MAX8744ETJ+ Maxim Integrated Products, MAX8744ETJ+ Datasheet - Page 31

• Manufacturer Part Number: MAX8744ETJ+
• Description: IC CNTRLR PWR SUP QUAD 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8744ETJ+

• Applications: Controller, Notebook Computers
• Voltage - Input: 6 ~ 26 V
• Number Of Outputs: 4
• Voltage - Output: 3.3V, 5V, 1 ~ 26 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Duty Cycle (max): 99 %
• Output Voltage: 3.315 V, 5.015 V, 2 V to 5.5 V
• Mounting Style: SMD/SMT
• Switching Frequency: 200 KHz, 300 KHz, 500 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost, Flyback, Forward
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Choose a low-side MOSFET (N
sible on-resistance (R
package (i.e., 8-pin SO, DPAK, or D
ably priced. Ensure that the MAX8744/MAX8745 DL_
gate driver can supply sufficient current to support the
gate charge and the current injected into the parasitic
drain-to-gate capacitor caused by the high-side MOSFET
turning on; otherwise, cross-conduction problems may
occur. Switching losses are not an issue for the low-side
MOSFET since it is a zero-voltage switched device when
used in the step-down topology.
Worst-case conduction losses occur at the duty-factor
extremes. For the high-side MOSFET (N
case power dissipation due to resistance occurs at mini-
mum input voltage:
Generally, use a small high-side MOSFET to reduce
switching losses at high input voltages. However, the
R
tion limits often limits how small the MOSFET can be. The
optimum occurs when the switching losses equal the
conduction (R
do not become an issue until the input is greater than
approximately 15V.
Calculating the power dissipation in high-side MOSFETs
(N
allow for difficult-to-quantify factors that influence the turn-
on and turn-off times. These factors include the internal
gate resistance, gate charge, threshold voltage, source
inductance, and PCB layout characteristics. The following
switching-loss calculation provides only a very rough esti-
mate and is no substitute for breadboard evaluation,
preferably including verification using a thermocouple
mounted on N
where C
the charge needed to turn on the N
is the peak gate-drive source/sink current (1A typ).
Switching losses in the high-side MOSFET can become
a heat problem when maximum AC adapter voltages
are applied, due to the squared term in the switching-
loss equation (C x V
chosen for adequate R
DS(ON)
H
) due to switching losses is difficult, since it must
PD N Switching
⎛
⎜
⎝
PD N
I
LOAD G SW
(
OSS
(
required to stay within package power-dissipa-
I
Supply Controllers for Notebook Computers
GATE
H
H
Q
Re
is the output capacitance of N
High-Efficiency, Quad-Output, Main Power-
H
DS(ON)
:
(
sistive
______________________________________________________________________________________
)
IN
) losses. High-side switching losses
DS(ON)
+
)
2 x f
)
=
C
=
DS(ON)
⎛
⎜
⎝
OSS IN MAX
V
SW
Power-MOSFET Dissipation
V
OUT
), comes in a moderate-sized
IN
V
). If the high-side MOSFET
2
L
) that has the lowest pos-
⎞
⎟
⎠
(
at low battery voltages
(
I
LOAD
H
2
PAK), and is reason-
MOSFET, and I
)
⎞
⎟
⎠
V
)
2
IN MAX SW
R
H
(
DS ON
), the worst-
H
, Q
(
)
f
G(SW)
)
GATE
is
becomes extraordinarily hot when subjected to
V
lower parasitic capacitance.
For the low-side MOSFET (N
dissipation always occurs at maximum battery voltage:
The absolute worst case for MOSFET power dissipation
occurs under heavy overload conditions that are
greater than I
exceed the current limit and cause the fault latch to trip.
To protect against this possibility, “overdesign” the cir-
cuit to tolerate:
where I
limit circuit, including threshold tolerance and sense-
resistance variation. The MOSFETs must have a
relatively large heatsink to handle the overload power
dissipation.
Choose a Schottky diode (D
drop low enough to prevent the low-side MOSFET’s
body diode from turning on during the dead time. As a
general rule, select a diode with a DC current rating
equal to a 1/3 the load current. This diode is optional
and can be removed if efficiency is not critical.
The boost capacitors (C
enough to handle the gate-charging requirements of
the high-side MOSFETs. Typically, 0.1µF ceramic
capacitors work well for low-power applications driving
medium-sized MOSFETs. However, high-current appli-
cations driving large, high-side MOSFETs require boost
capacitors larger than 0.1µF. For these applications,
select the boost capacitors to avoid discharging the
capacitor more than 200mV while charging the high-
side MOSFETs’ gates:
where Q
high-side MOSFET’s data sheet. For example, assume
the FDS6612A n-channel MOSFET is used on the high
side. According to the manufacturer’s data sheet, a sin-
gle FDS6612A has a maximum gate charge of 13nC
IN(MAX)
LIMIT
GATE
, consider choosing another MOSFET with
PD N
⎡
⎢
⎢
⎣
I
1
LOAD
is the peak current allowed by the current-
−
(
LOAD(MAX)
is the total gate charge specified in the
⎛
⎜
⎝
L
V
IN MAX
Re
=
V
C
OUT
(
I
LIMIT
sistive
BST
)
⎞
⎟
⎠
=
BST
−
⎤
⎥
⎥
⎦
but are not high enough to
)
Q
(
200
⎛
⎜
⎝
I
=
LOAD
Δ
GATE
) must be selected large
L
L
I
INDUCTOR
) with a forward-voltage
), the worst-case power
mV
Boost Capacitors
)
2
2
R
DS ON
(
⎞
⎟
⎠
)
31