Rainbow Electronics MAX1685 User Manual
Page 12
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
MAX1684
MAX1685
LX
AIN
IN
CVH
FB
L
C1
MBRS
130LT3
R1
R2
BOOT
PGND
AGND
CC
SHDN
STBY
0.1
µF
22
µF
0.1
µF
100
µF
-V
OUT
-1.25V
TO -5.5V
-V
OUT
= -1.25V
( )
V
IN
1
µF
0.1
µF
CVL
SYNC/PWM
REF
ILIM/SS
0.01
µF
0.01
µF
R2
+ 1
R1
V
IN
, MAX = 14V -
|
V
OUT
|
Figure 4. Inverting Output
Table 2. Inductor and Minimum Output Capacitor Selection
Table 3. Component Suppliers
22
220
MAX1684 (300kHz)
1.25 to 2.7
10
100
MAX1685 (600kHz)
L
(µH)
MIN C
OUT
(µF)
L
(µH)
MIN C
OUT
(µF)
V
OUT
(V)
22
100
10
47
2.7 to 4
47
68
22
33
4 to 6
68
47
33
22
6 to 14
capacitors have the lowest ESR of the high-value elec-
trolytic types. Use ceramic and OS-CON capacitors for
very compact, high-reliability, or wide-temperature
applications, 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 vari-
ous components 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.
12
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