Flipping Generator (Block1); Figure 16 Flip Circuit With Active Current Limiting - Philips KMZ51 Application Note

Philips Semiconductors
Electronic Compass Design using
KMZ51 and KMZ52
capacitor of some µF with low series resistance (e.g. Ta Elco) is recommended here. The time constant R21·C9
must allow C9 to charge up to approximately Vcc between the positive pulses, i.e. during the period of the clock
generator. The 5V sensor supply is derived from Vcc by series resistors R1 and R2. A symmetrical OP-Amp
supply is provided by connecting the OP-Amp supply pins to Vcc and GND respectively and connecting their
reference inputs to Vcc/2, generated by the Vref generator.

5.5.2 Flipping generator (block1)

This block generates the flipping current pulses at a repetition frequency, determined by R16 and C3. The
frequency is not critical, considering the flipping function itself. The choice of frequency is a trade-off between
average current consumption on the one hand and response time and output ripple on the other hand. A
frequency of approximately 1 kHz has proved to be a reasonable compromise. As the output of OP1 passes
from high to low, C4/R17 produce a pulse that switches TR2 on. This charges C6 and a short positive pulse is
passed to the flipping coil. For a low-to-high transition at the output of OP1, C5/R18 forces TR1 to conduct,
discharging C6 and providing a negative current pulse through the coil. KMZ51 and KMZ52 both require flip
current pulses of typically ±1A for a duration of 3µs. To drive that current controlled by the OP output, TR1 and
TR2 should be Darlington transistors. For a complete SCU, the flipping generator is only required once and the
flip coils of both sensors are connected in series. The function of decoupling network R21-C9 is described under
supply voltage considerations.
For product development, it is recommended to use an improved flipping circuit like that shown in Figure 16.
Here, an active current limiting provides, that the magnitude of the flip current pulses is independent of flip coil
resistance. Thus, this circuit is insensitive to spread of flip resistance or supply voltage.
Vcc
BC557A/PLP
R17
10k
C4
10n BAS32
C5
10n
BC547A/PLP
R18
10k
0
Figure 16 Flip circuit with active current limiting
R100
0.7
Q4
2
D1
Q2
0 BST62
1
1
D2
0
Q1
BAS32 BST52
Q3
2
R101
0.7
0 0
22
Application Note
AN00022
C6
1uF
Rflip
0