AN805 VISHAY [Vishay Siliconix], AN805 Datasheet - Page 2

• Manufacturer Part Number: AN805
• Description: PWM Optimized Power MOSFETs for Low-Voltage DC/DC Conversion
• Manufacturer: VISHAY [Vishay Siliconix]

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
r
dominate. At higher gate-source voltages, on-resistance becomes
almost a constant and the gate charge losses controlled by Q
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 r
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.
www.vishay.com FaxBack 408-970-5600
2
DS(on)
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.
DS(on)
of the MOSFET is high and therefore on-resistance losses
SYNC
into its constant region, but no further. Typically, this
2.2 K
R4
1-Cell
LiIon
C6
22 pF
C10
0.33 F
FIGURE 4. Si9160 Boost converter test circuit used to compare MOSFET technologies.
C4
0.1 F
C3
0.1 F
R2, 270
R1
10 k
0.1 F
C2
0.1 F
C5
2.2 k
R3
1
2
3
4
5
6
7
8
LS4148
FB
V
NC
D
COMP
NI
V
GND
DD
REF
MAX
D1
[7.]
Si9160
/SS
UVLO
ENABLE
P
C
R
GND
OSC
OSC
SET
D
D
g
V
12 k
S
R
S
D2
LS4148
R6
16
15
14
13
12
11
10
9
100 k
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:
Output voltage:
Output current:
Gate drive voltage: 4.5 V
Control scheme:
Switching frequency:
1.8 MHz
All results shown are with V
= 1 MHz unless otherwise stated.
R5
C11
36 pF
C10
0.1 F
ML
C1
10 F
C8
5600 pF
R9
100
2.7 V to 5 V (single-cell lithium ion battery is
2.7 V to 4.2 V)
5 V
1 A maximum
Constant frequency voltage mode control
ML
C2
10 F
1
2
3
4
R11
1.2 k
R10
3.6 k
D
S
S
G
IN
Si6801
1
1
1
1
= 3.6 V, V
Varied by RC value from 300 kHz to
C9
0.1 F
G
D
S
S
4.7 H
2
2
2
2
8
7
6
5
OUT
ML
C3
10 F
Document Number: 70649
= 5 V, I
ML
C4
10 F
OUT
January 1997
= 600 mA, f