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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