5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
MAX608
2.0
V
OUT
= 3.3V
L = 22µH
MAXIMUM OUTPUT CURRENT (A)
MAXIMUM OUTPUT CURRENT (A)
1.5
R
SENSE
= 25mΩ
R
SENSE
= 35mΩ
1.0
R
SENSE
= 50mΩ
3.5
3.0
2.5
2.0
1.5
1.0
0.5
R
SENSE
= 100mΩ
0
2.0
2.5
3.0
INPUT VOLTAGE (V)
3.5
0
2
3
4
INPUT VOLTAGE (V)
5
R
SENSE
= 50mΩ
V
OUT
= 5V
L = 22µH
R
SENSE
= 20mΩ
R
SENSE
= 25mΩ
R
SENSE
= 35mΩ
0.5
R
SENSE
= 100mΩ
Figure 4a. Maximum Output Current vs. Input Voltage
(V
OUT
= 3.3V)
3.5
MAXIMUM OUTPUT CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
2
4
R
SENSE
= 100mΩ
6
8
INPUT VOLTAGE (V)
10
12
R
SENSE
= 50mΩ
Figure 4b. Maximum Output Current vs. Input Voltage
(V
OUT
= 5V)
3.5
MAXIMUM OUTPUT CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
2
4
6
R
SENSE
= 100mΩ
8
10
12
INPUT VOLTAGE (V)
14
16
R
SENSE
= 50mΩ
V
OUT
= 12V
L = 22µH
R
SENSE
= 20mΩ
R
SENSE
= 25mΩ
R
SENSE
= 35mΩ
V
OUT
= 15V
L = 22µH
R
SENSE
= 20mΩ
R
SENSE
= 25mΩ
R
SENSE
= 35mΩ
Figure 4c. Maximum Output Current vs. Input Voltage
(V
OUT
= 12V)
Figure 4d. Maximum Output Current vs. Input Voltage
(V
OUT
= 15V)
current will ramp up to a high level before the current-lim-
it comparator can turn off the switch. The minimum on-time
for the switch (t
ON
(min)) is approximately 2µs; select an
inductor that allows the current to ramp up to I
LIM
.
The standard operating circuits use a 22µH inductor.
If a different inductance value is desired, select L such
that:
V
IN
(max) x 2µs
L
≥
—————----—--
I
LIM
Larger inductance values tend to increase the start-up
time slightly, while smaller inductance values allow the
coil current to ramp up to higher levels before the
switch turns off, increasing the ripple at light loads.
Inductors with a ferrite core or equivalent are recom-
mended; powder iron cores are not recommended for
use with high switching frequencies. Make sure the
inductor’s saturation current rating (the current at which
the core begins to saturate and the inductance starts to
fall) exceeds the peak current rating set by R
SENSE
.
However, it is generally acceptable to bias the inductor
into saturation by approximately 20% (the point where
the inductance is 20% below the nominal value). For
highest efficiency, use a coil with low DC resistance,
preferably under 20mΩ. To minimize radiated noise,
use a toroid, a pot core, or a shielded coil.
Table 1 lists inductor suppliers and specific recom-
mended inductors.
_______________________________________________________________________________________
9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
