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TD230

ELECTRONIC CIRCUIT BREAKER

STMicroelectronics 

TD230

where CISS is the input capacitance of the power

MOSFET1/2 and VTH, the threshold voltage of the

MOSFET (typically 5V).

This reconduction time can be extended with an

external soft start capacitor CSS1/2 as shown in

figure 1 CISS will therefore simply be replaced by

CISS + CSS 1/2.

Figure 1 : Dual Electronic Circuit Breaker

Application

RS 1

Vcc+

LBOO S T

CBO OS T

GND

CTRIP 1

CTRIP 2

Vcc-

P Vcc

1

R EF1

LBOOS T

GC1

2

CB OOS T

S E NSP

3

O S C GND

INHIBIT

4

5

P M1

S HUTDOWN

6

GND

S ENSN

7

P M2

NVcc

8

GC 2

R EF2

16

NMos

15

CSS 1

to BOARD

14

13

C ONTRO L

12

from BO ARD

11

NMOS

10

CS S2

9

RS2

If the fault (over current condition) still remains

after the reconduction state of the MOSFET1/2 has

been reached, the current through NMOS1/2 will

overpass the limitation given by equation (i), and

the NMOS 1/2 will immediately be switched off

again.

Figure 2 shows the current limitation which is

operated on every restart attempt.

Figure 2 : TD230 as Current Limitor

Trace A represents the Gate-Source Voltage of the

Power Mosfet (0 to 13,4V).

Trace B represents the voltage across the Sense

Resistor (68mΩ) in direct relation with the current

through it (0 to ~1A).

Note that the first current peak which is due to an

over current is limited only by the reaction time of

the TD230.

This off time is tied to the value of the external soft

start capacitor CSS 1/2 by equation (iii) :

• tOFF = RDSON x CSS (iii)

While in current limitation mode, the NMOS1/2 dis-

sipates low power due to the fact that the ON/OFF

cycle time rate is very low.

Note that the higher the value of CSS1/2 are, the

more the NMOS1/2 will stay in linear mode during

current limitation.

Note that at Power ON, or in the case of live

insertion, the inrush current is automatically limited

thanks to the slow gate charge of the MOSFET

which switches ON softly due to the time constant

given in equation (ii).

2.2. Fault Time Limitation

The repetitive switching off of the MOSFET will

come to an end under two conditions :

• either the fault has disappeared, and the cur-

rent through the shunt resistors RS 1/2 has

come back to its nominal value : the system

keeps running normally.

External line defaults (lightning, line breakage,

etc...) are usual causes for such temporary over

currents.

• either the repetitive switching off has lasted

over an externally adjustable time and the

TD230 has definitively switched off the corre-

sponding NMOS : the system waits to be re-

set.

Equipment faults (component short circuit, over

heat, etc ...) are usual causes for lasting over

currents.

This fault time supervision is done by the compari-

son of the output voltage to 75% of the nominal

supply voltage. As soon as the output voltage is

detected under 0.75xVcc(+/-), the corresponding

external capacitors CTRIP1/2 is charged by a fixed

current source IP/N2 - IP/N3 (3µA). When the voltage

across CTRIP1/2 reaches 1.20V, the corresponding

NMOS is definitively switched off and the SHUT-

DOWN pin is active low.

7/15

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