L9935

Functional description

Doc ID 5198 Rev 10

13/29

5.2

Full bridge function

Figure 5.

Displays a full bridge including the current sense circuit.

5.2.1

No current

Bit 5, bit 4 (corresponding bit 2 and bit1 for bridge B) both are HIGH, the current logic will

inhibit all drivers D11, D12, D21, D22 turning off M11, M12, M21, M22 independently from the

signal of the current sense comparator comp 1.

5.2.2

Turning on

Changing bit 5 or bit 4 or both to LOW will turn on either M11 and M22 or M21 and M12

(depending on the phase signal bit 3). Current will start to flow through the load. The current

will be sensed by the drop across R1.

The threshold of the comparator comp 1 depends on the current settings of bit 5 and bit 4.

The current will rise until it exceeds the turn off threshold of comp 1.

5.2.3

Chopping

Exceeding the threshold of comp 1 the drive logic will turn off the sink transistor (M12 or

M22). The sink transistor periodically is turned on again by the oscillator. Immediately after

turning on M12 or M22 the comparator comp 1 will be inhibited for a certain time to blank

switch over spikes caused by capacitive load components up to 5 nF.

Turning off for example M12 will yield a flyback current through D11. (So now the free

wheeling current flows through M21, the load and D11).

This leads to a slow current decay during flyback. Maximum duty cycles of more than 85%

(at fOSC = 25 kHz) are possible. In this case current flows of both bridges will overlap (not

shown in Figure 7).