Design procedure – Rainbow Electronics MAX1708 User Manual
Page 11
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
______________________________________________________________________________________
11
__________________Design Procedure
Inductor Selection (L1)
The MAX1708’s high switching frequency allows the
use of a small-size inductor. Use a 2.2µH inductor for
600kHz operation. If the MAX1708 is synchronized at a
different frequency, scale the inductor value with the
inverse of frequency (L
1
= 2.2µH
✕
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 3.3µH for 350kHz and
1.5µH 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 MAX1708’s full
load current is needed (see Electrical Characteristics
for ratings). For highest efficiency, use a coil with low
DC resistance, preferably under 20m
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
V
OUT
= output voltage
D' = (V
IN
) / (V
OUT
+ V
D
), neglecting switch voltage
drop
f = switching frequency
L1 = inductor value
I
LIM
= minimum value of switch current limit from
Electrical Characteristics or set by R1 of
Figure 1.
I
D I
D
V
V
V
L
OUT MAX
LIM
OUT
D
IN
(
)
'
'
=
−
+
−
× ƒ ×
2
1
SUPPLIER
PHONE
FAX
Central
631-435-1110
631-435-1824
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
Table 4. Component Suppliers
Table 3. Component Selection Guide
PRODUCTION
INDUCTORS
CAPACITORS
DIODES
Coiltronics UP2B-2R2
Sanyo 6TPC100M
Motorola MBRD1035CTL
Surface mount
Coilcraft DO3316P-222HC
Panasonic EEFUE0J151R
Central CMSH5-20
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
MAX1708