MAX8734 Maxim Integrated Products, MAX8734 Datasheet - Page 27

• Manufacturer Part Number: MAX8734
• Description: (MAX8732 - MAX8734) Main Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
(>5A) when using high-voltage (>20V) AC adapters.
Low-current applications usually require less attention.
Choose a high-side MOSFET (N1/N3) that has conduc-
tion losses equal to the switching losses at the typical
battery voltage for maximum efficiency. Ensure that the
conduction losses at the minimum input voltage do not
exceed the package thermal limits or violate the overall
thermal budget. Ensure that conduction losses plus
switching losses at the maximum input voltage do not
exceed the package ratings or violate the overall ther-
mal budget.
Choose a synchronous rectifier (N2/N4) with the lowest
possible R
high-side switch turning on due to parasitic drain-to-gate
capacitance, causing cross-conduction problems.
Switching losses are not an issue for the synchronous
rectifier in the buck topology since it is a zero-voltage
switched device when using the buck topology.
Worst-case conduction losses occur at the duty-factor
extremes. For the high-side MOSFET, the worst-case
power dissipation (PD) due to the MOSFET’s R
occurs at the minimum battery voltage:
Generally, a small high-side MOSFET reduces switch-
ing losses at high input voltage. However, the R
required to stay within package power-dissipation limits
often limits how small the MOSFET can be. The opti-
mum situation occurs when the switching (AC) losses
equal the conduction (R
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum battery volt-
age is applied, due to the squared term in the CV
switching-loss equation. Reconsider the high-side
MOSFET chosen for adequate R
voltages if it becomes extraordinarily hot when subject-
ed to V+
Calculating the power dissipation in N
switching losses is difficult since it must allow for quan-
tifying factors that influence the turn-on and turn-off
times. These factors include the internal gate resis-
tance, gate charge, threshold voltage, source induc-
tance, and PC board layout characteristics. The
PD N
(
H
(MAX)
Re
DS(ON)
Supply Controllers for Notebook Computers
sis
High-Efficiency, Quad-Output, Main Power-
.
tan
. Ensure the gate is not pulled up by the
ce
______________________________________________________________________________________
)
MOSFET Power Dissipation
=
Power MOSFET Selection
DS(ON)




V
V
IN MIN
OUT
(
) losses.
_
)
DS(ON)




(
I
LOAD
H
(N1/N3) due to
at low battery
)
2
R
DS ON
DS(ON)
DS(ON)
(
2
)
f
following switching-loss calculation provides only a
very rough estimate and is no substitute for bench eval-
uation, preferably including verification using a thermo-
couple mounted on N
where C
(N1/N3) and I
current.
For the synchronous rectifier, the worst-case power dis-
sipation always occurs at maximum battery voltage:
The absolute worst case for MOSFET power dissipation
occurs under heavy overloads that are greater than
I
the current limit and cause the fault latch to trip. To
protect against this possibility, “overdesign” the circuit
to tolerate:
where I
allowed by the current-limit circuit, including threshold
tolerance and resistance variation.
Current circulates from ground to the junction of both
MOSFETs and the inductor when the high-side switch is
off. As a consequence, the polarity of the switching
node is negative with respect to ground. This voltage is
approximately -0.7V (a diode drop) at both transition
edges while both switches are off (dead time). The drop
is I
The rectifier is a clamp across the synchronous rectifier
that catches the negative inductor swing during the dead
time between turning the high-side MOSFET off and the
synchronous rectifier on. The MOSFETs incorporate a
high-speed silicon body diode as an adequate clamp
diode if efficiency is not of primary importance. Place a
Schottky diode in parallel with the body diode to reduce
the forward voltage drop and prevent the N2/N4 MOSFET
body diodes from turning on during the dead time.
Typically, the external diode improves the efficiency by
1% to 2%. Use a Schottky diode with a DC current rating
equal to 1/3 of the load current. For example, use an
MBR0530 (500mA-rated) type for loads up to 1.5A, a
LOAD(MAX)
L
x R
I
LOAD
DS(ON)
PD N
LIMIT(HIGH)
RSS
( )
= I
but are not quite high enough to exceed
PD N Switching
(
is the reverse transfer capacitance of N
L
V
GATE
LIMIT(HIGH)
IN MAX
when the low-side switch conducts.
=
(
(



H
1
−
is the peak gate-drive source/sink
H
)
V
is the maximum valley current
)
V
IN MAX
2
(N1/N3):
OUT
(



+ (LIR / 2 ) x I
C
RSS SW LOAD
_
)
)



=
I
Rectifier Selection
GATE
f
I
LOAD
I
2
LOAD(MAX)
R
DS ON



(
)
27
H