Rainbow Electronics MAX1709 - Manual (Page 10)

MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
10 ______________________________________________________________________________________
where I 1 is the desired current limit in amperes, and
I
LIM
is the current limit value from the Electrical
Characteristics.
__________________Design Procedure
Inductor Selection (L1)
The MAX1709’s high switching frequency allows the
use of a small-size inductor. Use a 1.0µH inductor for
600kHz operation. If the MAX1709 will be synchronized
at a different frequency, scale the inductor value with
the inverse of frequency (L
1 = 1µH ✕ 600kHz / f SYNC ).
The PWM design tolerates inductor values within ±25%
of this calculated value, so choose the closest standard
inductor value. For example, use 1.5µH for 350kHz and
0.68µH for 1MHz).
Inductors with a ferrite core or equivalent are recom-
mended; powder iron cores are not recommended for
use at high switching frequencies. Ensure the induc-
tor’s saturation rating (the current at which the core
begins to saturate and inductance falls) exceeds the
internal current limit. Note that this current may be
reduced through SS/LIM if less than the MAX1709’s full
load current is needed (see Electrical Characteristics
for ratings). For highest efficiency, use a coil with low
DC resistance, preferably under 10m
Ω. To minimize
radiated noise, use a toroid, pot core, or shielded
inductor. See Tables 3 and 4 for a list of recommended
components and component suppliers. To calculate
the maximum output current (in amperes), use the fol-
lowing equation:
where:
V
IN = input voltage
V D = forward voltage drop of the Schottky diode at I LIM
current
V
OUT
= output voltage
D' = (V IN ) / (V OUT + V D ), assuming switch voltage drop
is negligible
f = switching frequency
L1 = inductor value
I D I D
V V V
L
OUT MAX LIM
OUT D IN
( )
' ' = −
+ −
× ƒ ×

 
 
 

2 1
SUPPLIER PHONE FAX
Coilcraft 847-639-6400 847-639-1489
Coiltronics 561-241-7876 561-241-9339
Motorola 602-303-5454 602-994-6430
Panasonic 714-373-7939 714-373-7183
STM-
Microelectronics
617-259-0300 617-259-9442
Table 4. Component Suppliers
Table 3. Component Selection Guide
PRODUCTION INDUCTORS CAPACITORS DIODES
Coilcraft DO3316P-102HC Panasonic EEFUE0J151R Motorola MBRD1035CTL
Surface mount
Coiltronics UP2B-1R0 Sanyo 6TPC100M
STM-Microelectronics
STPS8L30B
Figure 3. Simplified PWM Controller Functional Diagram
N
R
S
Q
LX
PGND
11m Ω
REF
SLOPE
COMP
FB
SS/LIM
12.5
OSCILLATOR
(LIMITED TO 100mV)
Figure 4. Adjustable Output Voltage
R4
R3
KEEP SHORT
FB
V IN
V OUT LX
MAX1709