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Automotive 12V to USB 5V 2A
output power adapter test results.
Normal operation
Nominal output voltage is given by Vout = Vfb*(1+Rtop1/Rbot1))
Vfb (V)
3.36
Rtop1 (Ohms)
1200
Rbot1 (Ohms)
2400
Vout_nom (V)
5.04
Measured output voltage and input current with resistive load.
Input
voltage
(V)
Load
Output
current voltage
(A)
(V)
Supply
current
(A)
Overall efficiency (%)
12
0
5.032
0.008
na
12
0.893
5.028
0.423
88.5
12
1.599
5.023
0.786
85.2
12
2.157
5.018
1.1
82.0
Overall efficiency includes the input current drawn by the input LED and the output currents drawn
by the TL431 resistor ladder and the output LED.
1
Overvoltage clamp operation
Output clamp voltage in response to regulator output voltage being forced to rise above normal set
voltage by placing 1k in parallel with R7 (Rbot1)
Rbot1//1k (Ohms)
Input
voltage (V)
705.9
Load
current
(A)
Output
voltage
(V)
Unclamped
forced Vout
0
9.072
Clamped
forced Vout
0
5.61
Supply current (A)
12
1.436
In this test, the instantaneous, pulse-by-pulse output current sunk in the clamp is much higher than
the measured (average) supply current.
Therefore the output of SMPS goes into pulse-by-pulse current limiting which limits the average
current drawn from the input supply.
In the event of the series switching MOSFET in the L4978 failing short circuit, the battery would be
effectively connected straight across the clamp.
A large current would then be sunk by the clamp, limited only by the resistances of the wiring,
chokes and the PCB tracks.
The STP36NF06L MOSFET used in the clamp is a 30A continuous/120A pulsed rated device.
After a few milliseconds, this current would blow the fuse.
2