Rainbow-electronics MAX652 Manual de usuario descargar pdf (Pagina 10)

__________________Design Procedure

Setting the Output Voltage

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:

OUT

R2 = R3 x

[

——— -1

]

REF

where V

REF

= 1.5V.

When using external resistors, it does no harm to con-

nect OUT and the output together, or to leave OUT

unconnected.

Current-Sense Resistor Selection

The current-sense resistor limits the peak switch cur-

rent to 210mV/R

SENSE

, where R

SENSE

is the value of

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.

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

= -4.5V.

MAX649/MAX651/MAX652

5V/3.3V/3V or Adjustable, High-Efficiency,

Low I

, Step-Down DC-DC Controllers

10 ______________________________________________________________________________________

3.0

2.5

MAXIMUM OUTPUT CURRENT (A)

2.0

MAX649-A26

INPUT VOLTAGE (V)

13 15

0.5

1.0

1.5

MAX651

OUT

= 3.3V

911

= 0.06Ω

= 0.07Ω

= 0.08Ω

= 0.10Ω

= 0.12Ω

= 0.14Ω

Figure 5b. MAX651 Current-Sense Resistor Graph

3.0

2.5

MAXIMUM OUTPUT CURRENT (A)

2.0

MAX649-A27

INPUT VOLTAGE (V)

13 15

0.5

1.0

1.5

MAX652

OUT

= 3.0V

911

= 0.06Ω

= 0.07Ω

= 0.08Ω

= 0.10Ω

= 0.12Ω

= 0.14Ω

Figure 5c. MAX652 Current-Sense Resistor Graph

3.0

2.0

2.5

MAXIMUM OUTPUT CURRENT (A)

916

MAX649-A25

INPUT VOLTAGE (V)

71210 11813

0.5

1.0

1.5

151445

MAX649

OUT

= 5V

= 0.06Ω

= 0.07Ω

= 0.08Ω

= 0.10Ω

= 0.12Ω

= 0.14Ω

Figure 5a. MAX649 Current-Sense Resistor Graph

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 16

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Rainbow-electronics MAX652 Manual de usuario Pagina 10

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