SEA05 Datasheet, PDF (5/10 Pages) STMicroelectronics – Advanced constant voltage and constant current controller

SEA05
5
Application information
Application information
Note:
Figure 4. Application information
ȝ)
9
9


9


9FF

 2XW
 9FWUO
 ^*QG

 P9 ,FWUO

,VHQVH
5
9R
5
9FVWK
5VHQVH
,
92 ^{5  5}  9
5
,2 PD[ # ^{ 9}
5VHQVH
A 15 Ω resistor in series to Ictrl pin helps to protect the IC in case of negative voltage that
exceed the AMR of Ictrl pin.
As example a potential dangerous phenomenon could happen during converter output
short-circuit.
Consider the steady state operation of the circuit during voltage mode regulation (i.e. the
output is at its nominal voltage). The output capacitor is fully charged at V_o. If an abrupt
short (i.e. with negligible impedance) is applied at the output, instantly the positive pin of the
electrolytic capacitor is connected to the SEA05 ground. Since the capacitor acts like a
battery, all its voltage is applied across the R_sense pin and therefore the Ictrl pin is pulled
down to –Vo. This could damage the IC in case the Ictrl pin AMR is violated.
In reality the short is not so severe because it has a some impedance, the electrolytic
capacitor has an ESR and it starts discharging as soon as the short is applied. The Ictrl pin
is brought to a negative voltage anyway. The pin internal structure has been design to be
robust against negative voltage but, since the severity of this phenomenon is proportional to
the output voltage, for some applications an external resistor in series with Ictrl pin helps
protect the IC.
The resistor added in series with Ictrl pin introduces an error in the current sense threshold
voltage.
This error can be calculated considering the Ictrl pin current: this current multiplied by the
value of the external resistor gives the current sense threshold variation.
As example if we add a 15 Ω resistor in series to Ictrl pin, we have
I_ctrl current = Ibias = 6 µA and therefore the error
6 µA x 15 Ω = 80 µV, the error is 80 µV / 50 mV = 0.16%
Doc ID 17014 Rev 1
5/10