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參數(shù)資料
| 型號(hào)： | MAX746MJE |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Parallel-Load 8-Bit Shift Registers 16-CDIP -55 to 125 |
| 中文描述： | SWITCHING CONTROLLER, CDIP16 |
| 封裝： | CERDIP-16 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大?。?/td> | 157K |
| 代理商： | MAX746MJE |
M
High-Effic ienc y, PWM, S tep-Down,
N-Channel DC-DC Controller
12
______________________________________________________________________________________
Next, determine the value of R
SENSE
such that:
LIMIT(min)
R
SENSE
= _V
125mV
I
PK
I
PK
For example, to obtain 5V at 3A, I
PK
= 3.3A and
R
SENSE
= 125mV/3.3A = 38m
.
The sense resistor should have a power rating greater
than (I
PK2)
(R
SENSE
) with an adequate safety margin.
With a 3A load current, I
PK
= 3.3A and R
SENSE
= 38m
.
The power dissipated by the resistor (assuming an 80%
duty cycle) is 331mW. Metal-film resistors are recom-
mended. Do not use wire-wound resistors because
their inductance will adversely affect circuit operation.
The duty cycle (for continuous conduction) is determined
from the following equation:
OUT
+ V
DIODE
Duty Cycle (%) = ___V
V+ - V
SW
+ V
DIODE
where V
SW
is the voltage drop across the external
N-FET and sense resistor. V
SW
can be approximated
as [I
LOAD
x (r
DS(ON)
+ R
SENSE
)].
Induc tor S elec tion
Once the sense-resistor value is determined, calculate
the inductor value (L) using the following equation. The
correct inductor value ensures proper slope compen-
sation. Continuing from the equations above:
(
SENSE
) (
V
OUT
)
L = ____R
(
V
RAMP(max)
) (
f
OSC
)
(
) (
5V
)
(
50mV
) (
100kHz
)
= _____38m
where V
RAMP(max)
is the 50mV peak value of the slope-
compensation linear ramp signal.
Although 38μH is the calculated value, the component
used may have a tolerance of ±30% or more.
Inductors with molypermalloy powder (MPP), Kool Mμ,
or ferrite are recommended. Inexpensive iron-powder
core inductors are not suitable, due to their increased
core losses, especially at switching frequencies in the
100kHz range. MPP and Kool Mμ cores have low per-
meability, allowing larger currents.
For highest efficiency, use a coil with low DC resis-
tance. To minimize radiated noise, use a toroid, a pot
core, or a shielded coil.
It is customary to select an inductor with a saturation
rating that exceeds the peak current set by R
SENSE
,
but inductors are often specified very conservatively.
If the inductor’s core losses do not cause excessive
temperature rise (inductor wire insulation is usually
rated for +125°C) and the associated efficiency loss-
es are minimal, inductors with lower current ratings
are acceptable.
In the 3.3V Standard Application Circuit (Figure 1b), the
inductor selected has a 2.2A current rating even
though the peak current is 3.3A. This inductor was
selected for two reasons: it is the highest-rated readily
available surface-mount inductor of its size, and lab
tests have verified that the core-loss increase is mini-
mal. With a 3A load current, the inductor current does
not begin showing significant losses due to saturation
until the supply voltage increases to 10V (the maximum
supply for this circuit is 6V).
GND
V+
FB
15
6
16
V
IN
V
OUT
R4 = 10k
TO 60k
OUT
9
R5
R4
C7
*
R5 = R4 V
V
REF
= 2.0V NOMINAL
*
SEE COMPENSATIONCAPACITORSECTION.
OUT
V
REF
MAX746
EXT
FB
OUT
D1
26.1k (1%)
V
OUT
L
C1
N
12
6
9
R5
C7
R4b
22.6k (1%)
N
R4a
17.4k (1%)
5V/3.3V
V
OUT
= VREF R5
R5
MAX746
SELECT WITH FET OFF:
V
OUT
= VREF +1
SELECT WITH FET OFF:
V
REF
= 2.0V NOMINAL
Figure 6. Adjustable Output Circuit
Figure 7. 3.3V/5V Ajustable Output Circuit
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