Datasheet

1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX669
LDO
CS+
REF
FREQ
V+
SYNC/
SHDN
PGND
FB
GND
N1
EXT
V
IN
= 1.8V to 12V
C3
C2
C1
C4
R4
R1
R2
R3
C7
D1
C5
C6
C8
3
5
7
6
8
2
4
9
1
10
V
OUT
= 12V @ 0.5A

L1
Figure 2. MAX669 High-Voltage Bootstrapped Configuration
MAX669
LDO
CS+
REF
FREQ
V
CC
SYNC/
SHDN
PGND
FB
GND
N1
EXT
V
IN
= 1.8V to 5V
C3
0.22µF
C2
1µF
R4
100k
1%
R1
0.02Ω
R2
75k
1%
R3
24.9k
1%
C7
220pF
D1
MBRS340T3
C4
68µF
10V
C5
68µF
10V
C6
0.1µF
3
5
7
6
8
2
4
9
1
10
V
OUT
= 5V @ 1A
C1
68µF
10V
L1
4.7µH
FDS6680
IRF7401
Figure 3. MAX669 Low-Voltage Bootstrapped Configuration
Bootstrapped Operation
With bootstrapped operation, the IC is powered from
the circuit output (V
OUT
). This improves efficiency
when the input voltage is low, since EXT drives the FET
with a higher gate voltage than would be available from
the low-voltage input. Higher gate voltage reduces the
FET on-resistance, increasing efficiency. Other (unde-
sirable) characteristics of bootstrapped operation are
increased IC operating power (since it has a higher
operating voltage) and reduced ability to start up with
high load current at low input voltages. If the input volt-
age range extends below 2.7V, then bootstrapped
operation with the MAX669 is the only option.
With V
CC
connected to V
OUT
, as in Figure 2, EXT volt-
age swing is 5V when V
CC
is 5.2V or more, and V
CC
-
0.2V when V
CC
is less than 5.2V. If the output voltage
does not exceed 5.5V, the on-chip regulator can be
disabled by connecting V
CC
to LDO (Figure 3). This
eliminates the LDO forward drop and supplies maxi-
mum gate drive to the external FET.
10
Maxim Integrated
MAX668/MAX669