Rainbow Electronics MAX1627 - Manual (Page 11)

Diode Selection
The MAX1626/MAX1627’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, such
as the MUR series, are acceptable. 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+. Schottky diodes are preferred for heavy
loads due to their low forward voltage, especially in
low-voltage applications. For high-temperature applica-
tions, some Schottky diodes may be inadequate due to
their high leakage currents. In such cases, ultra-high-
speed rectifiers are recommended, 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.
Equivalent series resistance (ESR) in the capacitor is a
major contributor to output ripple, so low-ESR capaci-
tors are recommended. Sanyo OS-CON capacitors are
best, and low-ESR tantalum capacitors are second
best. Low-ESR aluminum electrolytic capacitors are tol-
erable, but do not use standard aluminum electrolytic
capacitors.
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
To simplify selection, assume initially that two-thirds of
the ripple results from ESR and one-third results from
capacitor value. Voltage ripple as a consequence of
ESR is approximated by:
Estimate input and output capacitor values for given
voltage ripple as follows:
where I ∆ L is the change in inductor current (around
0.5I PEAK under moderate loads).
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 start-up inrush and peak switching
currents.
Pursuing output ripple lower than the error compara-
tor’s hysteresis (0.5% of the output voltage) is not prac-
tical, since the MAX1626/MAX1627 will switch as
needed, until the output voltage crosses the hysteresis
threshold. 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
ripple on V+ and CS, caused by the circuit’s switching
action. Use a low-ESR capacitor. Two smaller-value
low-ESR capacitors can be connected in parallel for
lower cost. Choose input capacitors with working volt-
age ratings higher than the maximum input voltage.
Place a surface-mount ceramic capacitor very close to
V+ and GND, as shown in Figure 7. This capacitor
bypasses the MAX1626/MAX1627, and prevents spikes
and ringing on the power source from obscuring the
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
RIPPLE,ESR
≈ ( )( ) R
ESR IPEAK
V
RIPPLE
≈ +
, ,
V V
RIPPLE ESR RIPPLE C
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
______________________________________________________________________________________ 11
Table 2. Component Suppliers
COMPANY PHONE FAX
(803) 946-0690
AVX USA or (803) 626-3123
(800) 282-4975
Coilcraft USA (847) 639-6400 (847) 639-1469
Coiltronics USA (516) 241-7876 (516) 241-9339
Dale USA (605) 668-4131 (605) 665-1627
International
USA (310) 322-3331 (310) 322-3332
Rectifier
IRC USA (512) 992-7900 (512) 992-3377
Motorola USA (602) 303-5454 (602) 994-6430
Nichicon USA (847) 843-7500 (847) 843-2798
Japan 81-7-5231-8461 81-7-5256-4158
Nihon USA (805) 867-2555 (805) 867-2698
Japan 81-3-3494-7411 81-3-3494-7414
Sanyo USA (619) 661-6835 (619) 661-1055
Japan 81-7-2070-6306 81-7-2070-1174
(408) 988-8000
Siliconix USA or (408) 970-3950
(800) 554-5565
Sprague USA (603) 224-1961 (603) 224-1430
Sumida USA (847) 956-0666 (847) 956-0702
Japan 81-3-3607-5111 81-3-3607-5144
United
USA (714) 255-9500 (714) 255-9400
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