Datasheet

MAX8543/MAX8544
Step-Down Controllers with Prebias Startup,
Lossless Sensing, Synchronization, and OVP
______________________________________________________________________________________ 19
A good compromise between size and efficiency is an
LIR of 0.3. Once all the parameters are chosen, the
inductor value is determined as follows:
where f
S
is the switching frequency. Choose a standard-
value inductor close to the calculated value. The exact
inductor value is not critical and can be adjusted to
make trade-offs among size, cost, and efficiency. Lower
inductor values minimize size and cost, but they also
increase the output ripple and reduce the efficiency due
to higher peak currents. On the other hand, higher induc-
tor values increase efficiency, but eventually resistive
losses due to extra turns of wire exceed the benefit
gained from lower AC current levels. This is especially
true if the inductance is increased without also increas-
ing the physical size of the inductor. Find a low-loss
inductor with the lowest possible DC resistance that fits
the allotted dimensions. Ferrite cores are often the best
choice, although powdered iron is inexpensive and can
work well at 300kHz. The chosen inductor’s saturation
current rating must exceed the peak inductor current
determined as:
Setting the Current Limits
Valley Current Limit
The valley current limit employs a current foldback
scheme. The MAX8543 has a fixed valley current-limit
threshold of 130mV, and a fixed foldback ratio (P
FB
) of
23%. The foldback ratio is the current-limit threshold
when the output is at 0V (output shorted to ground),
divided by the threshold when the output is at its nominal
regulated value. Thus, the minimum output current limit
(I
LIM
) and maximum short-circuit current (I
SC
) is calculat-
ed as:
where R
DS(ON)
is the maximum on-resistance of the
low-side MOSFET at the highest expected operating
junction temperature, and I
P-P
is the inductor ripple cur-
rent, calculated as:
Ensure that I
LIM
is equal to or greater than the maxi-
mum load current at peak current limit (see the Peak
Current Limit section):
where 40mV is the maximum current-limit threshold
when the output is shorted (V
OUT
= 0V).
The MAX8544 has an adjustable valley current limit and
can be selected for foldback with automatic recovery,
or constant current with latch-up. To set the current limit
for foldback mode, connect a resistor from ILIM2 to the
output (R
FOBK
), and another resistor from ILIM2 to
GND (R
ILIM
). See Figure 6. The values of R
FOBK
and
R
ILIM
are calculated as follows:
1) First, select the percentage of foldback (P
FB
). This
percentage corresponds to the current limit when
V
OUT
equals zero, divided by the current limit when
V
OUT
equals a nominal voltage. A typical value of
P
FB
is in the range of 15% to 40%. A lower value of
P
FB
yields lower short-circuit current. The following
equations are used to calculate R
FOBK
and R
ILIM:
where I
VALLEY
is the value of the inductor valley
current at maximum load (I
LOAD(MAX)
- 1/2 I
P-P
),
and R
DS(ON)
is the maximum on-resistance of the
low-side MOSFET at the highest operating junction
temperature.
R
RI PR
VRI P
ILIM
DS ON VALLEY FB FOBK
OUT DS ON VALLEY FB
=
×××−
()
×
−× × ×−
()
[]
51
51
()
()
R
PV
AP
FOBK
FB OUT
FB
=
×
×−
()
51μ
I
V
R
I
SC
DS ON
PP
=+
−
004
2
.
()
I
VV V
fLV
PP
IN OUT OUT
SIN
−
=
−
()
×
××
I
V
R
I
LIM
DS ON
PP
=+
−
011
2
.
()
II
LIR
I
PEAK LOAD MAX LOAD MAX
=+×
() ()
2
L
VVV
V f I LIR
OUT IN OUT
IN S LOAD MAX
=
×−
×× ×
()
()
MAX8544
R
FOBK
R
ILIM
LX
OUT
ILIM2
Figure 6. ILIM2 Resistor Connections