Design procedure – Rainbow Electronics MAX8728 User Manual
Page 22
MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
22
______________________________________________________________________________________
Fault Protection
During steady-state operation, if any output of the four
regulators (step-down regulator, step-up regulator,
positive charge-pump regulator, and negative charge-
pump regulator) does not exceed its respective fault-
detection threshold, the MAX8728 activates an internal
fault timer. If any condition or the combination of condi-
tions indicates a continuous fault for the fault-timer
duration (50ms typ), the MAX8728 sets a fault latch. If
the fault is caused by the step-up regulator or one of
the charge pumps (LCD fault), the MAX8728 shuts
down all the outputs except VL, REF, and the step-
down regulator. Once the fault condition is removed,
toggle EN or
SHDN, or cycle the input voltage to clear
the LCD fault latch and restart the LCD supplies. If the
fault is caused by the step-down regulator, the
MAX8728 shuts down all the outputs except VL and
REF. Once the fault condition is removed, toggle
SHDN
or cycle the input voltage to clear the step-down fault
latch and restart the supplies.
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the MAX8728.
When the junction temperature exceeds T
J
= +160°C, a
thermal sensor immediately activates the fault protec-
tion, which shuts down all the outputs except the refer-
ence, allowing the device to cool down. Once the
device cools down by approximately 15°C, the
MAX8728 automatically restarts all the supplies.
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 Design
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 a 30% ripple-current to
load-current ratio (LIR = 0.3), which corresponds to a
peak inductor current 1.15 times the DC load current:
where I
OUT1(MAX)
is the maximum DC load current, and
the switching frequency f
SW
is 1.5MHz when FSEL is
tied to GND, 1MHz when FSEL is tied to V
CC
, and
500kHz when FSEL is tied to REF. 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 high-
er peak currents. On the other hand, higher inductor
values increase efficiency, but at some point resistive
losses due to extra turns of wire will 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 usually the best choice, espe-
cially at the higher frequency settings. Shielded-core
geometries help keep noise, EMI, and switching wave-
form jitter low.
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
OUT1
, which occurs at
V
IN
= 2 x V
OUT1
.
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 correspond-
ing to the maximum load current.
I
I
V
V
V
V
RMS
OUT
OUT
IN
OUT
IN
=
Ч
Ч
−
(
)
1
1
1
I
V
V
V
f
L
V
I
I
I
OUT
RIPPLE
OUT
IN
OUT
SW
OUT
IN
OUT
PEAK
OUT MAX
OUT
RIPPLE
1
1
1
1
1
1
1
2
_
_
(
)
_
=
Ч
−
(
)
Ч
Ч
=
+
L
V
V
V
V
f
I
LIR
OUT
OUT
IN
OUT
IN
SW
OUT MAX
1
1
1
1
(
)
=
Ч
−
(
)
Ч
Ч
Ч