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参数资料
| 型号： | CS-51031D8 |
| 厂商： | ON SEMICONDUCTOR |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 700 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 0.150 INCH, SO-8 |
| 文件页数： | 6/8页 |
| 文件大小： | 156K |
| 代理商： | CS-51031D8 |
CS-51031
6
Applications Information
where VSAT = Rds(on) IOUT max. and Rds(on)is the value at
TJ 100C.
If VF = 0.60V and VSAT = 0.60V then the above equation
becomes:
DMAX =
= 0.62
DMIN =
= 0.40
2) Switching frequency and on and off time calculations
Given that fSW = 200kHz and DMAX = 0.80
T =
= 5s
TON(max) = T DMAX = 5s 0.62 @ 3s
TON(min) = T DMIN = 5s 0.40 = 2s
TOFF(max) = TON(min) = 5s - 2s = 3s
3) Oscillator Capacitor Selection
The switching frequency is set by COSC, whose value is
given by:
COSC in pF =
95
10-6
Fsw
(1+ -()
2
)
4) Inductor selection
The inductor value is chosen for continuous mode opera-
tion down to 0.3Amps.
The ripple current I = 2
IOUTmin = 2 0.3A = 0.6A.
Lmin =
=
=28H
This is the minimum value of inductor to keep the ripple
current to <0.6A during normal operation.
A smaller inductor will result in larger ripple current.
Ripple current at a minimum off time is
I =
=
=0.4A
The core must not saturate with the maximum expected
current, here given by:
IMAX = IOUT + I/2 = 3A+0.4A/2 = 3.2A
5) Output capacitor
The output capacitor and the inductor form a low pass fil-
ter. The output capacitor should have a low ESL and ESR.
Low impedance aluminum electrolytic, tantalum or organ-
ic semiconductor capacitors are a good choice for an out-
put capacitor. Low impedance aluminum are less expen-
sive. Solid tantalum chip capacitors are available from a
number of suppliers and are the best choice for surface
mount applications.
The output capacitor limits the output ripple voltage. The
CS-51031 needs a maximum of 20mV of output ripple for
the feedback comparator to change state. If we assume that
all the inductor ripple current flows through the output
capacitor and that it is an ideal capacitor (i.e. zero ESR), the
minimum capacitance needed to limit the output ripple to
50mV peak to peak is given by:
C =
=
= 7.5F
The minimum ESR needed to limit the output voltage rip-
ple to 50mV peak to peak is:
ESR =
=
= 83m
The output capacitor should be chosen so that its ESR is
less than 83m.
During the minimum off time, the ripple current is 0.4A
and the output voltage ripple will be:
V=ESR I = 83m 0.4 = 33mV.
6) VFB divider
VOUT = 1.25V
()=1.25V( +1)
The input bias current to the comparator is 4A. The resis-
tor divider current should be considerably higher than this
to ensure that there is sufficient bias current. If we choose
the divider current to be at least 250 times the bias current
this permits a divider current of 1mA and simplifies the
calculations.
= R1 + R2 = 5k
Let R2 = 1K
Rearranging the divider equation gives:
R1 = R2
(
- 1
)=1k( -1)=3k
5V
1.25
VOUT
1.25
5V
1mA
R1
R2
R1 + R2
R2
50
10-3
0.6A
V
I
0.6A
8
(200 103Hz) (50 10-3V)
I
8
fSW V
5.6V
2s
28H
(VOUT + VF) TOFF(min)
LMIN
5.6V
3s
0.6A
(VOUT + VD) TOFF(max)
I
30
10 3
Fsw
3
10 6
1
fSW
5.6
13.8
5.6
9
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