MB39A104_06 Datasheet, PDF (18/38 Pages) Fujitsu Component Limited. – 2-ch DC/DC Converter IC with Overcurrent Protection

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.
ON interval 450 (ns)
≤
VO (V)
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
VIN
(Rs)
Error Amp
Q1
21 ^(VS2)
4 ^VS1
Overcurrent Detection Function Operating Range
1600
1400
1200
VO = set to 3.3 V
1000
800
(ILIM2)
20 _ILIM1
5
Connect to RS
when
using RS
400
200
0
6
Operation Range
8
10 12 14 16 18 20
VCC (V)
• Method to detect by mean current (Possible to detect at 2 V or more of output voltage)
VIN
Q1
21 ^(VS2)
4 ^VS1
(ILIM2)
20 _ILIM1
5
Error Amp
RS
Overcurrent Detection Function Operating Range
1600
1400
1200
VO = set to 3.3 V
1000
800
600
Operation Range
400
200
0
6
8
10
12
14
16
18
20
VCC (V)
18