SI3232 datasheet(44/128 Pages) SILABS

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Si3232

Preliminary Rev. 0.96

4.12. Relay Driver Considerations

The Si3232 includes a general-purpose driver output for

each channel (GPOa, GPOb) to drive external test

relays. In most applications, the relay can be driven

directly from the Si3232 with no external relay drive

circuitry required. Figure 21 illustrates the internal relay

driver circuitry using a 3 V relay.

Figure 21. Si3232 Internal Relay Drive Circuitry

The internal driver logic and drive circuitry is powered

from the same 3.3 V supply as the chip’s main supply

(VDD1–VDD4 pins). When operating external relays

from a VCC supply that is equal to the chip’s VDD supply,

an internal diode network provides protection against

overvoltage conditions caused by flyback spikes when

the relay is opened. Only 3 V relays may be used in the

configuration shown in Figure 21, and either polarized

or non-polarized relays are acceptable provided both

VCC and VDD are powered by a 3.3 V supply. The input

impedance, RIN, of the relay driver pins is a constant

Ω while sinking less than the maximum rated 85 mA

into the pin.

If the desired operating voltage of the relay, VCC, is

higher than the Si3232’s VDD supply voltage, an

external drive circuit is required to eliminate leakage

from VCC to VDD through the internal protection diode.

In this configuration, a polarized relay is recommended

to provide optimal overvoltage protection with minimal

external components. Figure 22 illustrates the required

external

drive

circuit,

and

Table 26

provides

recommended values for RDRV for typical relay

characteristics

and

VCC supplies. The output

impedance, ROUT, of the relay driver pins is a constant

Ω while sourcing less than the maximum rated

28 mA out of the pin.

Figure 22. Driving Relays with VCC > VDD

The maximum allowable RDRV value can be calculated

using the following equation:

where

βQ1,MIN ~ 30 for a 2N2222

Si3232

Relay

Driver

Logic

GND

GPOa/b

3 V/5 V Relay

(polarized or

non-polarized)

Table 26. Recommended RDRV Values

ProSLIC

VDD

Relay

VCC

Relay

RCOIL

Maximum

RDRV

Recom-

mended

5% Value

3.3 V

±5%

3.3 V

±5%

Ω

Not

Required

—

3.3 V

±5%

178

Ω

2718

Ω

2.7 k

Ω

3.3 V

±5%

12 V

±10%

1028

Ω

6037

Ω

5.6 k

Ω

3.3 V

±5%

24 V

±10%

2880

Ω

8364

Ω

8.2 k

Ω

3.3 V

±5%

48 V

±10%

7680

Ω

11092

Ω

11 k

Ω

Si3232

GPOa

GPOb

Polarized

relay

DRV

MaxRDRV

VDD,MIN 0.6 V

–

() R

RELAY

() β

Q1,MIN

()

VCC,MAX 0.3 V

–

------------------------------------------------------------------------------------------------- RSOURCE

–

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