MAX1836/MAX1837 Stability

Commonly, instability is caused by excessive noise on

the feedback signal or ground due to poor layout or

improper component selection. When seen, instability

typically manifests itself as “motorboating,” which is

characterized by grouped switching pulses with large

gaps and excessive low-frequency output ripple during

no-load or light-load conditions.

PC Board Layout and Grounding

High switching frequencies and large peak currents

make PC board layout an important part of the design.

Poor layout may introduce switching noise into the

feedback path, resulting in jitter, instability, or degrad-

ed performance. High-power traces, bolded in the typi-

cal application circuits (Figures 1 and 2), should be as

short and wide as possible. Additionally, the current

L1, and D1) should be as tight as possible to avoid

radiated noise. Connect the ground pins of these

power components at a common node in a star-ground

configuration. Separate the noisy traces, such as the

LX node, from the feedback network with grounded

copper. Furthermore, keep the extra copper on the

board, and integrate it into a pseudoground plane.

When using external feedback, place the resistors as

close to the feedback pin as possible to minimize noise

coupling. The MAX1837 evaluation kit shows the rec-

ommended layout.

Applications Information

High-Voltage Step-Down Converter

The typical application circuits’ (Figures 1 and 2) com-

ponents were selected for 9V battery applications.

However, the MAX1836/MAX1837 input voltage range

allows supply voltages up to 24V. Figure 6 shows a

modified application circuit for high-voltage applica-

tions. When using higher input voltages, verify that the

that the inductor value exceeds the minimum induc-

tance recommended in the Inductor Selection section.

Inverter Configuration

Figure 7 shows the MAX1836/MAX1837 in a floating

ground configuration. By connecting what would nor-

mally be the output to the supply-voltage ground, the

IC’s ground pin is forced to regulate to -5V

(MAX183_EUT50) or -3.3V (MAX183_EUT33). Avoid

exceeding the maximum ratings of 24V between IN and

GND, and 5.5V between OUT and GND. Other negative

voltages may be generated by placing a resistive

divider across the output capacitor and connecting the

tap to FB in the same manner as the normal step-down

configuration.

24V Internal Switch, 100% Duty Cycle,

Step-Down Converters

______________________________________________________________________________________

11

SUPPLIER

PHONE

FAX

WEBSITE

INDUCTORS

Coilcraft

847-639-6400

847-639-1469

www.coilcraft.com

Coiltronics

561-241-7876

561-241-9339

www.coiltronics.com

Sumida USA

847-956-0666

847-956-0702

www.sumida.com

Toko

847-297-0070

847-699-1194

www.tokoam.com

CAPACITORS

AVX

803-946-0690

803-626-3123

www.avxcorp.com

Kemet

408-986-0424

408-986-1442

www.kemet.com

Panasonic

847-468-5624

847-468-5815

www.panasonic.com

Sanyo

619-661-6835

619-661-1055

www.secc.co.jp

Taiyo Yuden

408-573-4150

408-573-4159

www.t-yuden.com

DIODES

Central Semiconductor

516-435-1110

516-435-1824

www.centralsemi.com

International

310-322-3331

310-322-3332

www.irf.com

Nihon

847-843-7500

847-843-2798

www.niec.co.jp

On Semiconductor

602-303-5454

602-994-6430

www.onsemi.com

Zetex

516-543-7100

516-864-7630

www.zetex.com

Table 1. Component Suppliers