MAX8741EAI+T Maxim Integrated, MAX8741EAI+T Datasheet - Page 23

• Manufacturer Part Number: MAX8741EAI+T
• Description: Current & Power Monitors & Regulators 500kHz Multi-Out Pwr Supply Controller
• Manufacturer: Maxim Integrated

Specifications of MAX8741EAI+T

• Rohs: yes
• Product: Power Monitors
• Supply Voltage - Max: 30 V
• Supply Voltage - Min: 4.2 V
• Input Voltage Range: 4.2 V to 30 V

where:
V
put voltage
V
rectifier
V
age (from the Electrical Characteristics tables)
V
synchronous-rectifier MOSFET
V
resistor
In positive-output applications, the transformer sec-
ondary return is often referred to the main output volt-
age, rather than to ground, to reduce the needed turns
ratio. In this case, the main output voltage must first be
subtracted from the secondary voltage to obtain V
The high-current n-channel MOSFETs must be logic-
level types with guaranteed on-resistance specifica-
tions at V
specifications are better (i.e., 2V max rather than 3V
max). Drain-source breakdown voltage ratings must at
least equal the maximum input voltage, preferably with
a 20% derating factor. The best MOSFETs have the
lowest on-resistance per nanocoulomb of gate charge.
Multiplying R
comparing various MOSFETs. Newer MOSFET process
technologies with dense cell structures generally per-
form best. The internal gate drivers tolerate >100nC
total gate charge, but 70nC is a more practical upper
limit to maintain best switching times.
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I
both high-side and low-side MOSFETs. I
distributed between Q1 and Q2 according to duty fac-
tor (see the following equations). Generally, switching
losses affect only the upper MOSFET, since the
Schottky rectifier clamps the switching node in most
cases before the synchronous rectifier turns on. Gate-
charge losses are dissipated by the driver and do not
heat the MOSFET. Calculate the temperature rise
according to package thermal-resistance specifications
to ensure that both MOSFETs are within their maximum
junction temperature at high ambient temperature. The
worst-case dissipation for the high-side MOSFET
2
SEC
OUT(MIN)
FWD
RECT
SENSE
R power losses are the greatest heat contributor for
500kHz Multi-Output Power-Supply Controllers
= the minimum required rectified secondary out-
= the forward drop across the secondary
= the on-state voltage drop across the
= the voltage drop across the sense
= the minimum value of the main output volt-
GS
DS(ON)
= 4.5V. Lower gate-threshold
______________________________________________________________________________________
Selecting Other Components
✕
Q
G
provides a good figure for
with High Impedance in Shutdown
MOSFET Switches
2
R losses are
SEC
.
occurs at both extremes of input voltage, and the
worst-case dissipation for the low-side MOSFET occurs
at maximum input voltage:
where:
on-state voltage drop V
C
I
20ns = DH driver inherent rise/fall time
During short circuit, the MAX8741/MAX8742s' output
undervoltage shutdown protects the synchronous recti-
fier under output short-circuit conditions.
To reduce EMI, add a 0.1µF ceramic capacitor from the
high-side switch drain to the low-side switch source.
The rectifier diode is a clamp across the low-side
MOSFET that catches the negative inductor swing dur-
ing the 60ns dead time between turning one MOSFET
off and each low-side MOSFET on. The latest genera-
tions of MOSFETs incorporate a high-speed Schottky
diode, which serves as an adequate clamp diode. For
MOSFETs without integrated Schottky diodes, place a
Schottky diode in parallel with the low-side MOSFET.
Use a Schottky diode with a DC current rating equal to
1/3rd the load current. The Schottky diode’s rated
reverse breakdown voltage must be at least equal to
the maximum input voltage, preferably with a 20% der-
ating factor.
A signal diode such as a 1N4148 works well in most
applications. If the input voltage can go below +6V, use
a small (20mA) Schottky diode for slightly improved
efficiency and dropout characteristics. Do not use
large-power diodes, such as 1N5817 or 1N4001, since
high junction capacitance can pump up V
sive voltages.
GATE
RSS
= MOSFET reverse transfer capacitance
= DH driver peak output current capability (1A typ)
PD
PD
upperFET
upperFET
DUTY
=
=
=
I
I
(
+
⎛
⎜
⎝
LOAD
LOAD
V
V
V
OUT
IN
IN
Q_
I
GATE
×
×
2
2
= I
C
+
I
×
LOAD
×
RSS
V
LOAD
R
R
Q
DS ON
DS ON
2
) (
Rectifier Clamp Diode
+
/
(
(
× ×
Boost-Supply Diode
20
V
✕
f
IN
ns
)
)
R
-
×
×
DS(ON)
⎞
⎟
⎠
V
DUTY
Q
(
1
1
-
)
DUTY
L
to exces-
)
23