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参数资料
| 型号： | APW7093QAI-TRL |
| 厂商： | Anpec Electronics Corporation |
| 英文描述： | 3A, 1MHz, Step Down DC/DC Regulator |
| 中文描述： | 第3A，1MHz的，下步直流/直流稳压器 |
| 文件页数： | 12/20页 |
| 文件大小： | 728K |
| 代理商： | APW7093QAI-TRL |
Copyright
ANPEC Electronics Corp.
Rev. A.5 - Jun., 2005
APW 7093
www.anpec.com.tw
12
Application I nformation( Cont.)
Programming the Switching Frequency and
Off-Time and On-Time
The APW7093 features a programmable PWM-mode
switching frequency, which is set by the input and
output voltage and the value of R
TOFF
, connected from
T
OFF
to GND. R
TOFF
sets the PMOS power switch
off-time in PWM mode. Use the following equation to
select the off-time while sourcing current according to
the desired switching frequency in PWM mode:
(
)
(
)
NMOS
V
PMOS
V
IN
SW
f
PMOS
V
+
OUT
V
IN
V
OFF
T
V
=
where:
T
OFF
= the programmed off-time
V
IN
= the input voltage
V
OUT
= the output voltage
V
PMOS
= the voltage drop across the internal PMOS
power switch
|
I
OUT X
R
PMOS
|
V
NMOS
= the voltage drop across the internal NMOS
synchronous-rectifier switch
|
I
OUT X
R
NMOS
|
f
SW
= switching frequency
Make sure that T
ON
and T
OFF
are greater than 400ns
when sourcing current. Select R
TOFF
according to the
formula:
)
s
/1.00
(109k
s)
0.18
(T
R
OFF
TOFF
m
m
×
=
Setting the Output Voltage (Cont.)
Recommended values for R
TOFF
range from 24k
410k
for off-times of 0.4μs to 4μs.
Often the switching
frequency is set as high as possible, and the inductor value
is reduced to minimize the energy transferred from
inductor to capacitor during load-step recovery.
The operating frequency of the APW7093 is determined
primarily by T
OFF
(set by R
TOFF
), V
IN
, and V
OUT
as
shown in the following formula:
(
(
PMOS
IN
OFF
V
V
T
to
)
)
NMOS
V
PMOS
V
+
OUT
V
IN
SW
f
V
=
However, as the output current increases, the voltage
drop across the NMOS and PMOS switches increases
and the voltage across the inductor decreases. This
causes the frequency to drop. Assuming R
PMOS
=
R
NMOS
, the change in frequency can be approximated
with the following formula:
I
=
OFF
T
IN
PMOS
R
OUT
V
SW
f
×
×
where R
PMOS
is the resistance of the internal
MOSFETs (70m
typ).
APW7093
EXTREF
FB
LX
V
OUT
=V
EXTREF
V
EXTREF
FIG.4 Adjsting the Output Voltage using EXTREF
APW7093
EXTREF
FB
LX
R2
R1
V
OUT
REF
FIG.5 Adjsting the Output Voltage using FB
=
1
V
V
1
R
2
R
EXTREF
OUT
where V
EXTREF
= V
REF
= 1.1V
1.7V
V
V
1.1V
IN
EXTREF
≤
≤
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