Datasheet

MAX17113
Low-Cost, Multiple-Output
Power Supply for LCD TVs
24 ______________________________________________________________________________________
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the MAX17113.
When the junction temperature exceeds T
J
= +160°C, a
thermal sensor immediately activates the fault protec-
tion that shuts down all the outputs except the refer-
ence and latches off, allowing the device to cool down.
Once the device cools down by at least approximately
15°C, cycle the input voltage to clear the fault latch and
restart the MAX17113.
The thermal-overload protection protects the controller
in the event of fault conditions. For continuous opera-
tion, do not exceed the absolute maximum junction
temperature rating of T
J
= +150°C.
Design Procedure
Step-Down Regulator
Inductor Selection
Three key inductor parameters must be specified:
inductance value (L), peak current (I
PEAK
), and DC
resistance (R
DC
). The following equation includes a
constant, LIR, which is the ratio of peak-to-peak induc-
tor ripple current to DC load current. A higher LIR value
allows smaller inductance, but results in higher losses
and higher ripple. A good compromise between size
and losses is typically found at about 20% to 50% rip-
ple-current to load-current ratio (LIR):
where I
OUT(MAX)
is the maximum DC load current, and
the switching frequency f
SW
is 600kHz when FSEL is
connected to VL or 450kHz when FSEL is connected to
AGND. 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 inductor values increase effi-
ciency, but at some point resistive losses due to extra
turns of wire exceed the benefit gained from lower AC
current levels.
The inductor’s saturation current must exceed the peak
inductor current. The peak current can be calculated by:
The inductor’s DC resistance should be low for good
efficiency. Find a low-loss inductor having the lowest
possible DC resistance that fits in the allotted dimen-
sions. Ferrite cores are often the best choice. Shielded-
core geometries help keep noise, EMI, and switching
waveform jitter low.
Considering the typical operating circuit in Figure 1, the
maximum load current (I
OUT(MAX)
) is 2A with a 3.3V
output and a typical 12V input voltage. Choosing an
LIR of 0.4 at this operating point:
At that operating point, the ripple current and the peak
current are:
Input Capacitors
The input filter capacitors reduce peak currents drawn
from the power source and reduce noise and voltage
ripple on the input caused by the regulator’s switching.
They are usually selected according to input ripple cur-
rent requirements and voltage rating, rather than
capacitance value. The input voltage and load current
determine the RMS input ripple current (I
RMS
):
The worst case is I
RMS
= 0.5 x I
OUT
, which occurs at
V
IN2
= 2 x V
OUT
.
For most applications, ceramic capacitors are used
because of their high ripple current and surge current
capabilities. For optimal circuit long-term reliability,
choose an input capacitor that exhibits less than +10°C
temperature rise at the RMS input current corresponding
to the maximum load current.
Output Capacitor Selection
Since the MAX17113’s step-down regulator is internally
compensated, it is stable with any reasonable amount
of output capacitance. However, the actual capacitance
and equivalent series resistance (ESR) affect the regu-
lator’s output ripple voltage and transient response. The
rest of this section deals with how to determine the out-
put capacitance and ESR needs according to the
ripple-voltage and load-transient requirements.
II
VVV
V
RMS OUT
OUT IN OUT
IN
=×
×
()
2
2
-
IA
A
A
OUT PEAK_
.
.=+ =2
08
2
24
I
VVV
kHz μH
OUT RIPPLE_
..
.
=
×
()
××
≈
33 12 33
600 5 0 12
-
008.A
L
VVV
VkHzA
μH
OUT
=
×
()
×××
≈
33 12 33
12 600 2 0 4
50
..
.
.
-
II
I
OUT PEAK OUT MAX
OUT RIPPLE
_()
_
=+
2
I
VVV
fL V
OUT RIPPLE
OUT IN OUT
SW OUT IN
_
=
×
()
××
2
2
-
L
VVV
VfI LIR
OUT
OUT IN OUT
IN SW OUT MAX
=
×
()
×× ×
2
2
-
()