Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
MAX1684/MAX1685
offer the lowest ESR overall. Sanyo OS-CON capacitors
have the lowest ESR of the high-value electrolytic
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.
Ensure that the minimum capacitance value and maxi-
mum ESR values are met:
C
OUT >
I
OUT MAX
/ (V
OUT
·
AC Load Reg
·
f
OSC
)
R
ESR
< 2
·
AC Load Reg
·
V
OUT
/I
OUT MAX
where I
OUT MAX
= 1A, AC Load Reg
≅
1.4%, and
f
OSC
= 300kHz (MAX1684) or 600kHz (MAX1685).
Output Diode Selection
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 diode operates before the internal N-channel
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.
Table 2. Inductor and Minimum Output Capacitor Selection
MAX1684 (300kHz)
V
OUT
(V)
1.25 to 2.7
2.7 to 4
4 to 6
6 to 14
L
(µH)
22
22
47
68
MIN C
OUT
(µF)
220
100
68
47
L
(µH)
10
10
22
33
MAX1685 (600kHz)
MIN C
OUT
(µF)
100
47
33
22
Table 3. Component Suppliers
SUPPLIER
CAPACITORS
AVX
Matsuo
Sanyo
Sprague
INDUCTORS
Sumida
Coilcraft
Murata-Erie
Sumida
TDK
DIODES
Motorola
12
602-303-5454
602-994-6430
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
V
IN
, MAX = 14V -
|
V
OUT
|
R2
-V
OUT
= -1.25V R1 + 1
0.01µF
1µF
0.1µF
PHONE
FAX
0.1µF
803-946-0690
714-969-2591
619-661-6835
603-224-1961
803-626-3123
714-960-6492
619-661-1055
603-224-1430
AIN
V
IN
22µF
0.1µF
IN
CVH
MAX1684
MAX1685
MBRS
130LT3
LX
C1
L R1
-V
OUT
-1.25V
TO -5.5V
100µF
SHDN
STBY
CVL
SYNC/PWM
REF
ILIM/SS
FB
R2
BOOT
PGND
AGND
CC
0.01µF
(
)
Figure 4. Inverting Output
______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
