HMC1021S-TR Honeywell, HMC1021S-TR Datasheet - Page 10

• Manufacturer Part Number: HMC1021S-TR
• Manufacturer: Honeywell

Specifications of HMC1021S-TR

• Lead Free Status / Rohs Status: Compliant

HMC1001/1002/1021/1022
The three strap resistances are chosen at 1.8 ohms, or the worst-case high resistance points. Since they require a
minimum of 3 amperes peak, the series combination requires at least 16.2 volts, so a circuit Vdd of about 18 volts would
about the right level to drive the strap load and allocate for losses in the C1 capacitor ESR and the MOSFET switches X1
and X2. C1’s value is also chosen at 0.22 micro-farad so the circuit time constant is at least around 1 micro-second.
Supply reservoir capacitor C2 is chosen to many times the value of C1 and is also picked for small size, working voltage,
and low ESR relative to the strap load resistance. C2 typically will be in the 1 to 10 micro-farad range and best to error on
the high capacitance side since C2 now supplies additional X1 gate drive circuitry. Resistor R5 is then chosen after C2 to
set the recharge time constant and to limit peak supply current. These capacitors should be chosen to have a low ESR
characteristic of around 0.2 ohms per capacitor.
Working backwards from the strap load resistance, MOSFETs X1 and X2 are chosen as IRF7105 due to the total
packaged size (both X1 and X2 in one SOIC-8), and meeting the requirements for operating voltages, peak currents, and
low on resistances. X2 is directly driven from digital logic denoted as “Vset”, and “Vreset” drives the level shifting sub-
circuit to X1. Note that Vreset turns off X2 first prior to X1 being driven on by Vset, and also X1 is turned off before X2 is
turned on. While one logic line could control the operation of Vset and Vreset, the additional inverter stages and pulse
delay components may be too space and cost consuming compared to two logic ports in a microcontroller. See Figure 4
in Application Note 201 for the discrete Vset and Vreset pulse forming circuit.
Transistors Q1 and Q2 in Figure 4 are chosen to be generic BJTs to force MOSFET X1’s gate charge quickly into on and
off states. Resistors R1 and R2 are selected as nominal 1000 ohm values that can pump or dump X1’s gate charge by
supplying Q1 and Q2 with enough base drive currents to flip their on and off states. Transistor Q3 is also chosen as a
generic, but reasonably fast switch transistor to perform the level shift function with resistors R1 and R2. Components R3,
C3 and D1 are chosen to properly drive Q3 from a logic level source, with C3 and D1 denoted as a “speed-up” network to
quickly switch Q3 within a few nanoseconds of logic transitions.
10
Vset
Figure 4 – Higher Voltage Set/Reset Circuit for HMC1001 & HMC1002
7
C3
1N
R3
1000
D1
1N4148
6
4
Q3
2N2222
R1
1000
R2
1000
3
Vreset
Q1
2N2222
Q2
2N2907
1
8
2
5
X1
IRF7105P
X2
IRF7105N
C1
0.22U
C2
10U
R5
220
9
Rsr1
1.8
10
Vdd
11
Rsr2
1.8
12
Rsr3
1.8
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