MAX774CPA View Datasheet(PDF) - Maxim Integrated

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MAX774CPA

-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers

Maxim Integrated 

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

High-Efficiency, Low IQ Inverting DC-DC Controllers

2500

RSENSE = 0.05Ω

2000

RSENSE = 0.06Ω

1500

1000

RSENSE = 0.07Ω

RSENSE = 0.08Ω

500

RSENSE = 0.09Ω

VOUT = -5V

0

3 4 5 6 7 8 9 10 11 12 13 14 15

INPUT VOLTAGE (V)

Figure 6. MAX774 Maximum Output Current vs. Input Voltage

(VOUT = -5V)

1000

800

RSENSE = 0.05Ω

RSENSE = 0.06Ω

600 RSENSE = 0.07Ω

VOUT = -12V

400

200

RSENSE = 0.08Ω

RSENSE = 0.09Ω

0

3

4

5

6

7

8

9

INPUT VOLTAGE (V)

Figure 7. MAX775 Maximum Output Current vs. Input Voltage

(VOUT = -12V)

Choosing an Inductor

Practical inductor values range from 10µH to 50µH.

The maximum inductor value is not particularly critical.

For highest current at high VOUT to V+ ratios, the

inductor should not be so large that the peak current

never reaches the current limit. That is:

L(max) ≤ __[_V_+_(_m__i_n_)_-__V_S_W__(_m_a__x_)_]_x__1_2_µ__s_

ILIM(max)

This is only important if

__V_IN____

<

1

—

=

_t_O__F_F_(m__i_n_)_

VOUT

6 tON(max)

More important is that the inductor not be so small that the

current rises much faster than the current-limit comparator

can respond. This would be wasteful and reduce effi-

ciency. Calculate the minimum inductor value as follows:

L(min) ≥ _[_V__+_(_m__a_x_)_-__V_S_W__(_m__in__)]__x_0__.3_µ__s_

δ(I) x ILIM(min)

Where L is in µH, 0.3µs is an ample time for the com-

parator response, ILIM is the current limit (see Current-

Sense Resistor section), and δ(I) is the allowable per-

centage of overshoot. As an example, Figure 2's circuit

uses a 3A peak current. If we allow a 15% overshoot

and 15V is the maximum input voltage, then L(min) is

16µH. The actual value of L above this limit has minimal

effect on this circuit's operation.

For highest efficiency, use a coil with low DC resistance.

Coils with 30mΩ or lower resistance are available. To

minimize radiated noise, use a torroid, pot-core, or shield-

ed-bobbin inductor. Inductors with a ferrite core or equiv-

alent are recommended. Make sure that the inductor’s

saturation current rating is greater than ILIM(max).

Diode Selection

The ICs’ high switching frequencies demand a high-

speed rectifier. Schottky diodes such as the 1N5817 to

1N5822 families are recommended. Choose a diode

with an average current rating approximately equal to

or greater than ILIM (max) and a voltage rating higher

than VIN(max) + VOUT. For high-temperature applica-

tions, Schottky diodes may be inadequate due to their

high leakage currents; instead, high-speed silicon

diodes may be used. At heavy loads and high temper-

ature, the benefits of a Schottky diode’s low forward

voltage may outweigh the disadvantages of its high

leakage current.

Current-Sense Resistor

The current-sense resistor limits the peak switch cur-

rent to 210mV/RSENSE, where RSENSE is the value of

the current-sense resistor, and 210mV is the current-

sense comparator threshold (see Current-Limit Trip

Level in the Electrical Characteristics).

To maximize efficiency and reduce the size and cost of

external components, minimize the peak current.

However, since the output current is a function of the

peak current, do not set the limit too low. Refer to

Figures 6–9 to determine the sense resistor (and, there-

fore, peak current) for the required load current.

12 ______________________________________________________________________________________

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