ISL1591

16

FN7625.1

October 31, 2012

Power Control Function

constructed from amplifiers A and B. If both control lines are

unconnected externally, the internal 50k

Ω pull-up will switch the

differential pairs to divert the 100µA tail currents into the supply

turning off the amplifiers. Taking both control pins low will pass

are low, the typical 14mA total quiescent current for a port is

shown in the “Electrical Specification” tables on page 4. Taking

low will reduce the current in a port to a typical 7.4mA supply

current. Table 2 summarizes the operation modes for ISL1591

for each port.

Performance Considerations

Driving Capacitive Loads

All closed loop op amps are susceptible to reduced phase margin

when driving capacitive loads. This shows up as peaking in the

frequency response that can, in extreme situations, lead to

oscillations. The ISL1591 is designed to operate successfully

with small capacitive loads such as layout parasitics. As the

parasitic capacitance increases, it is best consider a small

resistor in series with the output to isolate the phase margin

effects of the capacitor. Figure 23 on page 9 shows the effect of

capacitive load. With 22pF on each output, we see about 6dB

Output Headroom Model

Driving high voltages into heavy loads will require a careful

consideration of the available output swing vs. load. Figure 51

shows a useful model for predicting the available output swing. If

the output is modeled as ideal NPN and PNP transistors, the

The no load headrooms can be found in the “Electrical

Specifications” table on page 5 as 24V - 22.4V = 1.6V giving 0.8V

to each supply.

The equivalent impedances for this model can be extracted from

the reduced swings shown in the specification table for the 100

Ω

load. Looking at the typical 100

Ω load swings, we see a +21.2V

swing. Solving for the two resistors in the Headroom model

shown in Figure 51 gives Equation 7:

For the differential configuration, Figure 52 shows the Headroom

model that can be used to predict the maximum available swing

For equal bipolar supplies, the available peak output swing will

be given by Equation 8:

For example, to worst case the design using +24V supplies with

±5% supply tolerance and a minimum expected load of 50

Ω, a

TABLE 2. POWER MODES OF THE ISL1591

OPERATION

00

01

10

1

1

Power-Down

CO

C1

+3.3V

+VCC IBIAS

+1.4V

IBIAS +VCC

+3.3V

+1V

50k

Ω

50k

Ω

C1

+3.3V

+VCC IBIAS

+1.4V

IBIAS +VCC

+3.3V

50k

Ω

50k

Ω

CO

50k

Ω

FIGURE 50. BIAS CONTROL CIRCUIT

FIGURE 51. HEADROOM MODEL

+

–

+

–

GND

(EQ. 7)

FIGURE 52. HEADROOM MODEL

-

+

-

+

GND

L

n

p

n

p

s

p

R

R

R

V

V

V

V

+

+

−

−

=

1

)

(

2

(EQ. 8)

peak

L

n

p

n

p

s

peak

V

R

R

R

V

V

V

V

1

.

19

50

8

.

2

8

.

2

1

)

6

.

1

8

.

22

(

1

)

(

=

Ω

Ω

+

Ω

+

−

=

+

+

−

−

=

(EQ. 9)