Datasheet
MAX1709
4A, Low-Noise, High-Frequency,
Step-Up DC-DC Converter
10 ______________________________________________________________________________________
from its nominal value (see
Electrical Characteristics
). A
resistor (R1 in Figure 1) between SS/LIM and ground
reduces the current limit as follows:
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 10mΩ. 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 following
equation:
where:
V
IN
= input voltage
IDID
VVV
L
OUT MAX LIM
OUT D IN
()
''=−
+−
׃×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
21
Rk
I
I
Rk
LIM
1 312 5 1 312 5
1
. .=×≤
()
ΩΩ
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
11mΩ
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