Rainbow Electronics MAX1837 - Manual (Page 11)

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
loops formed by the power components (C
IN , C OUT ,
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
input capacitor’s voltage rating exceeds V
IN(MAX) and
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.
MAX1836/MAX1837
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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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