ISL55211 datasheet(13/20 Pages) RENESAS

13 / 20 page

ISL55211

FN7868 Rev 0.00

Page 13 of 20

June 21, 2011

normal single-ended VFA application, but no common mode

signal related terms.

The examples shown are using the transformer to convert from

single to differential. However, if the source is already

differential, these same transformer input circuits can drive the

transformer differentially still providing impedance scaling if

needed and common mode rejection for both DC and AC

common mode issues. A good example would be differential

mixer outputs or SAW filter outputs. Those differential sources

could also be connected into the ISL55211 RG resistors through

blocking caps as well eliminating the input transformer. The AC

termination impedance for the differential source will then be

the sum of the two RG resistors when simple blocking caps are

used.

Amplifier I/O Range Limits

The ISL55211 is intended principally to give the lowest IM3

performance on the lowest power for a differential I/O

application. The amplifier will work DC coupled and over a

relatively wide supply range of 3.0V to 4.2V supplies. The outputs

have both a differential and common mode operating range

limits while the input pins internal to the ISL55211 have a

common mode voltage operating range. For single supply

operation, the -Vs pins are at ground as is the exposed metal pad

on the underside of the package. The ISL55211 can operate split

supply where then -Vs will be a negative supply voltage and the

exposed metal pad is either connected to this negative supply or

left unconnected on an insulating board layer.

Briefly, the I/O and VCM limits are as follows:

1. Maximum VCM setting = -Vs +2V

2. Input common mode operating range (internal summing

junction pints of the ISL55211) of -Vs + 1.1V or to output VCM

+ 0.5V

3. Output VO minimum (on each side) is either -Vs + 0.3V or

output VCM - 0.9V

4. Output VO maximum (on each side) is +Vs - 1.5V

The output swing limits are often asymmetrical around the VCM

voltage. The maximum single-ended swings are set by these two

limits - VO(MIN) is either -Vs + 0.3V or VCM - 0.9V, whichever is

less. So for instance, on a single 3.3V supply with the default VCM

voltage of 1.2V, these two limits give the same result and the

output pins can swing down to 0.3V above -Vs (= 0V). If, however,

the VCM pin is raised to 1.5V, then the minimum output voltage

will become 1.5V - 0.9V = 0.6V.

VO(MAX) is set by a headroom limit to the positive supply to be

-VO(MAX) = +Vs - 1.5V. Again, on a 3.3V single supply and the

default 1.2V VCM setting, this means the maximum referenced to

ground output pin voltages can be 3.3V - 1.5V = +1.8V or 0.6V

above the default VCM voltage.

Using these default conditions, and the maximum positive

excursion of 0.6V above the 1.2V output VCM setting, the

maximum differential VP-P swing will be 4x this 0.6V

single-ended limit or 2.4VP-P. Where +Vs is increased, the limit

then becomes the 0.9V below VCM, but then the absolute

maximum differential VP-P is then 4 x 0.9V to 3.6VP-P. So for

instance, to get this maximum output swing, increase the supply

voltage until +Vs - 1.5V > VCM + 0.9V. If we assume a VCM voltage

of 1.3V for instance, then 1.3V + 0.9V + 1.5V = 3.7V will give an

unclipped 3.6VP-P output capability. The VP-P reported in

Figure 27 is an asymmetrically clipped maximum swing. Going

10% above this 3.7V target to 4.1V will be within the

recommended operating range and give some tolerancing

headroom that would also suggest the VCM voltage be moved up

to approximately 1.5V, which coincides with the default output

VCM from Figure 27. Operating at +4.1V single supply in a

Figure 29 type configuration will give the maximum linear

differential output swing of 3.6VP-P.

The differential inputs internal to the ISL55211 also have

operating range limits relative to the supply voltages. Operating

in an AC-coupled circuit like Figure 29 will produce an input

common mode voltage equal to the outputs. The inputs can

operate with full linearity with this VCM voltage down to 1.1V

above the -Vs supply. On the default 1.2V output VCM on +3.3V

supplies this gives a 100mV guardband on the input VCM

voltages. Overriding the default VCM by applying a control voltage

to the VCM pin should be done with care in going towards the

negative supply due to this limit. On the + side, the maximum

input VCM above the -Vs supply is 2V so there is more room to

move the output VCM up than down from the default value.

Power Supply, Shutdown, and Thermal

Considerations

The ISL55211 is intended for single supply operation from 3.0V

to 4.2V with an absolute maximum setting of 4.5V. The 3.3V

supply current is trimmed to be nominally 35mA at +25°C

ambient. Figure 28 shows the supply current for nominal +25°C

and -40°C to +85°C operation over the specified maximum

supply range. The input stage is biased from an internal voltage

reference from the negative supply giving the exceptional 90dB

low frequency PSRR shown in Figure 26.

Since the input stage bias is from a re-regulated internal supply,

a simple approach to single +5V operation can be supported as

shown in Figure 33. Here, a simple IR drop from the +5V supply

will bring the operating supply voltage for the ISL55211 into its

allowed range. Figure 33 shows example calculations for the

voltage range at the ISL55211 +Vs pin assuming a ±5%

tolerance on the +5V supply and a 35mA to 55mA range on the

total supply current. Considering the 34mA to 44mA quiescent

current range from Figure 28 over the -40°C to +85°C ambient,

and the 3.4V to 4.4V supply voltage range assumed here, this is

designing for a 1mA to 11mA average load current, which should

be adequate for most intended application loads. Good supply

decoupling at the device pins is required for this simple solution

to still provide exceptional HD performance.

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