1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668/MAX669
Table 2. Bootstrapped and Non-Bootstrapped Configurations
CONFIGURATION
FIGURE
USE
WITH:
INPUT
VOLTAGE
RANGE* (V)
OUTPUT
VOLTAGE
RANGE (V)
COMMENTS
High-Voltage,
Bootstrapped
Figure
2
MAX669
1.8 to 28
3V to 28
Connect V
CC
to V
OUT
. Provides maximum external
FET gate drive for low-voltage (Input <3V) to high-
voltage (output >5.5V) boost circuits. V
OUT
cannot
exceed 28V.
Connect V
OUT
to V
CC
and LDO.
Provides maxi-
mum possible external FET gate drive for low-volt-
age designs, but limits V
OUT
to 5.5V or less.
Connect V
IN
to V
CC
. Provides widest input and out-
put range, but external FET gate drive is reduced for
V
IN
below 5V.
Connect V
IN
to V
CC
and LDO.
FET gate-drive
amplitude = V
IN
for logic-supply (input 3V to 5.5V) to
high-voltage (output >5.5V) boost circuits. IC oper-
ating power is less than in Figure 4, since IC current
does not pass through the LDO regulator.
Connect V
CC
and LDO to a separate supply
(V
BIAS
) that powers only the IC.
FET gate-drive
amplitude = V
BIAS
. Input power source (V
IN)
and
output voltage range (V
OUT
) are not restricted,
except that V
OUT
must exceed V
IN
.
Low-Voltage,
Bootstrapped
Figure
3
MAX669
1.8 to 5.5
2.7 to 5.5
High-Voltage,
Non-Bootstrapped
Figure
4
MAX668
3 to 28
V
IN
to
∞
Low-Voltage,
Non-Bootstrapped
Figure
5
MAX668
2.7 to 5.5
V
IN
to
∞
Extra IC supply,
Non-Bootstrapped
None
MAX668
Not
Restricted
V
IN
to
∞
*
For standard step-up DC-DC circuits (as in Figures 2, 3, 4, and 5), regulation cannot be maintained if V
IN
exceeds V
OUT
. SEPIC
and transformer-based circuits do not have this limitation.
In addition to the configurations shown in Table 2, the
following guidelines may help when selecting a config-
uration:
1) If V
IN
is ever below 2.7V, V
CC
must
be boot-
strapped to V
OUT
and the MAX669 must be used. If
V
OUT
never exceeds 5.5V, LDO may be shorted to
V
CC
and V
OUT
to eliminate the dropout voltage of
the LDO regulator.
2) If V
IN
is greater than 3.0V, V
CC
can be powered
from V
IN
, rather than from V
OUT
(non-bootstrapped).
This can save quiescent power consumption, espe-
cially when V
OUT
is large. If V
IN
never exceeds
5.5V, LDO may be shorted to V
CC
and V
IN
to elimi-
nate the dropout voltage of the LDO regulator.
3) If V
IN
is in the 3V to 4.5V range (i.e., 1-cell Li-Ion or
3-cell NiMH battery range), bootstrapping V
CC
from
V
OUT
, although not required, may increase overall
efficiency by increasing gate drive (and reducing
FET resistance) at the expense of quiescent power
consumption.
4) If V
IN
always exceeds 4.5V, V
CC
should be tied to
V
IN
, since bootstrapping from V
OUT
does not
increase gate drive from EXT but does increase
quiescent power dissipation.
12
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