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MAX774CPA  15V or adjustable, highefficiency, low IQ inverting DCDC controller  Maxim Integrated 
MAX774CPA Datasheet PDF : 16 Pages

5V/12V/15V or Adjustable,
HighEfficiency, Low IQ Inverting DCDC 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 currentlimit 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, potcore, or shield
edbobbin 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 hightemperature applica
tions, Schottky diodes may be inadequate due to their
high leakage currents; instead, highspeed 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.
CurrentSense Resistor
The currentsense resistor limits the peak switch cur
rent to 210mV/RSENSE, where RSENSE is the value of
the currentsense resistor, and 210mV is the current
sense comparator threshold (see CurrentLimit 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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