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參數(shù)資料
| 型號： | IRU3073 |
| 廠商： | International Rectifier |
| 英文描述： | SYNCHRONOUS PWM CONTROLLER WITH OVER-CURRENT PROTECTION / LDO CONTROLLER |
| 中文描述： | 同步PWM控制器，帶有過電流保護/ LDO控制器 |
| 文件頁數(shù)： | 7/21頁 |
| 文件大?。?/td> | 236K |
| 代理商： | IRU3073 |
IRU3073
7
Rev. 1.0
09/17/03
www.irf.com
If the over-current condition is temporary and goes away
quickly, the IRU3073 will resume its normal operation.
If output is shorted or over-current condition persists,
the output voltage will keep going down until it is below
0.4V. Then the output under-voltage lock out comparator
goes high and turns off both MOSFETs. The operation
waveforms are shown in Figure 6.
Figure 6 - Diagram of over-current operation.
Feedback
voltage
Switching
frequency
High Side MOSFET
turn on time (t
ON
)
Average Inductor
Current
I
OUT
I
OUT
I
OUT
I
OUT
D
MAX
/F
S(NOM)
F
S(NOM)
0.4V
V
REF
<I
L
>=I
OUT
Normal
operation
Over Current
Limit Mode
Shutdown
by UVLO
I
O(LIM)
I
O(MAX)
V
OUT
F
S(NOM)
×
V
IN
Operation in current limit is shown in Figure 7, the high
side MOSFET is turned off and inductor current starts to
decrease. Because the output inductor current is higher
than the current limit setpoint (I
SET
), the over-current com-
parator keeps high until the inductor current decreases
to be below I
SET
. Then another cycle starts.
During over-current mode, the valley inductor current is:
The peak inductor current is given as:
To avoid undesirable trigger of over-current protection,
this relationship must be satisfied:
I
SET
/
I
O(NOM)
-
I
PK-PK(NOM)
2
i
L(VALLEY)
= I
SET
I
L(PEAK)
= I
SET
+(V
IN
-V
OUT
)
×
t
ON
/L ---(3)
I
SET
=i
L(VALLEY)
i
L(PEAK)
i
L(AVG)
t
ON
t
OFF
Current Limit
Comparator Output
Inductor
Current
HDrv
I
SET
= I
O(LIM)
- (V
2
×
f
S
×
L
×
V
IN
(
)
IN
-V
OUT
)
×
V
OUT
(V
IN
- V
OUT
)
×
V
OUT
V
IN
×
L
×
f
S
I
PK-PK(LIM)
=
I
O(LIM)
= I
SET
+ ---(4)
I
2
R
SET
= I
×
I
O(LIM)
- (V
R
OCSET
[
(
)]
IN
-V
OUT
)
×
V
OUT
2
×
f
S
×
L
×
V
IN
Figure 7 - Operation waveforms during current limit.
From Figure 7, the average inductor current during the
current limit mode is:
The inductor's ripple current can be expressed as:
Combination of above equation and (4) results in:
Combination of equations (5) and (2) results in the rela-
tionship between R
SET
and output current limit:
From the above analysis, the current limit is not only
dependent on the current setting resistor R
SET
and R
DS(ON)
of low side MOSFET but it is also dependent on the
input voltage, output voltage, inductance and switching
frequency as well.
The cycle-by-cycle over-current limit will hold for a cer-
tain amount of time, until the output voltage drops below
0.4V, the under-voltage lock out activates and latches
off the output driver. The operation waveform is shown in
Figure 4. Normal operation will resume after IRU3073 is
powered up again.
Where:
I
O(LIM)
= The Output Current Limit -typical is 50%
higher than nominal output current.
V
IN
= Maximum Input Voltage
V
OUT
= Output Voltage
f
S
= Switching Frequency
L = Output Inductor
R
DS(ON)
= R
DS(ON)
of Low Side MOSFET
I
OCSET
= OC Threshold Set Current
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