ambient temperature rise. P
(EQ. 16)
?/DIV>
JA
depends primarily on the amount of PCB area
devoted to heat sinking.
Selection of MOSFET Snubber Circuit
The Switch node (SW) ringing is caused by fast switch-
ing transitions due to the energy stored in the parasitic
elements. This ringing on the SW node couples to other
circuits around the converter if they are not handled
properly. To dampen this ringing, an R-C snubber is con-
nected across the SW node and the source of the low-
side MOSFET.
R-C components for the snubber are selected as follows:
a) Measure the SW node ringing frequency (F
ring
) with a
low capacitance scope probe.
b) Connect a capacitor (C
SNUB
) from SW node to GND
so that it reduces this ringing by half.
c) Place a resistor (R
SNUB
) in series with this capacitor.
R
SNUB
is calculated using the following equation:
(EQ. 17)
d) Calculate the power dissipated in the snubber resistor
as shown in the following equation:
(EQ. 18)
where, V
IN(MAX)
is the maximum input voltage and FSW
is the converter switching frequency.
The snubber resistor chosen should be de-rated to han-
dle the worst-case power dissipation. Do not use wire-
wound resistors for R
SNUB
.
Loop Compensation
Typically, the closed loop crossover frequency (F
cross
),
where the overall gain is unity, should be selected to
achieve optimal transient and steady-state response to
disturbances in line and load conditions. It is recom-
mended to keep F
cross
below fifth of the switching fre-
quency of the converter. Higher phase margin tends to
have a more stable system with more sluggish response
to load transients. Optimum phase margin is about 60? a
good compromise between steady state and transient
responses. A typical design should address variations
over a wide range of load conditions and over a large
sample of devices.
P
Gate
Q
G
V
CC
F
SW
?/DIV>
?/DIV>
=
P
COND
1   D
(
)   I
OUT
2
?/DIV>
R
DS ON
(    )
?/DIV>
=
P
D MAX
(
)
T
J MAX
(
)
T
A MAX
(
)
?/DIV>
JA
-------------- ----------------- ----------------- -
=
R
SNUB
2
?nbsp  F
ring
C
SNUB
?/DIV>
?/DIV>
--------- ----------------- ----------------- --- -
=
P
R SNUB
(
)
C
SNUB
V
IN MAX
(
)
2
F
SW
?/DIV>
?/DIV>
=
Figure 24. Closed-Loop System with Type-3 Network
PWM
&
DRIVER
Summing
Amplifier
?/DIV>
Ramp
Generator
Current
Sense
Amplifier
Q1
Q2
V
OUT
L
V
IN
C
R
DC
R
ES
R
L
C2
R3
C3
C1
R2
R1
R
RAMP
V
IN
Reference
R
BIAS
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參數(shù)描述 |
FAN5069MTCX |
功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 PWM and ULDO Controller Combo RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
FAN5070 |
制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:High Performance Programmable Synchronous DC-DC Controller |
FAN5070M |
制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:Voltage-Mode SMPS Controller |
FAN5071 |
制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:High Performance Programmable Synchronous DC-DC Controller for Multi-Voltage Platforms |
FAN5071AB WAF |
制造商:Fairchild Semiconductor Corporation 功能描述: |