AN805 查看數據表（PDF） - Vishay Semiconductors
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AN805 Datasheet PDF : 7 Pages
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AN805
Vishay Siliconix
Figure 2 shows the respective contribution of on-resistance and
gate charge to overall losses for the p-channel Si6801DQ at three
different switching frequencies. At low gate-source voltages, the
rDS(on) of the MOSFET is high and therefore on-resistance losses
dominate. At higher gate-source voltages, on-resistance becomes
almost a constant and the gate charge losses controlled by Qg
dominate. Gate losses increase with the switching frequency,
causing a narrowing in the optimum gate voltage. Therefore, the
optimum drive voltage will be at a level which is just enough to take
the rDS(on) into its constant region, but no further. Typically, this
drive voltage is between 3 and 5 V, which is what most controller
ICs provide.
Figure 3 compares the power losses, at a switching frequency of 1
MHz, of Vishay Siliconix’s PWM optimized Si6801DQ, a
conventional power MOSFET (Si6542DQ), and a low-threshold
power MOSFET (Si6552DQ).
Power losses for the PWM optimized MOSFET at gate drives
between 2.5 and 5.5 V are significantly lower than both
conventional and low-threshold MOSFETs, making the optimized
device the obvious choice for all switching applications.[7.]
The PWM Optimized MOSFET in a Real Application
The PWM optimized power MOSFET is best viewed in the context
of a real application. In the example used here, the Si6801DQ is
paired with the Si9160BQ switching regulator IC to create a
synchronous boost converter for cellular telephones with the
following specifications:
Input voltage: 2.7 V to 5 V (single-cell lithium ion battery is
2.7 V to 4.2 V)
Output voltage: 5 V
Output current: 1 A maximum
Gate drive voltage: 4.5 V
Control scheme: Constant frequency voltage mode control
Switching frequency:
Varied by RC value from 300 kHz to
1.8 MHz
All results shown are with VIN = 3.6 V, VOUT = 5 V, IOUT = 600 mA, f
= 1 MHz unless otherwise stated.
1-Cell
LiIon
SYNC
D1
LS4148
D2
LS4148
R5
100 k
C10
0.33 mF
C2
0.1 mF
R4
2.2 K
C3
0.1 mF
R3
2.2 k
R2, 270 W
R1
10 k
C6 C4
22 pF 0.1 mF
C5
0.1 mF
Si9160
1
16
2 VDD
VS 15
NC
3
DR 14
4 DMAX/SS
DS 13
COMP
5
PGND 12
FB
6
UVLOSET 11
NI
7
COSC 10
8 VREF
ROSC 9
GND ENABLE
R6
12 k
C10
0.1 mF
C11
36 pF
ML ML
C1 C2
10 mF 10 mF
4.7 mH
1
2 D1
3 S1
4 S1
G1
8
D2 7
S2
S2
G2
6
5
Si6801
R9
100 W
R10
3.6 k
C9
0.1 mF
C8
5600 pF
R11
1.2 k
ML ML
C3
C4
10 mF 10 mF
FIGURE 4. Si9160 Boost converter test circuit used to compare MOSFET technologies.
7. Neither Figure 2 nor Figure 3 is intended for exacting power loss calculations. These figures should only be used as a comparative measure for various MOSFET technologies.
www.vishay.com S FaxBack 408-970-5600
2
Document Number: 70649
January 1997
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