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參數(shù)資料
| 型號： | IRU3048CF |
| 廠商： | International Rectifier |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | DUAL SYNCHRONOUS PWM CONTROLLER CIRCUITRY AND LDO CONTROLLER |
| 中文描述： | 雙同步PWM控制器電路和LDO控制器 |
| 文件頁數(shù)： | 6/18頁 |
| 文件大小： | 154K |
| 代理商： | IRU3048CF |
6
Rev. 1.7
09/12/02
IRU3048
www.irf.com
Output Capacitor Selection
The criteria to select the output capacitor is normally
based on the value of the Effective Series Resistance
(ESR). In general, the output capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy sta-
bility requirements. The ESR of the output capacitor is
calculated by the following relationship:
V
I
O
Where:
V
O
= Output Voltage Ripple
I
O
= Output Current
V
O
=75mV and
I
O
=3A, results to: ESR=25m
The Sanyo TPC series, PosCap capacitor is a good
choice. The 6TPC150M 150
μ
F, 6.3V has an ESR 40m
.
Selecting two of these capacitors in parallel for each
output, results to an ESR of
20m
which achieves our
low ESR goal.
The capacitor value must be high enough to absorb the
inductor's ripple current. The larger the value of capaci-
tor, the lower will be the output ripple voltage.
The resulting output ripple current is smaller then each
channel ripple current due to the 180
8
phase shift. These
currents cancel each other. The cancellation is not the
maximum because of the different duty cycle for each
channel.
Inductor Selection
The inductor is selected based on output power, operat-
ing frequency and efficiency requirements. Low induc-
tor value causes large ripple current, resulting in the
smaller size, but poor efficiency and high output noise.
Generally, the selection of inductor value can be reduced
to desired maximum ripple current in the inductor (
i);
the optimum point is usually found between 20% and
50% ripple of the output current.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
i
V
V
IN
×
i
×
f
S
Where:
V
IN
= Max Input Voltage
V
OUT
= Output Voltage
i = Inductor Ripple Current
f
S
= Switching Frequency
If the high value feedback resistors are used, the input
bias current of the Fb pin could cause a slight increase
in output voltage. The output voltage set point can be
more accurate by using precision resistor.
Soft-Start Programming
The soft-start timing can be programmed by selecting
the soft start capacitance value. The start up time of the
converter can be calculated by using:
t
START
= 75
×
Css (ms) ---(2)
Where:
C
SS
is the soft-start capacitor (
μ
F)
For a start-up time of 7.5ms, the soft-start capacitor will
be 0.1
μ
F. Choose a ceramic capacitor at 0.1
μ
F.
Boost Supply Vc
To drive the high-side switch it is necessary to supply a
gate voltage at least 4V greater than the bus voltage.
This is achieved by using a charge pump configuration
as shown in Figure 9. The capacitor is charged up to
approximately twice the bus voltage. A capacitor in the
range of 0.1
μ
F to 1
μ
F is generally adequate for most
applications.
Input Capacitor selection
The input filter capacitor should be based on how much
ripple the supply can tolerate on the DC input line. The
ripple current generated during the on time of control
MOSFET should be provided by input capacitor. The RMS
value of this ripple is expressed by:
Where:
D is the Duty Cycle, simply D=V
OUT
/V
IN.
I
RMS
is the RMS value of the input capacitor current.
I
OUT
is the output current for each channel.
For V
IN1
=12V, I
OUT1
=4A and D1=0.275
Results to: I
RMS1
=1.78A
And for V
IN2
=5V, I
OUT2
=4A and D2=0.36
Results to: I
RMS2
=1.92A
For higher efficiency, a low ESR capacitor is recom-
mended.
For V
IN1
=12V, choose one Poscap from Sanyo
16TPB47M (16V, 47
μ
F, 70m
, 1.4A)
For V
IN2
=5V, choose one 6TPC150M (6.3V, 150
μ
F,
40m
, 1.9A).
I
RMS
= I
OUT
D
×
(1-D)
---(3)
ESR
≤
---(4)
V
IN
- V
OUT
= L
×
t
t = D
×
f
1
S
V
OUT
V
IN
L = (V
IN
- V
OUT
)
×
---(5)
t = Turn On Time
D = Duty Cycle
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