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MCR03EZPFX4021 数据表(PDF) 19 Page - International Rectifier

部件名 MCR03EZPFX4021
功能描述  HIGHLY EFFICIENT INTEGRATED 4A, SYNCHRONOUS BUCK REGULATOR
PDF  35 Pages
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制造商  IRF [International Rectifier]
网页  http://www.irf.com
标志 IRF - International Rectifier

MCR03EZPFX4021 数据表(HTML) 19 Page - International Rectifier

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Rev 4.0
19
PD-97516
IR3853MPbF
Application Information
Design Example:
The following example is a typical application for
IR3853. The application circuit is shown on page
25.
Enabling the IR3853
As explained earlier, the precise threshold of the
Enable lends itself well to implementation of a
UVLO for the Bus Voltage.
For a maximum Enable threshold of V
EN = 1.36 V
For a V
in (min)=10.2V, R1=49.9K and R2=7.5K is a
good choice.
Programming the frequency
For F
s = 600 kHz, select Rt = 23.7 kΩ, using
Table. 1.
Output Voltage Programming
Output voltage is programmed by reference
voltage and external voltage divider. The Fb pin
is the inverting input of the error amplifier, which
is internally referenced to 0.7V. The divider is
ratioed to provide 0.7V at the Fb pin when the
output is at its desired value. The output voltage
is defined by using the following equation:
Fb
IR3624
VOUT
R9
R8
IR3853
When an external resistor divider is connected to
the output as shown in figure 12.
Equation (6) can be rewritten as:
For the calculated values of R8 and R9 see
feedback compensation section.
Soft-Start Programming
The soft-start timing can be programmed by
selecting the soft-start capacitance value. From
(1), for a desired start-up time of the converter,
the soft start capacitor can be calculated by
using:
Where T
start is the desired start-up time (ms).
For a start-up time of 3.5ms, the soft-start
capacitor will be 0.099μF. Choose a 0.1μF
ceramic capacitor.
Bootstrap Capacitor Selection
To drive the Control FET, it is necessary to
supply a gate voltage at least 4V greater than
the voltage at the SW pin, which is connected
the source of the Control FET . This is achieved
by using a bootstrap configuration, which
comprises the internal bootstrap diode and an
external bootstrap capacitor (C6), as shown in
Fig. 13. The operation of the circuit is as follows:
When the lower MOSFET is turned on, the
capacitor node connected to SW is pulled down
to ground. The capacitor charges towards V
cc
through the internal bootstrap diode, which has a
forward voltage drop V
D. The voltage Vc across
the bootstrap capacitor C6 is approximately
given as
When the upper MOSFET turns on in the next
cycle, the capacitor node connected to SW rises
to the bus voltage V
in. However, if the value of
C6 is appropriately chosen, the voltage Vc
kHz
600
=
Vo
of
5%
A
4
=
V
1.8
=
max)
13.2V
(
V
12
=
s
o
o
o
in
F
ΔV
I
V
V
±
≤
.(8)
..........
..........
1
9
8
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
∗
=
R
R
V
V
ref
o
Fig. 12. Typical application of the IR3853 for
programming the output voltage
(9)
....
..........
..........
..........
8
9
V
V
V
R
R
ref
o
ref
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
∗
=
V
V
V
D
cc
c
(11)
......
..........
..........
−
≅
)
(
T
)
(
C
start
SS
(10)
..........
0.02857
ms
μF
×
=
IR3853
Enable
Vin
R2
R
1
V
R
R
R
*
V
EN
(min)
in
(6)
......
1.36V.....
2
1
2
=
=
+
V
V
V
R
R
EN
)
min
in(
EN
(7)
..........
1
2
−
=



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