5 V/3 .3 V o r Ad ju s t a b le , 1 0 0 % Du t y-Cyc le ,
Hig h -Effic ie n c y, S t e p -Do w n DC-DC Co n t ro lle rs
/MAX1627
best, and low-ESR tantalum capacitors are second
Table 2. Component Suppliers
best. Low-ESR aluminum electrolytic capacitors are tol-
erable, but do not use standard aluminum electrolytic
capacitors.
COMPANY
PHONE
FAX
(803) 946-0690
or
(800) 282-4975
AVX
USA
(803) 626-3123
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
Coilcraft
Coiltronics
Dale
USA
USA
USA
(847) 639-6400
(516) 241-7876
(605) 668-4131
(847) 639-1469
(516) 241-9339
(605) 665-1627
V
≈
V
+ V
RIPPLE
RIPPLE,ESR RIPPLE,C
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:
International
Rectifier
USA
(310) 322-3331
(310) 322-3332
IRC
USA
USA
(512) 992-7900
(602) 303-5454
(512) 992-3377
(602) 994-6430
Motorola
Nichicon
V
≈ (R
)(I )
RIPPLE,ESR
ESR PEAK
USA
Japan
(847) 843-7500
81-7-5231-8461
(847) 843-2798
81-7-5256-4158
Estimate input and output capacitor values for given
voltage ripple as follows:
Nihon
Sanyo
USA
Japan
(805) 867-2555
81-3-3494-7411
(805) 867-2698
81-3-3494-7414
2
1
2
LI
USA
Japan
(619) 661-6835
81-7-2070-6306
(619) 661-1055
81-7-2070-1174
∆L
C
C
=
IN
V
V
RIPPLE,CIN IN
(408) 988-8000
or
(800) 554-5565
2
∆L
1
2
Siliconix
USA
USA
(408) 970-3950
LI
V
IN
=
OUT
V
V
V − V
IN OUT
RIPPLE,COUT OUT
Sprague
Sumida
(603) 224-1961
(603) 224-1430
USA
Japan
(847) 956-0666
81-3-3607-5111
(847) 956-0702
81-3-3607-5144
where I is the change in inductor current (around
∆L
0.5I
under moderate loads).
PEAK
United
Chemi-Con
USA
(714) 255-9500
(714) 255-9400
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.
Dio d e S e le c t io n
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
Pursuing output ripple lower than the error compara -
tor’s hysteresis (0.5% of the output voltage) is not prac-
tic a l, s inc e the MAX1626/MAX1627 will s witc h a s
needed, until the output voltage crosses the hysteresis
threshold. Choose an output capacitor with a working
voltage rating higher than the output voltage.
limit set by R
, and that its breakdown voltage
SENSE
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.
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.
Ca p a c it o r S e le c t io n
Choose filter capacitors to service input and output
p e a k c urre nts with a c c e p ta b le volta g e rip p le .
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
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
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
