2 8 V, Lo w -P o w e r, Hig h -Vo lt a g e ,
Bo o s t o r In ve rt in g DC-DC Co n ve rt e r
MAX629
limit threshold. Resistor R3 increases the usable input
voltage range by limiting the peak injected current. The
V
OUT
2Ω re sistor shown provide s a usa b le input volta ge
R1
range beyond V = 15V. In applications with a different
IN
R
B
inp ut volta g e ra ng e , R3 ma y b e inc re a s e d or
decreased as necessary, with a resulting efficiency
change of roughly 0.5%/Ω.
FB
V
BIAS
MAX629
R2
La yo u t Co n s id e ra t io n s
Proper PC board layout is essential due to high current
levels and fast switching waveforms that radiate noise.
It is recommended that initial prototyping be performed
us ing the MAX629 e va lua tion kit or e q uiva le nt PC
board-based design. Breadboards or proto-boards
should never be used when prototyping switching reg-
ulators.
GND
(REF)
( ) ARE FOR NEGATIVE OUTPUT VOLTAGE CONFIGURATIONS.
Figure 4. Adjustable Output Voltage
increasing usable battery life. Using separate supplies
for V and V also reduces noise injection onto V
CC
It is imp orta nt to c onne c t the GND p in, the inp ut
bypass-capacitor ground lead, and the output filter-
capacitor ground lead to a single point (star ground
configuration) to minimize ground noise and improve
regulation. Also, minimize lead lengths to reduce stray
capacitance, trace resistance, and radiated noise, with
preference given to the feedback circuit, the ground
circuit, and LX. Place R1 and R2 as close to the feed-
back pin as possible. Place the input bypass capacitor
CC
IN
by isolating it from the switching transients, allowing a
smaller, less-expensive input filter capacitor to be used
in many applications. If input voltages below 2V will be
common, reducing the inductor to 22µH may improve
performance in this voltage range, at the potential cost
of some decrease in maximum load current and effi-
ciency.
as close as possible to V and GND.
In the negative configuration shown in Figure 3, the
inverting charge pump injects current into LX with each
c yc le . The a mount of c ha rg e inje c te d inc re a s e s a t
CC
Refer to the MAX629 evaluation kit data sheet for an
example of proper board layout.
higher V , and may prematurely trip the internal current-
IN
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9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
