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
Typical Operating Circuit is recommended for most
a p p lic a tions . La rg e r ind uc ta nc e s re d uc e the p e a k
inductor current, but may limit output current capability
at low input voltages and provide slower start-up times.
Smaller inductances require less board space, but may
cause greater peak current due to current-sense com-
parator propagation delay. If input voltages below 2V
will b e c ommon, re d uc ing the ind uc ta nc e to 22µH
might improve performance; however, maximum load
current and efficiency may decline. It is important to
thoroughly test operation under all input and output
conditions to ensure proper component selection.
S e t t in g t h e Cu rre n t Lim it
External current-limit selection provides added control
over the MAX629’s output performance. A higher cur-
rent limit increases the amount of energy stored in the
inductor during each cycle, which provides a higher
output current capability. For higher output current
applications, choose the 500mA current-limit option by
connecting ISET to V . When lower output current is
CC
required, the 250mA current limit can provide several
advantages. First, a smaller inductor can be used,
which saves board area and cost. Second, the smaller
energy transfer per cycle reduces output ripple for a
given capacitor, providing design flexibility between
board area, cost, and output ripple by allowing cheap-
er, higher-ESR capacitors. Connect ISET to GND to
select the 250mA current-limit option.
Inductors with a ferrite core or equivalent are recom-
mended; powder iron cores are not recommended for
use with high switching frequencies. The inductor’s
incremental saturation rating must exceed the selected
current limit. For highest efficiency, use an inductor with
a low DC resistance (under 100mΩ). See Table 1 for a
list of inductor suppliers.
In d u c t o r S e le c t io n
The MAX629’s high switching frequency allows for the
use of a small inductor. The 47µH inductor shown in the
V
+0.8V
TO +15V
IN
V
IN
+0.8V
TO +24V
V
C1
10µF
35V
CC
C1
10µF
35V
V
*
+2.7V
TO +5.5V
CC
+2.7V
TO +5.5V
*
L1
47µH
L1
47µH
C3
0.1µF
C3
0.1µF
C5
2.2µF
R3
2Ω
D1
MBR0540L
V
+24V
OUT
V
CC
V
CC
D1 = D2 = MBR0540L
D2
D1
SHDN
LX
SHDN
LX
FB
V
C2
10µF
35V
OUT
R1
576k
1%
C
F
150pF
ISET
POL
-20V
MAX629
GND
ISET
POL
C2
10µF
35V
CF
150pF
R1
576k
1%
R2
31.6k
1%
MAX629
FB
R2
35.7k
1%
REF
C4
REF
0.1µF
GND
C4
0.1µF
* FOR SINGLE-SUPPLY OPERATION
* FOR SINGLE-SUPPLY OPERATION
*FOR SINGLE-SUPPLY OPERATION
*FOR SINGLE-SUPPLY OPERATION
Figure 2. +24V for a Positive LCD Bias
Figure 3. -20V for a Negative LCD Bias
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7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
