Rainbow Electronics MAX1745 - Manual (Page 12)

MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controller in µMAX
12 ______________________________________________________________________________________
Diode Selection
The MAX1744/MAX1745’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817–1N5822 family or surface-mount equiva-
lents, are recommended. Ultra-high-speed rectifiers
with reverse recovery times around 50ns or faster
should be used for high output voltages, where the
increased forward drop causes less efficiency degra-
dation. Make sure that the diode’s peak current rating
exceeds the peak current limit set by R
SENSE , and that
its breakdown voltage exceeds V IN . Schottky diodes
are preferred for heavy loads due to their low forward
voltage, especially in low-voltage applications. For
high-temperature applications, some Schottky diodes
may be inadequate due to their high leakage currents.
In such cases, ultra-high-speed rectifiers are recom-
mended, although a Schottky diode with a higher
reverse voltage rating can often provide acceptable
performance.
Capacitor Selection
Choose filter capacitors to service input and output
peak currents with acceptable voltage ripple. ESR in
the output capacitor is a major contributor to output rip-
ple, so low-ESR capacitors are recommended. Low-
ESR tantalum, polymer, or ceramic capacitors are best.
Low-ESR aluminum electrolytic capacitors are tolera-
ble, but standard aluminum electrolytic capacitors are
not recommended.
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
For tantalum capacitors, the ripple is determined by the
ESR, but for ceramic capacitors, the ripple is mostly
due to the capacitance. Voltage ripple as a conse-
quence of ESR is approximated by:
The ripple due to the capacitance is approximately:
Estimate input and output capacitor values for given
voltage ripple as follows:
where I ∆L is the change in inductor current.
These equations are suitable for initial capacitor selec-
tion; final values should be set by testing a prototype or
evaluation kit. When using tantalum capacitors, use
good soldering practices to prevent excessive heat
from damaging the devices and increasing their ESR.
Also, ensure that the tantalum capacitors’ surge-current
ratings exceed the startup inrush and peak switching
currents.
Pursuing output ripple lower than the error compara-
tor’s hysteresis (0.6% of the output voltage) is not prac-
tical, since the MAX1744/MAX1745 will switch at slower
frequencies, increasing inductor ripple current thresh-
old. Choose an output capacitor with a working voltage
rating higher than the output voltage.
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
C
LI
V V
C
LI
V V
V
V V
IN
L
RIPPLE CIN IN
OUT
L
RIPPLE COUT OUT
IN
IN OUT
=
=
−

 

1
2
2
1
2
2
∆
∆
,
,
V
LI
2CV
RIPPLE,C
PEAK
O
≈
2
V
RIPPLE,ESR
≈
− ( ) R
ESR Ip p ∆
V
RIPPLE
≈ +
, ,
V V
RIPPLE ESR RIPPLE C
Table 1. Component Suppliers
COMPANY COUNTRY PHONE FAX
803-946-0690
or AVX USA
800-282-4975
803-626-3123
Coilcraft USA 847-639-6400 847-639-1469
Coiltronics USA 516-241-7876 516-241-9339
Dale/Vishay USA 402-564-3131 402-563-6418
Kemet USA 408-986-0424 408-986-1442
Inter nati onal
Recti fi er
USA 310-322-3331 310-322-3332
IRC USA 512-992-7900 512-992-3377
Motorola USA 602-303-5454 602-994-6430
Nichicon
USA
Japan
847- 843- 7500
81- 7- 5231- 8461
847-843-2798
81-7-5256-4158
Nihon
USA
Japan
805- 867- 2555
81- 3- 3494- 7411
805-867-2698
81-3-3494-7414
Sanyo
USA
Japan
619- 661- 6835
81- 7- 2070- 6306
619-661-1055
81-7-2070-1174
408-988-8000
or Siliconix USA
800-554-5565
408-970-3950
Sprague USA 603-224-1961 603-224-1430
Sumida
USA
Japan
847- 956- 0666
81- 3- 3607- 5111
847-956-0702
81-3-3607-5144
United
Chemi-Con
USA 714-255-9500 714-255-9400