L9935 datasheet(7/19 Pages) STMICROELECTRONICS

7 / 19 page

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 cur-

rent sense comparator comp 1.

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.

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

= 25kHz) are possible. In this case current flows

of both bridges will overlap (not shown in Fig. 5).

Reversing phase:

Suppose the current flowed via M21, the load and

M12 before reversing phase. Reversing phase

M21 and M12 will be turned off. So now the cur-

rent will flow through D22, the load and D11. This

leads to a fast current decay.

Chopper control by oscillator

Both chopping circuits work with offset phase.

One chopper will switch on the bridge at the

maximum voltage of the oscillator while the other

chopper will switch on the bridge at minimum

voltage of the oscillator.

MS1 and MS2 blank switching spikes that could

lead to errors of the current control circuit.

SRA

SRB

OSC

COSC

MS1

MS2

Comp1

inhibit

D99AT420

Comp2

Dr1

OSCILLATOR

Dr2

RES2

RES1

MOS DRIVERS

RESET

DOMINANT

RESET

DOMINANT

RSFF1

RSFF2

fOSC=

iOSC

2.46V · COSC

Figure 4. Principal chopper control circuit.

L9935

7/19

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