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Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
V RIPPLE ( C ) =
V RIPPLE ( ESL ) =
and I P - P = ? IN OUT 1 ? ? OUT 1 ?
V IN ? ?
P Q 2 CC = ? 1 - × I OUT 1 2 × R DS _ ON
The peak-to-peak output voltage ripple as a conse-
quence of the ESR, ESL, and output capacitance is:
V RIPPLE ( ESR ) = I P - P × R ESR
I P - P
8 × C OUT × f S
where C OUT is C4 in the Typical Applications Circuits .
V IN × ESL
L 1 A + ESL
? V - V ? ? V ?
? f S L ? ? V IN ?
where I P-P is the peak-to-peak inductor current (see the
Inductor Selection section). An approximation of the
overall voltage ripple at the output is:
V RIPPLE = V RIPPLE ( C ) + V RIPPLE ( ESR ) + V RIPPLE ( ESL )
While these equations are suitable for initial capacitor
selection to meet the ripple requirement, final values may
also depend on the relationship between the LC double-
pole frequency and the capacitor ESR zero. Generally,
the ESR zero is higher than the LC double pole (see the
Compensation Design section). Solid polymer electrolyt-
ic or ceramic capacitors are recommended due to their
low ESR and ESL at higher frequencies. Higher output
current may require paralleling multiple capacitors to
meet the output voltage ripple.
The MAX8513/MAX8514s’ response to a load transient
depends on the selected output capacitor. After a load
transient, the output instantly changes by (ESR ×
? I OUT1 ) + (ESL × dI OUT1 / dt). Before the controller can
respond, the output deviates further depending on the
inductor and output capacitor values. After a short peri-
od of time (see the Typical Operating Characteristics ),
the controller responds by regulating the output voltage
back to its nominal state. The controller response time
depends on the closed-loop bandwidth. With a higher
bandwidth the response time is faster, preventing the
output capacitor from further deviation from its regulat-
ing value. Be sure not to exceed the capacitor’s voltage
or current ratings.
MOSFET Selection
The MAX8513/MAX8514 drive two external, logic-level,
? Maximum drain-to-source voltage (V DS ) should be at
least 20% higher than the input supply rail at the
high-side MOSFET’s drain.
? For gate charges (Q GS , Q GD , Q DS ), the lower the
better.
Choose the MOSFETs with rated R DS_ON at V GS =
4.5V. For a good compromise between efficiency and
cost, choose the high-side MOSFET (Q1 in the Typical
Applications Circuits ) that has conduction loss equal to
switching loss at nominal input voltage and maximum
output current. For the low-side MOSFET (Q2 in the
Typical Applications Circuits ), make sure that it does
not spuriously turn on due to dV/dt caused by Q1 turn-
ing on as this results in shoot-through current degrad-
ing the efficiency. MOSFETs with a lower Q GD / Q GS
ratio have higher immunity to dV/dt.
For proper thermal management, the power dissipation
must be calculated at the desired maximum operating
junction temperature, maximum output current, and
worst-case input voltage. For Q2, the worst case is at
V IN_MAX . For Q1, it could be either at V IN_MIN or
V IN_MAX . Q1 and Q2 have different loss components
due to the circuit operation. Q2 operates as a zero volt-
age switch, where major losses are the channel con-
duction loss (P Q2CC ) and the body-diode conduction
loss (P Q2DC ).
? V OUT 1 ?
?
P Q 2 DC = 2 × I OUT 1 × V F × t dt × f S
where V F is the body-diode forward voltage drop, t dt =
50ns is the dead time between Q1 and Q2 switching
transitions, and f S is the switching frequency.
The total losses for Q2 are:
P Q 2 _ TOTAL = P Q 2 CC + P Q 2 DC
Q1 operates as a duty-cycle control switch and has the
following major losses: the channel conduction loss
(P Q1CC ), the V I overlapping switching loss (P Q1SW ),
and the drive loss (P Q1DR ). Q1 does not have body-
diode conduction loss because the diode never con-
ducts current.
N-channel MOSFETs as the circuit switch elements.
The key selection parameters are:
P Q 1 CC =
V OUT1
V IN
× I OUT 1 2 × R DS _ ON
? For on-resistance (R DS_ON ), the lower the better.
where R DS_ON is at the maximum operating junction
temperature.
20
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