iru3072 International Rectifier Corp., iru3072 Datasheet - Page 11

• Manufacturer Part Number: iru3072
• Description: 20-pin Synchronous Pwm Controller/ 3 Ldo Controller -
• Manufacturer: International Rectifier Corp.

The LDO and PWM output can be shutdown by using a
transistor to pull down the pin V
Figure 12. Because the V
LDO input voltage for the power UVLO block, a high im-
pedance resistor such as 4.7K has to be inserted be-
tween V
The input voltage UVLO operation will not be affected
due to the high input impedance nature of V
pin. The operation waveforms is shown as follows:
One feature of shutdown by pulling down V
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
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:
When voltage V
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:
Rev. 1.0
3/25/04
Figure 13 - Shutdown by pulling down pin V
V
V
I
SET
DS(ON)
OCSet
OCSet
SEN33
= i
= 20mA3R
=20mA3R
L
of the low side MOSFET. A resistor, R
(critical) = 20mA3R
/SDB pin and the input of LDO such as 3.3V.
OCSet
is below zero, the current sensing
SET
SET
-i
-i
L
SEN33
3R
L
3R
/SDB pin also senses the
DS(ON)
DS(ON)
SET
SEN33
/R
= 0
/SDB as shown in
DS(ON)
SEN33
SEN33
---(2)
SEN33
/SDB is
/SDB.
SET
/SDB
www.irf.com
, is
The operation is illustrated in Figure 15.
During the normal operation mode, the synchronous buck
converter operates in fixed frequency F
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
time of the high side MOSFET is given as:
Oscillator
t
ON
Figure 15 - Operation of IRU3072 current limit
High Side MOSFET
Enb
IRU3072
Figure 14 - IRU3072 current limit diagram.
turn on time (t
(normal) D3T
Average Inductor
S
R
Err Comp
Feedback
voltage
Current
Q
Switching
frequency
CS Comp
ON
V
0.4V
REF
)
F
20uA
F
S(NOM)
operation
and UVLO.
Normal
S(NOM)
<I
S(NOM)
V
L
OUT
>=I
3V
3V
OCSet
OUT
0.4V
IN
I
= V
O(NOM)
Over Current
Limit Mode
OUT
HDrv
LDrv
Fb1
/(F
I
R
O(LIM)
SET
S(NOM)
V
IRU3072
REF
B U S
S(NOM)
Q1
Q2
Shutdown
by UVLO
. The turn on
3V
F
L
D
S(NOM)
MAX
, which is
I
I
I
OUT
I
OUT
OUT
OUT
IN
Rf1
Rf2
V
)
O U T
11