FUJITSU MB39A104 - Manual (Page 18)

MB39A104
18
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.
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.
ON interval 450 (ns)
≤
V
O
(V)
V
IN
(V)
× f
OSC
(Hz)
−
+ 20
4
21
ILIM1
(ILIM2)
VS1
(VS2)
V IN
Error Amp
Q1
5
(Rs)
1600
1400
1200
1000
800
400
200
0
6 8 10 12 14
VCC (V)
f OSC
(kHz)
16 18 20
Overcurrent Detection Function Operating Range
V
O
= set to 3.3 V
Operation Range
• Method to detect by current when external FET(Q1) is turned on
Connect to RS
when
using RS
−
+
20
4
21
ILIM1
(ILIM2)
VS1
(VS2)
VIN
R S
Error Amp
Q1
5
1600
1400
1200
1000
800
600
400
200
0
6 8 10 12 14
VCC (V)
f OSC
(kHz)
16 18 20
Overcurrent Detection Function Operating Range
V
O
= set to 3.3 V
Operation Range
• Method to detect by mean current ( Possible to detect at 2 V or more of output voltage )