Typical FETKY Applications

AND8265/D
Typical FETKY Applications
Prepared by: Han Zou
ON Semiconductor
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APPLICATION NOTE
Introduction
In consumer power electronics, it is not unusual to see
both power MOSFETs and Schottky diodes operating side
by side as main circuit elements. The reasons of placing
power MOSFETs in conjunction with the Schottky diodes
can be versatile, such as reverse current protection, DC/DC
converter free−wheeling rectification (Asynchronous),
switching efficiency improvement and so on. Working
together, they could provide a more desirable system
performance.
Needless to say, the integration of these two devices into
one standard package could bring benefits. From circuit
design point of view, it can minimize the parasitic influence,
save the board space, add convenience of PCB routing, and
lowers down the total system cost. The integration results in
a new device category: FETKY. This short note tries to cover
typical applications of the FETKY devices in brief.
Applications
• DC/DC circuits: MOSFET + Schottky free−wheeling
diode
L1
Vout
Vin
Q1
D1
C1
C2
PWM
NTLJF3117P
Gnd
Gnd
Figure 1. Buck Converter Circuit with mCoolt NTLJF3117P FETKY device
Two simple topologies frequently seen in the portable
electronic DC/DC circuits probably are asynchronous buck
and boost circuit where the integrated Schottky diode acts as
a free−wheeling rectification diode to carry the current
© Semiconductor Components Industries, LLC, 2006
June, 2006 − Rev. 0
during the time the FET is turned off. A low gate charge
MOSFET and a low forward voltage Schottky diode ensure
both fast switching and minimum power loss when the
loading current is high.
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D1
L1
Vout
Vin
C1
PWM
C2
Q1
R1
NTLJF4156N
Gnd
Gnd
Figure 2. Boost Converter Circuit mCoolt NTLJF4156N FETKY device
dropped on the Schottky diode at 2 A is less than 0.55 V, so
the Schottky diode also works in its safe region. Overall, it
is an applicable design if there is a secure thermal contact
between FETKY and the PC board.
Routing of FETKY device in a DC/DC buck circuit:
In order to achieve good power efficiency, the drain pin of
MOSFET needs to be placed as close as possible to the
cathode pin of the Schottky diode to minimize the switching
transient caused by the parasitic inductance. A
recommendation for routing the FETKY device properly is
illustrated as below:
The following example describes the method to estimate
and evaluate the FETKY performance in an asynchronous
DC/DC circuit design:
The objective is to design a 5 V input and 2.5 V output
converter with a 45°C ambient temperature and 2 A
maximum loading. The PWM controller operates at
700 kHz constant frequency with conversion duty cycle
calculated as:
V
D + out + 50%
Vin
(eq. 1)
For a 700 kHz operating frequency with 50% duty cycle,
the on−time is 0.7 ms.
Referring to the thermal transient response curve in the
FETKY device datasheet (WDFN6 2x2 NTLJF3117P), the
thermal resistance for 50% duty cycle and 0.7 ms on−time
would be 80°C/W.
The elevated ambient temperature is 45°C, therefore, the
max power loss on the FET is:
Pdmax +
Tj(max) * TA
RqJA
+
150 * 45
+ 1.31 W
80
(eq. 2)
Assume that the gate voltage allows P−channel MOSFET
to be fully conducted, the maximum RDS(on) according to
the datasheet is 100 mW. With a 45°C ambient temperature,
RDS(on) will be 8% higher than normal which is about
108 mW (Figure 5 in the datasheet). The maximum loading
current for the converter is 2 A. Therefore the real power
consumption on the device can be calculated as:
PFET + RDS(on) @ I max 2 + 0.45 W
Figure 3. Recommended routing for the FETKY
(WDFN6 2x2, NTLJF3117P) in a buck converter
(eq. 3)
In conclusion, the FET would work in a safe region. From
Figure 13 in the datasheet, the typical forward voltage
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Load Switches and Charging Circuits with Reverse Protection
A Schottky can be used in series with a power MOSFET
shorted accidentally. The routing for FETKY in charging
as a blocking unit as shown in Fig. 4. This topology is often
circuit is a bit different from the DC/DC circuit with drain
used in the Li−ion battery linear charging circuit for cellular
pin of the P−channel MOSFET connected to the anode of the
phones. The power MOSFET acts as a charging element and
Schottky diode from outside of the package, no switch node
the voltage across it varies with the battery voltage. The
exists, but the distance between Drain and Anode needs to
Schottky diode is then used to prevent the reverse current
be as short as possible. A recommended routing scheme for
flow on the battery side when the charge input becomes
this application is presented in Fig. 5.
D1
Vout
Vin
Q1
NTLJF3117P
C1
On/Off
C2
R1
Gnd
Gnd
Figure 4. FETKY load switch with blocking diode
The FETKY structure can also be utilized to protect itself
from being damaged. If multiple power sources exist or the
output voltage could be higher than the input voltage, the
reverse current may flow through the body diode of the
P−channel MOSFET causing irreversible damage to the
MOSFET. A low voltage Schottky diode (0.4 V @ 1 A) in
parallel with the FET will bypass the current and keep the
reverse voltage across the switch less than a diode voltage
drop, therefore greatly reduce the risk of damaging the
power device.
Figure 5. Recommended routing for FETKY (WDFN6
2x2, NTLJF3117P) in a linear battery charging circuit
NTLJF3117P
D1
Vout
Vin
Q1
C1
C2
On/Off
R1
Gnd
Gnd
Figure 6. FETKY load switch with by−pass diode
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AND8265/D
Switching Efficiency Improvement
In synchronous DC/DC buck converter circuit, a Schottky
diode is placed in parallel with the synchronous rectifying
MOSFET to improve efficiency. Due to the fast recovering
feature of the Schottky diode, it becomes conducted first
during the dead−time between the conduction of the two
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on and storing charge during
the dead−time, which could cost as much as 2% in
efficiency.
L1
Vout
Vin
Q1
D1
C1
C2
CTL
Q2
NTLJF4156N
Gnd
Gnd
Figure 7. FETKY for efficiency improvement of synchronous DC/DC converter
Featured Devices:
mCOOLt 2x2 mm
P−channel:
NTLJF3117P
MOSFET: 20 VDS/8 VGS, 75 mW RDS(on),
Schottky: [email protected], 2 A maximum current
NTLJF1103P
MOSFET: 8 VDS/6 VGS, 70 mW RDS(on),
Schottky: [email protected]−channel:, 2 A maximum current
N−channel
NTLJF4156N
MOSFET: 30 VDS/8 VGS, 47 mW RDS(on),
Schottky: [email protected], 2 A maximum current, 30 V reverse
voltage
NTLJF3118N
Figure 8. Recommended layout for FETKY (WDFN6
2x2, NTLJF4156N) in a synchronous converter
MOSFET: 20 VDS/12 VGS, 50 mW RDS(on),
Schottky: [email protected], 2 A maximum current
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