Rainbow Electronics MAX8514 User Manual
Page 20
MAX8513/MAX8514
Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
20
______________________________________________________________________________________
The peak-to-peak output voltage ripple as a conse-
quence of the ESR, ESL, and output capacitance is:
where C
OUT
is C4 in the Typical Applications Circuits.
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:
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,
N-channel MOSFETs as the circuit switch elements.
The key selection parameters are:
• For on-resistance (R
DS_ON
), the lower the better.
• 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
).
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:
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.
where R
DS_ON
is at the maximum operating junction
temperature.
P
V
V
I
R
Q CC
OUT
IN
OUT
DS ON
1
1
1
2
_
=
Ч
Ч
P
P
P
Q
TOTAL
Q CC
Q DC
2
2
2
_
=
+
P
V
V
I
R
P
I
V
t
f
Q CC
OUT
IN
OUT
DS ON
Q DC
OUT
F
dt
S
2
1
1
2
1
1
2
2
=
Ч
Ч
=
Ч
Ч
Ч
Ч
_
-
V
V
V
V
RIPPLE
RIPPLE C
RIPPLE ESR
RIPPLE ESL
=
+
+
( )
(
)
(
)
V
V
ESL
L A
ESL
and I
V
V
f L
V
V
RIPPLE ESL
IN
P P
IN
OUT
S
OUT
IN
(
)
=
×
+
=
1
1
1
-
-
V
I
R
V
I
C
f
RIPPLE ESR
P P
ESR
RIPPLE C
P P
OUT
S
(
)
( )
=
Ч
=
Ч
Ч
-
-
8