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参数资料
| 型号： | APW7057 |
| 厂商： | Anpec Electronics Corporation |
| 英文描述： | High Power Step-Down Synchronous DC/DC Controller |
| 中文描述： | 高功率降压型同步DC / DC控制器 |
| 文件页数： | 9/14页 |
| 文件大小： | 154K |
| 代理商： | APW7057 |
Copyright
ANPEC Electronics Corp.
Rev. A.3 - Oct., 2003
APW 7057
www.anpec.com.tw
9
Application I nformation
Component Selection Guidelines
Output Capacitor Selection
The selection of C
OUT
is determined by the required
effective series resistance (ESR) and voltage rating
rather than the actual capacitance requirement. There-
fore select high performance low ESR capacitors that
are intended for switching regulator applications. In
some applications, multiple capacitors have to be
paralled to achieve the desired ESR value. If tantalum
capacitors are used, make sure they are surge tested
by the manufactures. If in doubt, consult the capaci-
tors manufacturer.
Input Capacitor Selection
The input capacitor is chosen based on the voltage
rating and the RMS current rating. For reliable
operation, select the capacitor voltage rating to be at
least 1.3 times higher than the maximum input voltage.
The maximum RMS current rating requirement is ap-
proximately I
OUT
/2 , where I
OUT
is the load current.
During power up, the input capacitors have to handle
large amount of surge current. If tantalum capacitors
are used, make sure they are surge tested by the
manufactures. If in doubt, consult the capacitors
manufacturer.
For high frequency decoupling, a ceramic capacitor
between 0.1uF to 1uF can be connected between V
CC
and ground pin.
Inductor Selection
The inductance of the inductor is determined by the
output voltage requirement. The larger the inductance,
the lower the inductor’s current ripple. This will trans-
late into lower output ripple voltage. The ripple current
and ripple voltage can be approximated by:
I
RIPPLE
=
V
IN
- V
OUT
Fs x L
V
OUT
V
IN
x
V
OUT
= I
RIPPLE
x ESR
where Fs is the switching frequency of the regulator.
There is a tradeoff exists between the inductor’s ripple
current and the regulator load transient response time
A smaller inductor will give the regulator a faster load
transient response at the expense of higher ripple cur-
rent and vice versa. The maximum ripple current oc-
curs at the maximum input voltage. A good starting
point is to choose the ripple current to be approxi-
mately 30% of the maximum output current.
Once the inductance value has been chosen, select
an inductor that is capable of carrying the required
peak current without going into saturation. In some
type of inductors, especially core that is make of
ferrite, the ripple current will increase abruptly when it
saturates. This will result in a larger output ripple
voltage.
MOSFET Selection
The selection of the N-channel power MOSFETs are
determined by the R
DS(ON)
, reverse transfer capacitance
(C
RSS
) and maximum output current requirement.The
losses in the MOSFETs have two components: con-
duction loss and transition loss. For the upper and
lower MOSFET, the losses are approximately given
by the following :
P
UPPER
= I
out
(1+ TC)(R
DS(ON)
)D + (0.5)(I
out
)(V
IN
)(t
sw
)F
S
P
LOWER
= I
out
(1+ TC)(R
DS(ON)
)(1-D)
where I
OUT
is the load current
TC is the temperature dependency of R
DS(ON)
F
S
is the switching frequency
t
sw
is the switching interval
D is the duty cycle
2
2
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