MAX746 Maxim, MAX746 Datasheet - Page 12

• Manufacturer Part Number: MAX746
• Description: High-Efficiency / PWM / Step-Down / N-Channel DC-DC Controller
• Manufacturer: Maxim

High-Efficiency, PWM, Step-Down,
N-Channel DC-DC Controller
Figure 6. Adjustable Output Circuit
Next, determine the value of R
For example, to obtain 5V at 3A, I
R
The sense resistor should have a power rating greater
than (I
With a 3A load current, I
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:
where V
N-FET and sense resistor. V
as [I
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:
12
SENSE
*
Duty Cycle (%) = _____________________ x 100%
LOAD
SEE COMPENSATION CAPACITOR SECTION.
______________________________________________________________________________________
PK 2)
= 125mV/3.3A = 38mΩ.
SW
R
x (r
SENSE
(R
is the voltage drop across the external
DS(ON)
L = ______________________
SENSE
MAX746
15
16
= _____________________ = 38µH
V
GND
V+
IN
= _____________ = ________
(
) with an adequate safety margin.
+ R
V
V
(
(
RAMP(max)
50mV
V+ - V
R
LIMIT(min)
OUT
(
PK
SENSE
FB
V
SENSE
38mΩ
I
PK
OUT
= 3.3A and R
6
9
) (
SW
SW
SENSE
) (
100kHz
+ V
)].
) (
Inductor Selection
+ V
C7
V
) (
5V
can be approximated
DIODE
OUT
*
R4 = 10k
R5 = R4
V
f
)
REF
OSC
DIODE
such that:
)
= 2.0V NOMINAL
)
125mV
PK
)
R4
(
SENSE
I
PK
V
R5
V
TO 60k
OUT
REF
= 3.3A and
-1
)
= 38mΩ.
V
OUT
Figure 7. 3.3V/5V Ajustable Output Circuit
where V
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
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).
SELECT WITH FET OFF:
SELECT WITH FET OFF:
RAMP(max)
V
V
V
OUT
OUT
REF
= 2.0V NOMINAL
MAX746
= V REF
= V REF
(
(
R4a
R4a + R4b
R5
is the 50mV peak value of the slope-
OUT
EXT
FB
R5
+1
)
12
6
9
+1
)
5V/3.3V
N
D1
26.1k (1%)
17.4k (1%)
L
R4a
R5
N
R4b
22.6k (1%)
V
C1
C7
OUT
SENSE
,