MAX776CSA+ Maxim Integrated Products, MAX776CSA+ Datasheet - Page 12

• Manufacturer Part Number: MAX776CSA+
• Description: DC DC Cntrlr Single-OUT Inverting 3V to 16.5V Input 8-Pin SOIC N
• Manufacturer: Maxim Integrated Products
• Type: Invertingr

Specifications of MAX776CSA+

• Package: 8SOIC N
• Input Voltage: 3 to 16.5 V
• Output Voltage: -15 V
• Output Current: 0.4 A
• Number Of Outputs: 1
• Mounting Style: SMD/SMT
• Package / Case: SOIC-8 Narrow
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

inductor should not be so large that the peak current
never reaches the current limit. That is:
This is only important if
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 efficiency. Calculate the minimum inductor value
as follows:
Where L is in µH, 0.3µs is an ample time for the com-
parator response, I
Current-Sense Resistor section), and (I) is the allow-
able percentage 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 30m or lower resistance are available. To min-
imize radiated noise, use a torroid, pot-core, or shielded-
bobbin inductor. Inductors with a ferrite core or equivalent
-5V/-12V/-15V or Adjustable, High-Efficiency,
Low I
Figure 6. MAX774 Maximum Output Current vs. Input Voltage
(V
12
OUT
______________________________________________________________________________________
L(min)
= -5V)
L(max)
2500
2000
1500
1000
500
0
Q
3 4
_______ < — = ___________
V
_______________________________
V
V
OUT
[
OUT
V+(max) - V
Inverting DC-to-DC Controllers
IN
_______________________________
R
[
SENSE
= -5V
5 6 7
V+(min) - V
R
LIM
SENSE
= 0.06
INPUT VOLTAGE (V)
(I) x I
= 0.05
is the current limit (see the
1
6
8
I
R
SW
LIM
9 10 11 12 13 14 15
SENSE
LIM
SW
R
(min)
(max)
SENSE
t
t
(min)
R
= 0.09
ON
OFF
(max)
SENSE
= 0.08
(max)
]
(min)
= 0.07
x 0.3µs
]
x 12µs
are recommended. Make sure that the inductor’s satura-
tion current rating is greater than I
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 I
than V
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 tempera-
ture, the benefits of a Schottky diode’s low forward volt-
age may outweigh the disadvantages of its high leak-
age current.
The current-sense resistor limits the peak switch cur-
rent to 210mV/R
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. See Figures
6–9 to determine the sense resistor, as well as the peak
current, for the required load current.
Figure 7. MAX775 Maximum Output Current vs. Input Voltage
(V
OUT
= -12V)
IN
(max) + V
1000
800
600
400
200
0
3
R
LIM
SENSE
SENSE
R
SENSE
R
OUT
4
SENSE
= 0.07
(max) and a voltage rating higher
= 0.06
. For high-temperature applica-
, where R
= 0.05
INPUT VOLTAGE (V)
5
Current-Sense Resistor
6
R
SENSE
R
SENSE
LIM
SENSE
7
= 0.09
Diode Selection
V
= 0.08
(max).
OUT
= -12V
8
is the value of
9