AN279
APPLICATION NOTE
SHORT CIRCUIT PROTECTION ON L6203
by G. Scrocchi and G. Fusaroli
With devices like L6203 used as driver often interfacing the external world by means of wires, can be easy to
have short circuits.
A short circuit can occur for many reasons : a short on the load, a mistake during the connection of the wires
between the device and the load (i.e. L6203 driving a motor), an accidental short between the wires and so on.
The outputs of L6203 are not protected against the short circuit and if a short occurs, the big amount of current
flowing through the outputs can destroy the device.
To avoid this risk can be useful to add a circuitry to protect the device : in this case, to have a total protection,
we must consider three types of short circuit :
1 - output to output short circuit
2 - output to supply voltage short circuit
3 - output to ground short circuit
The first step is to sense the short circuit current. In output to output (fig 1) or output to supply (fig 2) short circuit
can be used the sensing resistor (Rsl) already used to set the current flowing in the load during the normal operation.
Figure 1.
Figure 2.
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AN279 APPLICATION NO TE
To sense the output to ground short circuit (fig 3) another sensing resistor (Rsu) must be added betw een the
supply pin and the supply voltage.
Figure 3.
The second step is to create a threshold over which the value of the current must be considered as short circuit:
for this way a transistor or a diode could be used.
The complete protection will be given by the ’ or ’ of the signal coming from the upper and the lower sensing
circuitry ; this signal can be used to act on the ENABLE pin of the L620 3 disabling the output stages. A com plete
protection schematic diagram is shown in fig. 4.
Figure 4.
In normal operation the circui t works up to 3A/40V. When a short circuit occur s the SCR is triggered and L6203
disabled : due to the SCR memory L6203 is kept disabled until the power is switched off and then on, if the
cause of short was removed.
The short circuit is detected when :
V
I
SU
BE T1()
-------------------- -
>
R
SU
0.6
------- - 6A==
0.1
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AN279 APPLICATION NOTE
VDV
+
----------------------------------------
>
I
SL
THSCMT
R
SC
The effective short circuit peak current is greater than Isu and Isl : this is due to the high dI/dT during the short
and to the delay between the short circuit detection and the ENABLE intervention :
Rsu and Rsl must be non inductive resistors.
R1 and R2 are used to scale the signal when the tr ansi stor goes on and in conj unction with C 1 to fil ter the shor t
circuit signals in order to avoid false trig ger of the SCR : this filtering should not be too much heavy to avoid to
introduce an excessive delay in the short circuit loop.
Isu and Isl must be calc ulated at the effect ive operatin g temperatur e being the Vbe and Vd temper ature dependent.
Instead of the SCR, a monostabl e wi th a long time c onstant (0.3 ¸ 0.5 sec) can be used : in the case, every time
a short circuit occurs, L6203 is disabled for the monostable time constant and then enabled, if the short is still
present L6203 is disabled again, if the short was removed L6203 returns in normal operation (fig 5).
Figure 5.
0.6 0.7+
---------------------- - 7.8A==
0.165
R
, R1, C1, RSU, RSL are choosen depending on the application.
1
The intervention of the protection circuit is determined by
-----------
I
> I
SU
R
V
= V
D
DIODE
V
BE
;
SU
; VIH = min VInput High T+Monostab
SL
VIHVD+
---------------------- -
>
R
SL
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AN279 APPLICATION NO TE
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implic ation or otherwise under any patent or paten t rights of ST M i croelectronics. Speci fications me ntioned in this publicat i on are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as crit i cal component s i n l i f e support devi ces or systems wi t hout express wri t ten approval of STMicroelectronics.
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