Rainbow Electronics MAX1734 - Manual (Page 7)

GND (Figure 1). Choose R2 to be less than 50k
Ω:
where V REF = 1.25V.
Inductor Selection
The MAX1733/MAX1734 are optimized to use a 10µH
inductor over the entire operating range. A 300mA
rated inductor is enough to prevent saturation for out-
put currents up to 250mA. Saturation occurs when the
inductor’s magnetic flux density reaches the maximum
level the core can support and inductance falls.
Choose a low DC-resistance inductor to improve effi-
ciency. Tables 1 and 3 list some suggested inductors
and suppliers.
Capacitor Selection
The MAX1733/MAX1734 require output voltage ripple
(approximately 30mVp-p) for stable switching behavior.
Use a 10µF to 47µF tantalum output capacitor with
about 200m
Ω to 300mΩ ESR to provide stable switch-
ing while minimizing output ripple. Choose input and
output capacitors to filter inductor currents for accept-
able voltage ripple. The input capacitor reduces peak
currents and noise at the voltage source. Input capaci-
tors must meet the input ripple requirements and volt-
age rating. Use the following equation to calculate the
maximum RMS input current:
Tables 2 and 3 list some suggested capacitors and
suppliers.
Using Ceramic C OUT with MAX1733
The circuit of Figure 3 is designed to allow the use of
ceramic output capacitors with the MAX1733.
Feedback is derived from the LX pin instead of the out-
put to remove the effects of phase lag in the feedback
loop. Compared to the standard applications circuit,
there are three benefits: 1) availability of ceramic vs.
tantalum; 2) size of 2.2µF 0805 vs. 22µF A-case; 3) out-
put ripple less than 10mVp-p vs. greater than 30mVp-p.
Increase the output capacitance to 4.7µF to further
reduce the output ripple. Note that this circuit exhibits
load regulation equal to the series resistance of the
inductor multiplied by the load current. This small
amount of load regulation is helpful in reducing over-
shoot of the output voltage during load transients.
I
I V V OUT
V
RMS
OUT OUT IN
IN
=
−
( )




1 2 /
R R
V
V
OUT
REF
1 2 1
= × −






MAX1733/MAX1734
Low-Voltage, Step-Down DC-DC Converters
in SOT23
_______________________________________________________________________________________ 7
INDUCTOR VALUE
(µH)
MANUFACTURER
PART
NUMBER
CR43-100
10 Sumida
CDRH4D18-100
10 Coilcraft DT1608C-103
Table 1. Suggested Inductors
Table 2. Suggested Capacitors
CAPACITOR
TYPE
MANUFACTURER
PART
NUMBER
Tantalum
(22µF)
Taiyo Yuden LMK212BJ225MG
Ceramic
(2.2µF)
AVX TAJA226M006R
Table 3. Component Suppliers
COMPANY PHONE FAX
AVX 843-946-0238 843-626-3123
Coilcraft 847-639-6400 847-639-1469
Kemet 408-986-0424 408-986-1442
Murata 814-237-1431 814-238-0490
USA 847-956-0666 847-956-0702
Sumida
Japan 81-3-3607-5111 81-3-3607-5144
Taiyo Yuden 408-573-4150 408-573-4159
FB
30k
1000pF
68k
2.2 µF
X7R
OUTPUT
1.8V at 250mA 10 µH
LX IN
MAX1733
SHDN OFF ON
2.2 µF
X7R
GND
Li+
2.7V TO 4.2V
Figure 3. Using a Ceramic Output Capacitor with the MAX1733