MAX8513 MAXIM [Maxim Integrated Products], MAX8513 Datasheet - Page 20

• Manufacturer Part Number: MAX8513
• Description: Wide-Input, High-Frequency, Triple-Output Supplies with Voltage Monitor and Power-On Reset
• Manufacturer: MAXIM [Maxim Integrated Products]

Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
The peak-to-peak output voltage ripple as a conse-
quence of the ESR, ESL, and output capacitance is:
where C
where I
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
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.
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
20
OUT1
V
RIPPLE
______________________________________________________________________________________
) + (ESL × dI
P-P
OUT
is the peak-to-peak inductor current (see the
V
=
V
and I
RIPPLE ESR
RIPPLE ESL
is C4 in the Typical Applications Circuits.
V
V
RIPPLE C
RIPPLE C
P P
-
(
(
=
OUT1
( )
( )



)
)
DS_ON
V
+
IN
=
=
=
/ dt). Before the controller can
V
8
RIPPLE ESR
-
I
f L
L A
P P
V
S
V
1
IN
-
×
OUT
), the lower the better.
MOSFET Selection
C
×
×
+
I
P P
OUT
1
(
-
R
ESL



ESL



ESR
V
)
OUT
V
×
+
IN
f
V
S
RIPPLE ESL
1



(
)
• Maximum drain-to-source voltage (V
• For gate charges (Q
Choose the MOSFETs with rated R
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
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
V
due to the circuit operation. Q2 operates as a zero volt-
age switch, where major losses are the channel con-
duction loss (P
loss (P
where V
50ns is the dead time between Q1 and Q2 switching
transitions, and f
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
and the drive loss (P
diode conduction loss because the diode never con-
ducts current.
where R
temperature.
IN_MAX
IN_MAX
Q1CC
least 20% higher than the input supply rail at the
high-side MOSFET’s drain.
better.
P
P
Q2DC
Q CC
Q DC
), the V I overlapping switching loss (P
F
. Q1 and Q2 have different loss components
DS_ON
P
2
2
. For Q1, it could be either at V
Q CC
is the body-diode forward voltage drop, t
1
).
P
Q
=
=
2
Q2CC



2
_
is at the maximum operating junction
S
=
1
TOTAL
×
is the switching frequency.
-
V
OUT
I
V
) and the body-diode conduction
V
OUT
IN
Q1DR
OUT
V
IN
=
GS
1
1
1
P
×
, Q
×

 ×

). Q1 does not have body-
Q CC
2
I
V
OUT
GD
F
I
OUT
×
, Q
+
1
2
1
t
P
dt
2
DS
×
Q DC
DS_ON
×
2
DS
R
×
), the lower the
R
DS ON
) should be at
DS ON
f
S
_
_
IN_MIN
GD
at V
Q1SW
/ Q
GS
dt
GS
or
=
=
),