MAX668 Maxim, MAX668 Datasheet - Page 10

• Manufacturer Part Number: MAX668
• Description: 1.8V to 28V Input / PWM Step-Up Controllers in MAX
• Manufacturer: Maxim

1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
Figure 2. MAX669 High-Voltage Bootstrapped Configuration
Figure 3. MAX669 Low-Voltage Bootstrapped Configuration
With bootstrapped operation, the IC is powered from
the circuit output (V
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
______________________________________________________________________________________
0.1 F
C3
0.22 F
1 F
C3
0.22 F
1 F
C4
C2
C2
V
IN
V
IN
= 1.8V to 12V
OUT
= 1.8V to 5V
10
R4
100k
1%
10
R4
100k
1%
1
9
4
2
1
9
4
2
SYNC/
SHDN
REF
FREQ
LDO
V
V
SYNC/
SHDN
REF
FREQ
LDO
CC
). This improves efficiency
CC
Bootstrapped Operation
MAX669
MAX669
PGND
PGND
68 F
GND
EXT
CS+
68 F
GND
EXT
CS+
20V
FB
10V
C1
FB
C1
5
8
6
7
3
8
6
7
5
3
0.02
R1
N1
IRF7401
FDS6680
IRF7401
R1
0.02
N1
L1
4.7 H
L1
4.7 H
MBRS340T3
MBRS340T3
age range extends below 2.7V, then bootstrapped
operation with the MAX669 is the only option.
With V
age swing is 5V when V
0.2V when V
does not exceed 5.5V, the on-chip regulator can be
disabled by connecting V
eliminates the LDO forward drop and supplies maxi-
mum gate drive to the external FET.
D1
D1
CC
220pF
220pF
C5
68 F
20V
C7
C4
68 F
10V
C7
connected to V
CC
is less than 5.2V. If the output voltage
C6
68 F
20V
C5
68 F
10V
R3
24.9k
1%
R2
218k
1%
R3
24.9k
1%
R2
75k
1%
CC
OUT
CC
is 5.2V or more, and V
, as in Figure 2, EXT volt-
to LDO (Figure 3). This
C8
0.1 F
C6
0.1 F
V
V
OUT
OUT
= 5V @ 1A
= 12V @ 0.5A
CC
-