apu3073o APEC, apu3073o Datasheet - Page 7

• Manufacturer Part Number: apu3073o
• Description: Synchronous Pwm Controller With Over-current Protection / Ldo Controller
• Manufacturer: APEC

If the over-current condition is temporary and goes away
quickly, the APU3073 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.
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
parator keeps high until the inductor current decreases
to be below I
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:
High Side MOSFET
turn on time (t
Average Inductor
i
I
I
L(VALLEY)
L(PEAK)
SET
Figure 6 - Diagram of over-current operation.
Feedback
voltage
Current
/ I
Switching
frequency
= I
O(NOM)
= I
SET
SET
ON
V
0.4V
SET
REF
)
. Then another cycle starts.
+(V
- DI
F
IN
F
S(NOM)
operation
PK-PK(NOM)
Normal
S(NOM)
-V
<I
V
L
2
OUT
OUT
>=I
3 V
OUT
)3t
IN
I
O(LIM)
ON
SET
/L
), the over-current com-
Over Current
Limit Mode
I
O(MAX)
---(3)
D
MAX
/F
Shutdown
by UVLO
S(NOM)
I
I
I
OUT
I
OUT
OUT
OUT
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
From the above analysis, the current limit is not only
dependent on the current setting resistor R
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 APU3073 is
powered up again.
Comparator Output
Inductor
Current
Figure 7 - Operation waveforms during current limit.
R
I
DI
I
Current Limit
Where:
I
higher than nominal output current.
V
V
f
L = Output Inductor
R
I
SET
O(LIM)
SET
O(LIM)
S
OCSET
IN
OUT
DS(ON)
PK-PK(LIM)
= Switching Frequency
I
SET
=
= I
= Maximum Input Voltage
=i
= Output Voltage
= I
R
= The Output Current Limit -typical is 50%
O(LIM)
= OC Threshold Set Current
L(VALLEY)
I
i
OCSET
DS(ON)
L(PEAK)
= R
i
L(AVG)
SET
HDrv
=
-
+
DS(ON)
(
(V
3 I
(V
DI
IN
23f
[
t
SET
ON
IN
PK-PK(LIM)
V
- V
O(LIM)
of Low Side MOSFET
-V
IN
2
and output current limit:
S
3L3f
OUT
OUT
3L3V
-
t
OFF
)3V
)3V
(
(V
S
23f
IN
OUT
OUT
IN
-V
---(4)
)
S
OUT
3L3V
APU3073
)3V
SET
OUT
IN
---(5)
)]
and R
---(6)
DS(ON)
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