MAX649 Maxim, MAX649 Datasheet - Page 10

• Manufacturer Part Number: MAX649
• Description: 5V/3.3V/3V or Adjustable / High-Efficiency /
• Manufacturer: Maxim

5V/3.3V/3V or Adjustable, High-Efficiency,
Low I
Figure 5a. MAX649 Current-Sense Resistor Graph
The MAX649/MAX651/MAX652 are preset for 5V, 3.3V,
and 3V output voltages, respectively. Tie FB to GND
for fixed-output operation. They may also be adjusted
from 1.5V (the reference voltage) to the input voltage,
using external resistors R2 and R3 configured as
shown in Figure 4. For adjustable-output operation,
150kΩ is recommended for resistor R3. 150kΩ is a
good value—high enough to avoid wasting energy, yet
low enough to avoid RC delays caused by parasitic
capacitance at FB. R2 is given by:
where V
When using external resistors, it does no harm to con-
nect OUT and the output together, or to leave OUT
unconnected.
The current-sense resistor limits the peak switch cur-
rent to 210mV/R
the current-sense resistor, and 210mV is the current-
limit trip level (see Electrical Characteristics ).
To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a func-
tion of the peak current, the peak current must not
be too low.
10
__________________Design Procedure
______________________________________________________________________________________
REF
3.0
2.5
2.0
1.5
1.0
0.5
0
3
= 1.5V.
Current-Sense Resistor Selection
Q
4
, Step-Down DC-DC Controllers
5
R2 = R3 x
SENSE
6
Setting the Output Voltage
INPUT VOLTAGE (V)
7
, where R
8
9
[
10 11
——— -1
V
V
OUT
REF
R
R
R
R
R
R
12
S
SENSE
S
S
S
S
S
MAX649
V
= 0.07
= 0.06
= 0.08
= 0.10
= 0.12
= 0.14
OUT
13
= 5V
14
]
15
is the value of
16
Figure 5b. MAX651 Current-Sense Resistor Graph
Figure 5c. MAX652 Current-Sense Resistor Graph
To choose the proper current-sense resistor for a par-
ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, referring
to Figures 5a, 5b, or 5c, using the minimum input volt-
age, find the curve with the largest sense resistor that
provides sufficient output current. It is not necessary
to perform worst-case calculations. These curves take
into account the worst-case values for sense resistor
(±5%), inductor (22µH ±10%), diode drop (0.6V), and
the IC’s current-sense trip level; an external MOSFET
on-resistance of 0.13Ω is assumed for V
3.0
2.5
2.0
1.5
1.0
0.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
3
3
4
4
5
5
6
6
INPUT VOLTAGE (V)
7
INPUT VOLTAGE (V)
7
8
8
9
9
10
10
11
11
12
12
R
R
R
R
R
R
R
R
R
R
R
R
MAX652
V
MAX651
V
S
S
S
S
S
S
S
S
S
S
S
S
OUT
OUT
13
13
= 0.06
= 0.07
= 0.08
= 0.10
= 0.12
= 0.14
= 0.06
= 0.07
= 0.08
= 0.10
= 0.12
= 0.14
= 3.0V
14
= 3.3V
14
GS
15
15
16
16
= -4.5V.