Inductor Selection

The MAX1878 is optimized to use a 10µH inductor over

the entire operating range. Smaller inductance values

typically offer smaller physical size for a given series

resistance or saturation current. Circuits using larger

inductance values may startup at lower input voltages

and exhibit less ripple, but also provide reduced output

power. This occurs when the inductance is sufficiently

large to prevent the maximum current limit from being

reached before the maximum on-time expires. The

inductor’s saturation current rating should be greater

than the peak switching current. However, it is generally

acceptable to bias the inductor into saturation by as

much as 20%, although this will slightly reduce efficien-

cy. Choose a low DC-resistance inductor to improve effi-

ciency. For the above reasons choose the step-up

converter inductor in the range of 10µH to 33µH

Step-Up Converter Diode Selection

The high maximum switching frequency of 500kHz

requires a high-speed rectifier such as the 1N4148. To

maintain high efficiency, the average current rating of

the diode should be greater than the peak switching

current. Choose a reverse breakdown voltage greater

than the output voltage. A Schottky diode is not recom-

mended as the lower forward voltage does little to

improve efficiency whereas the higher reverse leakage

current decreases efficiency.

Input Bypass Capacitors

ic capacitor to PGND and PGNDLCD as close to the IC

as possible. This input bypass capacitor reduces peak

currents and noise at the input voltage source. Connect

AIN1 and AIN2 together and bypass with a low-ESR

1µF surface-mount ceramic capacitor to AGND. A low

resistance (10Ω) from IN to AIN1 and AIN2 creates a

lowpass RC filter and provides low-noise analog input

power to the MAX1878.

Output Filter Capacitors

The MAX1878 is a voltage mode converter and requires

ripple at FB and FBLCD for stable regulation. For most

surface-mount capacitor to PGNDLCD. For small

ceramic capacitors, the output ripple voltage is domi-

nated by the capacitance value. If tantalum or electrolyt-

ic capacitors are used, the higher ESR increases the

output ripple voltage. Decreasing the ESR reduces the

output ripple voltage and the peak-to-peak transient

voltage. Surface-mount capacitors are generally pre-

ferred because they lack the inductance and resistance

10µF to 47µF tantalum capacitor to PGND. Choose a

capacitor with 200mΩ to 300mΩ ESR to provide stable

switching while minimizing output ripple. A 22µF filter

capacitor works well for most applications.

Ripple Regulation

For proper switching control the ripple at FB and FBLCD

must be greater than 25mV. Use R6 and C6 as shown in

Figure 1 to inject ripple into FB. To insure sufficient ripple

on FBLCD, connect C5 as shown in Figure 1.

PC Board Layout and Grounding

High switching frequencies make PC board layout a

very important part of design. Good design minimizes

excessive EMI on the feedback paths and voltage gra-

dients in the ground plane, both of which can result in

instability or regulation errors. Connect the inductors,

input filter capacitors, and output filter capacitors as

close to the device as possible, and keep their traces

short, direct, and wide. The external voltage-feedback

networks should be very close to the feedback pins,

within 0.2 inches (5mm). Keep noisy traces, such as LX

and LXLCD, away from the voltage feedback networks;

also keep them separate, using grounded copper.

The exposed backside pad and corner tabs of the QFN

package are internally connected to analog ground.

For heat dissipation, connect the exposed backside

pad to a large analog ground plane, preferably on a

surface of the board that receives good airflow.

Connect all power grounds and all analog grounds to

separate ground planes in a star ground configuration.

Connect the analog ground plane and the power

ground plane together at a single point. The MAX1878

evaluation kit data sheet includes a proper PC board

layout and routing scheme.

Chip Information

TRANSISTOR COUNT: 4131

EXPOSED PADDLE CONNECTED TO AGND

PROCESS: BiCMOS

Dual-Output Step-Down and LCD Step-Up

Power Supply for PDAs

10

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