-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 ______________________________________________________________________________________