iru3072 International Rectifier Corp., iru3072 Datasheet - Page 12

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

IRU3072
As the load current goes up, the inductor current in-
creases and the high side MOSFET’s turn on time in-
creases a little due to the voltage drop across the high
side MOSFET R
As the output current increases to limit current, I
which is set by the resistor R
go into cycle-by-cycle current limit mode. The operation
waveforms of IRU3072 during cycle-by-cycle current
mode is shown in Figure 16.
12
Comparator Output
High Side MOSFET
Figure 16 - Cycle-by-Cycle operation when IRU3072
Inductor
Comparator Output
at normal operation
Current
at current limit
Current Limit
Internal Clock
(a) Normal operation.
(b). Operation at current limit mode.
Internal Clock
Current Limit
Driver HDrv
I
Inductor
Current
SET
Internal Clock
D
Driver HDrv
3
=i
MOSFET
L(VALLEY)
T
i
L(PEAK)
S(NOM)
i
L(AVG)
is in over-current limit mode.
i
L(AVG)
t
I
SET
O N
DS(ON)
T
.
S(NOM)
t
OFF
i
L(PEAK)
SET
i
L(VALLEY)
. The buck converter will
L
=I
O(LIM)
www.irf.com
,
From Figure 16, first, the high side MOSFET is on for t
period and the inductor current increases during this time.
Then, the high side MOSFET is off and low side MOSFET
is on. Because the inductor current is higher than the
critical inductor current I
parator goes high and the low side MOSFET keeps on.
The inductor current is discharged by the output voltage.
When the inductor current is below setting current or
critical current I
low and enables the oscillator. The high side MOSFET
is turned on again and next cycle starts. The operation
frequency is only dependent on the current sensing com-
parator and the internal clock frequency is modified by
current limit.
In conclusion, from Figures 15 and 16, two big differ-
ences exist between normal operation and current limit
mode. First, during current limit mode, the valley induc-
tor current is determined by I
Second, in Figures 15 and 16, the frequency in current
limit mode, is lower than normal operation frequency.
In general, the output current is represented by:
Figure 15 shows that the operation frequency of the buck
converter decreases as output current goes up during
current limit mode. The on time of high side MOSFET is
controlled by the output voltage loop so that the voltage
at Fb pin, still equals the reference voltage, V
The output voltage is regulated to the desired voltage.
As a result:
Where V
determined by the feedback resistor and reference volt-
age as shown in Figure 14. The above equation indi-
cates that the operation frequency is inversely propor-
tional to the output current during the current limit mode.
For practical application, the most important is setting
up the over current limit threshold. From Figure 15, at
the current limit threshold I
equal to nominal operation frequency.
I
I
Where DI
ripple which is given by:
DI
t
I
ON
SET
OUT
OUT(Current Limit Mode)
PK_PK
= V
= i
= i
O(NOM)
L(VALLEY)
L(AVG)
O(NOM)
= i
L(PK)
=i
PK_PK
SET
is the nominal output voltage and it is
/V
L(VALLEY)
-i
, the current sensing comparator goes
L(VALLEY)
IN
is the peak to peak inductor current
/F
= I
S
+DI
SET
SET
= (V
+
PK_PK
O(LIM)
, the current sensing com-
(V
SET
IN
IN
(23L3V
-V
/2
.
-V
, the frequency is still
OUT
O(NOM)
)3t
)3V
ON
IN
3Fs)
/L
O(NOM)
FB
Rev. 1.0
=V
3/25/04
REF
ON
.