Rainbow Electronics MAX8742 User Manual
Page 23
MAX8741/
M
AX8742
500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
______________________________________________________________________________________
23
where:
V
SEC
= the minimum required rectified secondary out-
put voltage
V
FWD
= the forward drop across the secondary
rectifier
V
OUT(MIN)
= the minimum value of the main output volt-
age (from the Electrical Characteristics tables)
V
RECT
= the on-state voltage drop across the
synchronous-rectifier MOSFET
V
SENSE
= the voltage drop across the sense
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
SEC
.
Selecting Other Components
MOSFET Switches
The high-current n-channel MOSFETs must be logic-
level types with guaranteed on-resistance specifica-
tions at V
GS
= 4.5V. Lower gate-threshold
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
DS(ON)
✕
Q
G
provides a good figure for
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
2
R power losses are the greatest heat contributor for
both high-side and low-side MOSFETs. I
2
R losses are
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
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
Q_
= I
LOAD
✕
R
DS(ON)
C
RSS
= MOSFET reverse transfer capacitance
I
GATE
= DH driver peak output current capability (1A typ)
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.
Rectifier Clamp Diode
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.
Boost-Supply Diode
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
L
to exces-
sive voltages.
PD
I
R
DUTY
V
I
f
V
C
I
ns
PD
I
R
DUTY
DUTY
V
V
V
V
upperFET
LOAD
DS ON
IN
LOAD
IN
RSS
GATE
upperFET
LOAD
DS ON
OUT
Q
IN
Q
=
Ч
Ч
+
Ч
Ч Ч
Ч
+
=
Ч
Ч
=
+
(
) (
)
2
2
2
1
20
1
(
)
(
)
(
)
/
-
-