ST AN2229 APPLICATION NOTE

AN2229

APPLICATION NOTE

Startup Behavior L9903/L9904

Introduction

Purpose of the Application Note is to detail the startup behaviour of L9903 and L9904 devices and to
explain a correct initialization sequence, possibly being necessary when application configuration is
different from the one suggested inside datasheet.

In fact, in case an additional, external diagnostic structure is used, it may happen that after a first
connection of the supply voltage, and before first enabling of the device, the measurement results of
these diagnostic structure is not precise.

AN2229/10-05 1/8

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AN2229

Contents

1 Explanation of 2 possible Startup Behaviors . . . . . . . . . . . . . . . . . . . . . . . 3

2 Application example using standby voltage monitoring . . . . . . . . . . . . . . 4

3 Proposed initialization sequences for L9903: . . . . . . . . . . . . . . . . . . . . . . . 5

4 Proposed initialization sequences L9904: . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

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AN2229 1 Explanation of 2 possible Startup Behaviors

1 Explanation of 2 possible Startup Behaviors

Starting condition: the device is disconnected from the battery.

After switching on V
startup states:

a) Gate of high side driver is internally switched to ground (Fig. 1).

b) Gate of high side driver is internally switched to CBx (Fig. 2).

If the voltage (C

Figure 1. Startup behavior internal switched

to ground (state 1)

) exceeds 3.5V the device switches always like shown in figure 1

Boot

, as long as the voltage over C

S

is below 3.5V, the device can have 2

Boot

Figure 2. Startup behavior internal switch to

CBx (state 2.

After the first VS ramp up the outputs will enter one of the explained states.

If the device enters state 2 and the threshold of the external MOSFET is below 3.5V, the Sx pin will raise
and the charging of the bootstrap capacitor will be stopped. This state will persist until the L9903/04 will
be enabled by the EN pin. The only consequence of entering state 2 is an increased leakage current.
There is no influence on the standard application suggested inside datasheet. There is no influence on
operating condition (EN=HIGH). If the application requires a startup behavior like in figure 1, an
initialization is needed (see sections

3

and 4).

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