current (IPEAK). PC Board Layout and Grounding High switching frequencies and large peak currents make PC board layout an important part of design. Poor design can result in excessive EMI on the feedback paths and voltage gradients in the ground plane. Both of these factors can result in instability or regulation errors. The OUT pin must be bypassed directly to GND as close to the IC as possible (within 0.2 in. or 5mm). Place power components—such as the MAX1642/ MAX1643, inductor, input filter capacitor, and output filter capacitor—as close together as possible. Keep their traces short, direct, and wide ( ≥50 mil or 1.25mm), and place their ground pins close together in a star-ground configuration. Keep the extra copper on the board and integrate it into ground as a pseudo-ground plane. On multilayer boards, route the star ground using compo- nent-side copper fill, then connect it to the internal ground plane using vias. Place the external voltage-feedback network very close to the FB pin (within 0.2 in. or 5mm). Noisy traces, such as from the LX pin, should be kept away from the voltage- feedback network and separated from it using grounded copper. The evaluation kit manual shows an example PC board layout, routing, and pseudo-ground plane. Noise and Voltage Ripple EMI and output voltage ripple can be minimized by fol- lowing a few simple design rules. 1) Place the DC-DC converter and digital circuitry on an opposite corner of the PC board, away from sen- sitive RF and analog input stages. 2) Use a closed-core inductor, such as toroid or shielded bobbin, to minimize fringe magnetic fields. 3) Choose the largest inductor value that satisfies the load requirement to minimize peak switching cur- rent and resulting ripple and noise. 4) Use low-ESR input and output filter capacitors. 5) Follow sound circuit-board layout and grounding rules (see the PC Board Layout and Grounding section). 6) Where necessary, add LC pi filters, linear post-reg- ulators such as the MAX8863 and MAX8864 (SOT23 package), or shielding. The LC pi filter’s cutoff frequency should be at least a decade or two below the DC-DC converter’s switching frequency for the specified load and input voltage. MAX1642 GND BATT PFI VOUT = 2V TO 5.2V 0.88V to 1.65V INPUT *OPTIONAL COMPENSATION 100 µH PF0 100pF* SHDN LX OUT FB R1 R2 OUT 22 µF 0.1 µF MAX1642 MAX1643 PFI R3 VTH R4 Figure 4. Adjustable-Output Circuit Figure 5. Power-Fail Detection Circuit |
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