MAX5025(2001) Ver la hoja de datos (PDF) - Maxim Integrated
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MAX5025 Datasheet PDF : 13 Pages
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500kHz, 36V Output, SOT23, PWM Step-Up
DC-DC Converters
ULVO
OSCILLATOR
THERMAL
SHUTDOWN
3 FB
REF
2 GND
5 VCC
MAIN PWM
COMPARATOR
-A
+A
CONTROL
LOGIC
+C
-C
CURRENT-
LIMIT
COMPARATOR
SHDN 4
LX 6
N
PGND 1
MAX5025
MAX5026
Figure 1. Functional Diagram
R1
=
R2
VOUT
VREF
-1
where VREF is 1.25V
Determining Peak Inductor Current
If the boost converter remains in the discontinuous
mode of operation, then the approximate peak inductor
current, ILPEAK, is represented by the formula below:
( ) ILPEAK =
2 TS VOUT − VIN IOUT
ηL
where TS is the period, VOUT is the output voltage, VIN
is the input voltage, IOUT is the output current, and η is
the efficiency of the boost converter.
Determining the Inductor Value
47µH is the recommended inductor value when the out-
put voltage is 30V and the input voltage is 5V. In gener-
al, the inductor should have a current rating greater
than the current-limit value. For example, the inductor’s
current rating should be greater than 150mA to support
a 4mA output current. Equivalent series resistance
(ESR) should be below 1Ω for reasonable efficiency.
Due to the MAX5025–MAX5028’s high switching fre-
quency, inductors with a ferrite core or equivalent are
recommended. Powdered iron cores are not recom-
mended due to their high losses at frequencies over
500kHz. Table 1 shows a list of vendors and 47µH
inductor parts.
For 4mA output current and output voltages other than
30V, the inductor can be simply scaled in value
according to the following formula:
L
=
(47µH) (VOUT − ) VIN
(25V)
Use the following formula to calculate the upper bound
of the inductor value at different output voltages and
output currents. This is the maximum inductance value
for discontinuous mode operation.
( ) LUPPER
=
V2
IN
VOUT − VIN TSη
2 IOUT VO2UT
Calculate the lower bound, LLOWER, for the acceptable
inductance value using the following formula, which will
allow the maximum output current to be delivered with-
out reaching the peak current limit:
( ( )) LLOWER
=
2 TS VOUT − VIN IOUT
η
VIN
5
260mA
2
Notice that the switch current limit, (VIN/5)(260mA), is a
function of the input voltage, VIN. The current rating of
the inductor should be greater than the switch current
limit.
Table 1. Inductor Vendors
VENDOR
Coilcraft
Sumida
Toko
PHONE
847-639-6400
847-545-6700
847-297-0070
FAX
847-639-1469
847-545-6720
847-699-7864
PART NUMBER OF 47µH INDUCTOR
DT1608C-473
CDRH4D28-470
A915BY-470M
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