HMC1001/1002/1021/1022
The next circuit implementation is the classic set/reset design in which a push-pull output stage (totem pole stage) drives
one end of the HMC1001 set/reset strap, with the other end grounded. Figure 3 shows this circuit.
R1
200
C1
10U
VDD
8 Volts
R2
22K
X1
IRF7105P
C4
0.1U
Rsource
10
Vsource
C2
0.47U
Vsr
Rsr
1.5
X2
IRF7105N
C3
0.47U
Figure 3 – Totem Pole Set/Reset Circuit for HMC1001
The totem pole moniker comes from the stacked semiconductors between the positive supply voltage (VDD) and the
negative connection (Ground). In the above example circuit, the semiconductors depicted are two complementary power
MOSFETs, with the P-channel device on top and the N-channel device on the bottom. The International Rectifier IRF7105
part is chosen in this circuit as it contains both P-channel and N-channel MOSFET die in a very small package, and has
the electrical characteristics needed for this circuit. Other manufacturers can be used as well with the requirements that
they can be fully turned on/off with a 5-volt logic stimulus, handle the peak set/reset strap load currents, and present an
“on” resistance at those peak currents that is fairly small in comparison to the connected strap load resistance.
HIGHER VOLTAGE TOTEM POLE CIRCUITS
While the previous example uses the convenience o standard 5
f
-volt logic drive and modest supplies, many sensor
designs require higher applied voltages to the set/reset straps to achieve greater currents or because the straps are
series connected to assure even current distribution across all the straps pulsed. By creating series chains of straps,
variances in strap resistance are less likely to fall out of the minimum or maximum range for peak pulse currents. If the
straps are parallel connected, wide set/reset strap ohmic tolerances may be prone to “current hogging” and the straps will
provide dissimilar magnetic fields at each sensor, potentially creating non-uniform accuracies at each sensor axis.
The circuit in Figure 3 relies on MOSFETs that could predictably be turned off and on completely using logic level inputs.
At higher voltages, the P-channel device needs its gate drive voltage to approach the source voltage, which is higher than
usual logic levels. To perform this level shifting from logic levels to higher pulse source voltage supply levels, a BJT level
shifter sub-circuit is employed to perform this task. Figure 4 shows this higher voltage operating circuit.
From Figure 4, Rsr1, Rsr2, and Rsr3 are three strap resistances that are modeled from the HMC1001 or HMC1002
products. Three of these strap resistances are chosen since many users desire 3-axis magnetic field sensing that comes
from a pairing of a HMC1001 and a HMC1002. Also this combination of three series straps is also used on the HMC2003
hybrid sensor module and in the HMR2300 Smart Digital Magnetometer.
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