MB39A104
Overcurrent Protection Circuit: Range of Operation
When an overcurrent flow occurs, if the increased voltage between the drain and source of the FET is detected
by means of the external FET (Q1) resistor, operational stability is lost when the external FET (Q1) ON interval
determined by the oscillation frequency, input voltage, and output voltage falls below 450 ns.
Therefore, the circuit should be used within a range that ensures that the ON interval does not fall below 450ns,
according to the following formula.
VO (V)
ON interval 450 (ns) ≥
VIN (V) × fOSC (Hz)
If the ON interval of the external FET (Q1) is below 450ns, we recommend the use of an overcurrent detection
resistor RS to detect overcurrent, as shown below.
This example shows the range of operation of the overcurrent detection function with a setting of Vo = 3.3V.
• Method to detect by current when external FET(Q1) is turned on
Overcurrent Detection Function Operating Range
1600
VIN
(Rs)
ErrAmp
Q1
1400
VO = set to 3.3 V
(VS2)
1200
1000
800
400
200
0
21
4
VS1
Operation Range
(ILIM2)
20
−
+
ILIM1
ConnecttoRS
when
5
using RS
6
8
10
12
14
16
18
20
VCC (V)
• Method to detect by mean current
Overcurrent Detection Function Operating Range
VIN
ErrAmp
1600
RS
Q1
1400
VO = set to 3.3 V
(VS2)
21
1200
VS1
4
1000
800
Operation Range
600
400
200
0
(ILIM2)
−
+
20
ILIM1
5
6
8
10
12
14
16
18
20
VCC (V)
18
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