MAX770CSLASHD datasheet(15/20 Pages) MAXIM

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Determining RSENSE

The

Typical Operating Characteristics graphs show the

output current capability for various modes, sense

resistors, and input/output voltages. Use these graphs,

along with the theoretical output current curves shown

in Figures 7a-7d, to select RSENSE. These theoretical

curves assume that an external N-FET power switch is

used. They were derived using the minimum (worst-

case) current-limit comparator threshold value, and the

inductance value. No tolerance was included for

RSENSE. The voltage drop across the diode was

assumed to be 0.5V, and the drop across the power

switch rDS(ON) and coil resistance was assumed to be

0.3V. To use the graphs, locate the graph with the

appropriate output voltage or the graph having the

nearest output voltage higher than the desired output

voltage. On this graph, find the curve for the largest

sense-resistor value with an output current that is ade-

quate at the lowest input voltage.

Determining the Inductor (L)

Practical inductor values range from 10µH to 300µH.

20µH is a good choice for most applications. In appli-

cations with large input/output differentials, the IC’s

output current capability will be much less when the

inductance value is too low, because the IC will always

operate in discontinuous mode. If the inductor value

is too low, the current will ramp up to a high level

before the current-limit comparator can turn off the

switch. The minimum on-time for the switch (tON(min))

is approximately 2µs; select an inductor that allows

the current to ramp up to ILIM/2 in no less than 2µs.

Choosing a value of ILIM/2 allows the half-size current

pulses to occur, increasing light-load efficiency and

minimizing output ripple.

5V/12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controllers

______________________________________________________________________________________

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

RSENSE = 40mΩ

RSENSE = 50mΩ

RSENSE = 75mΩ

RSENSE = 200mΩ

RSENSE = 100mΩ

VOUT = 5V

L = 22

µH

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

RSENSE = 200mΩ

RSENSE = 100mΩ

RSENSE = 40mΩ

RSENSE = 50mΩ

RSENSE = 75mΩ

VOUT = 12V

L = 22

µH

Figure 7a. Maximum Output Current vs. Input Voltage

(VOUT = 5V)

Figure 7b. Maximum Output Current vs. Input Voltage

(VOUT = 12V)

Figure 7c. Maximum Output Current vs. Input Voltage

(VOUT = 15V)

Figure 7d. Maximum Output Current vs. Input Voltage

(VOUT = 24V)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

RSENSE = 200mΩ

RSENSE = 100mΩ

VOUT = 15V

L = 22

µH

RSENSE = 40mΩ

RSENSE = 50mΩ

RSENSE = 75mΩ

INPUT VOLTAGE (V)

0.8

0.2

0.4

0.6

VOUT = 24V

L =150

µH

RSENSE = 100mΩ

RSENSE = 200mΩ

RSENSE = 400mΩ

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