1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
MAX668/MAX669
V
IN
= 1.8V to 12V
C1
L1
1
C4
9
C2
LDO
EXT
8
6
V
OUT
= 12V @ 0.5A
N1
D1
C5
C6
C8
MAX669
V+
CS+
R1
7
PGND
FB
GND
5
3
C7
R2
10 SYNC/
SHDN
4
REF
C3
2
R4
FREQ
R3
Figure 2. MAX669 High-Voltage Bootstrapped Configuration
V
IN
= 1.8V to 5V
C1
68µF
10V
L1
4.7µH
1
C2
1µF
9
LDO
EXT
8
6
R1
0.02Ω
V
OUT
= 5V @ 1A
D1
MBRS340T3
FDS6680
IRF7401
N1
C4
68µF
10V
C5
68µF
10V
C6
0.1µF
R2
75k
1%
MAX669
V
CC
CS+
10 SYNC/
SHDN
4
REF
C3
0.22µF
2
FREQ
PGND
FB
GND
7
5
3
C7
220pF
R3
24.9k
1%
R4
100k
1%
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-
10
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.
______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
