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Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
Thermal-Overload Protection
Thermal-overload protection limits the total power dissi-
pation in the MAX8513/MAX8514. When the junction
temperature exceeds T J = +170°C, a thermal sensor
shuts down the device, forcing DL and DH low and
value allows for a smaller inductor but results in higher
losses and higher output ripple. A good compromise
between size and efficiency is a 30% LIR. Once all of
the parameters are chosen, the inductor value is deter-
mined as follows:
allowing the IC to cool. The thermal sensor turns the part
on again after the junction temperature cools by 25°C,
resulting in a pulsed output during continuous thermal-
overload conditions. During a thermal event, the main
L =
V OUT 1 × ( V IN - V OUT 1 )
V IN × f S × I OUT 1 _ MAX × LIR
R 1 = R 2 × ? OUT 1 - 1 ?
step-down converter and the linear regulators are turned
off, POR and PFO go low, and soft-start is reset.
Design Procedure
OUT1 Voltage Setting
The output voltage is set by a resistive-divider network
from OUT1 to FB1 to GND (see R1 and R2 in the
Typical Applications Circuits ). Select R2 between 5k ?
and 15k ? . Then R1 can be calculated by:
? V ?
? 1 . 25 V ?
Input Power-Fail Setting
The PFI input can monitor V IN to determine if it is falling.
When the voltage at PFI crosses 1.22V, the output
( PFO ) goes low. The input voltage value at the PFI trip
where V OUT1 is the main switching regulator output and
f S is the switching frequency.
Choose a standard value close to the calculated value.
The exact inductor value is not critical and can be
adjusted to make tradeoffs between size, cost, and effi-
ciency. Lower inductor values minimize size and cost,
but also increase the output ripple and reduce the effi-
ciency due to higher peak currents. On the other hand,
higher inductor values increase efficiency, but eventual-
ly resistive losses due to extra turns of wire exceed the
benefit gained from lower AC current levels. 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 up to 300kHz. The chosen inductor’s satu-
ration current rating must exceed the peak inductor cur-
rent as calculated below:
threshold, V PFI , is set by a resistive-divider network
from IN to PFI to GND (see the Typical Applications
Circuits ). Select R11, the resistor from PFI to GND
I PEAK = I OUT 1 _ MAX +
( V IN - V OUT1 ) × V OUT 1
2 × L × f S × V IN
between 10k ? and 40k ? . Then R10, the resistor from
PFI to IN, is calculated by:
This peak value should be smaller than the value set at
ILIM when V OUT1 is at its nominal regulated voltage (see
R 10 = R 11 × ?
- 1 ?
? V PFI
? 1 . 22 V
?
?
the Current Limit and Current-Limit Setting sections).
In applications where a multiple winding inductor (cou-
pled inductor) is used to generate the supply voltages
Switching-Frequency Setting
The resistor connected from FREQ to GND, R FREQ (R7
in the Typical Applications Circuits ), sets the switching
frequency, f S , as shown by the equation below:
for the LDOs, the inductance value calculated above is
for the winding connected to the DC-DC step-down
(primary windings) inductance. The inductance seen
from the other windings (secondary windings) is pro-
portional to the square of the turns ratio with respect to
f S =
15 × 10 9
R FREQ
Hz × ?
the primary winding.
The turns ratio is important since it sets the LDOs’ sup-
ply voltage values. The voltage generated by the sec-
where R FREQ is in ohms.
ondary winding (V SEC ) together with the rectifier diode
and output capacitor is calculated as follows:
) × ? ? nn 2 ? ? - V D2
Inductor Value
There are several parameters that must be examined
when determining which inductor to use: input voltage,
output voltage, load current, switching frequency, and
V SEC = ( V OUT 1 + V Q 2
? ?
1
LIR. LIR is the ratio of peak-to-peak inductor ripple cur-
rent to the maximum DC load current. A higher LIR
where V Q2 and V D2 are the voltage drops across the
low-side MOSFET on the primary side and the rectifier
18
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