Lo w -No is e , 1 4 V In p u t , 1 A, P WM
S t e p -Do w n Co n ve rt e rs
offer the lowest ESR overall. Sanyo OS-CON capacitors
ha ve the lowe s t ESR of the hig h-va lue e le c trolytic
types. Use ceramic and OS-CON capacitors for very
compact, high-reliability, or wide-temperature applica-
tions, where expense is justified. When using very low-
ESR capacitors, such as ceramic or OS-CON, check
for stability while examining load-transient response,
and increase the output compensation capacitor if
needed. Table 3 lists suppliers for the various compo-
nents used with the MAX1684/MAX1685.
whe re I
= 1A, AC Loa d Re g
1.4%, a nd
OUT MAX
f
= 300kHz (MAX1684) or 600kHz (MAX1685).
OSC
Ou t p u t Dio d e S e le c t io n
Use a 1A external Schottky diode (MBRS130LT3 or
equivalent) for the output rectifier to pass inductor cur-
rent during the start of the second half of each cycle.
This d iod e op e ra te s b e fore the inte rna l N-c ha nne l
MOSFET completely turns on and during high-current
operation. Use a Schottky diode to avoid forward bias-
ing the internal body diode of the N-channel MOSFET.
Ensure that the minimum capacitance value and maxi-
mum ESR values are met:
C
I
/ (V
· AC Load Reg · f
)
OUT > OUT MAX
OUT
OSC
OUT OUT MAX
R
< 2 · AC Load Reg · V
/I
ESR
Table 2. Inductor and Minimum Output Capacitor Selection
4/MAX1685
MAX1684 (300kHz)
MAX1685 (600kHz)
MIN C
V
(V)
OUT
L
(µH)
MIN C
L
(µH)
OUT
OUT
(µF)
220
100
68
(µF)
100
47
1.25 to 2.7
2.7 to 4
4 to 6
22
22
47
68
10
10
22
33
33
6 to 14
47
22
Table 3. Component Suppliers
SUPPLIER
CAPACITORS
AVX
PHONE
FAX
-V
-1.25V
TO -5.5V
OUT
MBRS
130LT3
0.1µF
MAX1684
MAX1685
AIN
IN
LX
FB
803-946-0690
714-969-2591
619-661-6835
603-224-1961
803-626-3123
714-960-6492
619-661-1055
603-224-1430
V
C1
IN
100µF
0.1µF
L
Matsuo
22µF
R1
R2
CVH
Sanyo
SHDN
Sprague
INDUCTORS
Sumida
STBY
CVL
BOOT
PGND
AGND
CC
SYNC/PWM
847-956-0666
847-639-6400
814-237-1431
847-956-0666
847-390-4373
847-956-0702
847-639-1469
814-238-0490
847-956-0702
847-390-4428
1µF
REF
Coilcraft
Murata-Erie
Sumida
0.1µF
ILIM/SS
0.01µF
0.01µF
TDK
R2
R1
DIODES
Motorola
+ 1
)
V , MAX = 14V - V
| OUT|
-V = -1.25V
(
IN
OUT
602-303-5454
602-994-6430
Figure 4. Inverting Output
12 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
