Rainbow Electronics MAX669 - Manual (Page 11)

Non-Bootstrapped Operation
With non-bootstrapped operation, the IC is powered
from the input voltage (V
IN ) or another source, such as
a logic supply. Non-bootstrapped operation (Figure 4)
is recommended (but not required) for input voltages
above 5V, since the EXT amplitude (limited to 5V by
LDO) at this voltage range is no higher than it would be
with bootstrapped operation. Note that non-boot-
strapped operation is
required if the output voltage
exceeds 28V, since this level is too high to safely con-
nect to V CC . Also note that only the MAX668 can be
used with non-bootstrapped operation .
If the input voltage does not exceed 5.5V, the on-chip
regulator can be disabled by connecting V
CC to LDO
(Figure 5). This eliminates the regulator forward drop
and supplies the maximum gate drive to the external
FET for lowest on-resistance. Disabling the regulator
also reduces the non-bootstrapped minimum input volt-
age from 3V to 2.7V.
MAX668/MAX669
1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
______________________________________________________________________________________ 11
MAX668
LDO
CS+
REF
FREQ
V CC
SYNC/
SHDN
PGND
FB
GND
N1 EXT
V IN = 2.7V to 5.5V
C3
0.22
µ F
C2
1 µ F
R4
100k
1%
R1
0.02 Ω
R2
218k
1%
R3
24.9k
1%
C7
220pF
D1
MBRS340T3
C4
68
µ F
20V
C5
68
µ F
20V
C6
0.1
µ F
3
5
7
6
8
2
4
9
1
10
V OUT = 12V @ 1A
C1
68 µ F
10V
L1
4.7
µ H
FDS6680
Figure 5. MAX668 Low-Voltage Non-Bootstrapped Configuration
MAX668
LDO
CS+
REF
FREQ
V CC
PGND
FB
GND
N1 EXT
V IN = 3V to 12V
C3
0.22
µ F
C4
1 µ F
C2
0.1 µ F
R4
100k
1%
R1
0.02 Ω
R2
218k
1%
R3
24.9k
1%
C7
220pF
D1
MBRS340T3 C5
68
µ F
20V
C6
68
µ F
20V
C8
0.1
µ F
3
5
7
6
8
2
4
9
1
10
V OUT = 12V @ 1A
C1
68 µ F
20V
L1
4.7
µ H
FDS6680
SYNC/
SHDN
Figure 4. MAX668 High-Voltage Non-Bootstrapped Configuration