ISL705ARH, ISL705BRH, ISL705CRH, ISL706ARH, ISL706BRH, ISL706CRH

11

FN7662.0

September 15, 2011

is at its maximum specified value. This allows the positive supply

to fluctuate within its acceptable range without signaling a reset

when configured as shown in Figure 16.

negative supply rail is set for -4.5V. Equation 2 can be used to

select the appropriate resistor values. R1 is selected arbitrarily as

the values into Equation 2 as shown in Equation 3 it can be seen

a resistor of 153.3kΩ is needed. The closest 1% resistor value is

154kΩ.

Figure 4 also has a general purpose NPN transistor in which the

base is connected to the PFO pin through a 100kΩ resistor. The

emitter is tied to ground and the collector is tied to MR signal.

This configuration allows the negative voltage sense circuit to

initiate a reset if it is not within its regulation window. A pull-up

on the MR ensures no false reset triggering when the negative

voltage is within its regulation window.

Assuring a Valid RST Output

current and is essentially an open circuit. As a result, this pin can

drift to undetermined voltages if left undriven. By adding a pull-down

resistor to the RST pin as shown in Figure 17, any stray charge or

leakage currents will be drained to ground and keep RST low when

should be large enough not to load RST and small enough to pull

RST to ground. A 100kΩ resistor would suffice, assuming there is no

load on the RST pin during that time.

Assuring a Valid RST Output

result, this pin can drift to undetermined voltages if left undriven. By

adding a pull-up resistor to the RST pin as shown in Figure 18, RST

however, it should be large enough not to exceed the sink capability

of RST pin at 1.2V. A 300kΩ resistor would suffice, assuming there

is no load on the RST pin during that time.

Selecting Pull-Up Resistor Values

The ISL705CRH and ISL706CRH have open drain active low reset

outputs (RST_OD). A pull-up resistor is needed to ensure RST_OD

must be chosen in order not to exceed the sink capability of the

RST_OD pin. The ISL705ARH has a sink capability of 3.2mA and

the ISL706CRH has a sink capability of 1.2mA. Equation 4 may

FIGURE 16. ±5V MONITORING

R2

R1 VPFI Vtrip

–

()

VDD VPFI

–

---------------------------------------------

=

(EQ. 2)

R2

100k 1.25

4.5

–

()

–

()

51.25

–

------------------------------------------------------

153.3k

Ω

=

=

(EQ. 3)

+5V

ISL705ARH

-5V

100k

100k

2N3904

MR

RST

PFO

PFI

R1

R2

FIGURE 17. RST VALID TO GROUND CIRCUIT

FIGURE 18. RST VALID TO GROUND CIRCUIT

FIGURE 19. RST_OD PULL-UP CONNECTION

ISL705ARH, ISL706ARH

100k

Ω

RST

ISL705BRH, ISL706BRH

300k

Ω

RST

R1

ISL706CRH, ISL705CRH

RST_OD