Rainbow Electronics MAX1817 - Manual (Page 11)

loss in efficiency. Choose a reverse breakdown voltage
greater than the output voltage.
Input Bypass Capacitor
The input supplies high currents to the inductors and
requires local bulk bypassing close to the inductors. A
low equivalent series resistance (ESR) input capacitor
connected in parallel with the battery will reduce peak
battery currents and input-reflected noise. Battery
bypassing is especially helpful at low input voltages
and with high-impedance batteries (such as alkaline
types). Benefits include improved efficiency and lower
useful end-of-life voltage for the battery. A single 10µF
low-ESR surface-mount capacitor is sufficient for most
applications.
Output Bypass Capacitors
For most applications, use a small surface-mount 22µF
or greater ceramic capacitor on the main converter out-
put, and a 1µF or greater ceramic capacitor on the LCD
output. For small ceramic capacitors, the output ripple
voltage is dominated by the capacitance value. If tanta-
lum or electrolytic capacitors are used, the ESR of the
capacitors dominates the output ripple voltage.
Decreasing the ESR reduces the output ripple voltage
and the peak-to-peak transient voltage.
LCD Compensation
The MAX1817’s LCD step-up converter feedback
requires a small 4.7pF feed-forward capacitor for the
typical application circuit. Circuits with adjustable V
OUT
(main converter) from 2.5V to 5.5V may require a larger
value LCD feed-forward capacitor to prevent multipuls-
ing of the LCD converter. Larger feed-forward capaci-
tors slightly degrade load regulation, so choose the
smallest value capacitor that provides stability.
Layout Considerations
The MAX1817’s high-frequency operation makes PC
board layout important for optimal performance. Use
separate analog and power ground planes. Connect
the two planes together at a single point as close as
possible to the IC. Use surface-mount components
where possible. If leaded components are used, mini-
mize lead lengths to reduce stray capacitance and
keep the components close to the IC to minimize trace
resistance. Where an external voltage-divider is used to
set output voltage, the traces from FB or FBLCD to the
feedback resistors should be extremely short (less than
0.2in or 5mm) to minimize coupling from LX and
LXLCD. Refer to the MAX1817 evaluation kit for a full
PC board example.
____________________Chip Information
TRANSISTOR COUNT: 2785
PROCESS: BiCMOS
MAX1817
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
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