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參數(shù)資料
| 型號： | IRU3072C |
| 廠商： | International Rectifier |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | 20-PIN SYNCHRONOUS PWM CONTROLLER/ 3 LDO CONTROLLER |
| 中文描述： | 20引腳同步PWM控制器/ 3 LDO控制器 |
| 文件頁數(shù)： | 11/24頁 |
| 文件大小： | 332K |
| 代理商： | IRU3072C |
IRU3072
11
Rev. 1.0
3/25/04
www.irf.com
The LDO and PWM output can be shutdown by using a
transistor to pull down the pin V
SEN33
/SDB as shown in
Figure 12. Because the V
SEN33
/SDB pin also senses the
LDO input voltage for the power UVLO block, a high im-
pedance resistor such as 4.7K has to be inserted be-
tween V
SEN33
/SDB pin and the input of LDO such as 3.3V.
The input voltage UVLO operation will not be affected
due to the high input impedance nature of V
SEN33
/SDB
pin. The operation waveforms is shown as follows:
Figure 13 - Shutdown by pulling down pin V
SEN33
/SDB.
One feature of shutdown by pulling down V
SEN33
/SDB is
that there is no negative voltage shown in the buck con-
verter output because both high side and low side
MOSFET drivers are off after shutdown.
Over Current Protection
The IRU3072 over current protection is achieved with a
cycle-by-cycle current limit and an output voltage under-
voltage lockout scheme. The diagram is shown in Figure
14. It includes an over current comparator and an output
voltage UVLO comparator. The current is sensed through
the R
DS(ON)
of the low side MOSFET. A resistor, R
SET
, is
connected from OCSet pin to the drain of the low side
MOSFET in order to set the over-current limit. When the
low side MOSFET Q2 is ON, the inductor current flows
through MOSFET Q2. The voltage at OCSet pin is given
as:
V
OCSet
=20
μ
A
×
R
SET
-i
L
×
R
DS(ON)
When voltage V
OCSet
is below zero, the current sensing
comparator flips and disables the oscillator. The high
side MOSFET is turned off and the low side MOSFET is
on until the inductor currents reduces to below current
setting value. The critical inductor current can be calcu-
lated by setting:
V
OCSet
= 20
μ
A
×
R
SET
-i
L
×
R
DS(ON)
= 0
I
SET
= i
L
(critical) = 20
μ
A
×
R
SET
/R
DS(ON)
---(2)
t
ON
(normal)
D
×
T
S(NOM)
= V
OUT
/(F
S(NOM)
×
V
IN
)
Figure 14 - IRU3072 current limit diagram.
The operation is illustrated in Figure 15.
Figure 15 - Operation of IRU3072 current limit
and UVLO.
During the normal operation mode, the synchronous buck
converter operates in fixed frequency F
S(NOM)
, which is
the normal operation switching frequency and it is deter-
mined by the external resistor Rt. The output voltage is
regulated to the desired voltage and the feedback volt-
age is equal to the reference voltage V
REF
. The turn on
time of the high side MOSFET is given as:
Enb
Oscillator
R
S
Q
3V
20uA
CS Comp
Err Comp
0.4V
IRU3072
V
BUS
OCSet
HDrv
LDrv
Fb1
R
SET
Q2
Q1
L
V
OUT
Rf1
Rf2
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)
V
OUT
F
S(NOM)
×
V
IN
I
O(NOM)
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