LM5035ASQSLASHNOPB datasheet(24/40 Pages) TI

24 / 40 page

1.25V x (R1 + RCOMBINED)

OVPoff

R3 =

RCOMBINED =

1.25V x R1

UVLOoff ± 1.25V

R1 =

UVLOon - UVLOoff

23 PA

UVLO

LM5035A

VPWR

STANDBY

OFF

STANDBY

1.25V

0.4V

23 PA

OVPoff = 1.25V x

R1 + R2 + R3

UVLOoff = 1.25V x

R1 + R2 + R3

R2 + R3

LM5035A

SNVS537F – JANUARY 2008 – REVISED APRIL 2013

www.ti.com

Table 1. UVO/OVP Divider Formulas

Outputs disabled due to VIN falling below UVLO threshold

(11)

Outputs enabled due to VIN rising above UVLO threshold

UVLOon = UVLOoff + (23 µA x R1)

Outputs disabled due to VIN rising above OVP threshold

(12)

Outputs enabled due to VIN falling below OVP threshold

OVPon = OVPoff - [23 µA x (R1 + R2)]

The typical operating ranges of undervoltage and overvoltage thresholds are calculated from the above

equations. For example, for resistor values R1 = 86.6k

Ω, R2 = 2.10kΩ and R3 = 1.40kΩ the computed

thresholds are:

UVLO turn-off = 32.2V

UVLO turn-on = 34.2V

OVP turn-on = 78.4V

OVP turn-off = 80.5V

Figure 25. Remote Standby and Disable Control

To maintain the threshold’s accuracy, a resistor tolerance of 1% or better is recommended.

The design process starts with the choice of the voltage difference between the UVLO enabling and disabling

thresholds. This will also approximately set the difference between OVP enabling and disabling regulation:

(13)

Next, the combined resistance of R2 and R3 is calculated by choosing the threshold for the UVLO disabling

threshold:

(14)

Then R3 is determined by selecting the OVP disabling threshold:

(15)

Finally, R3 is subtracted from RCOMBINED to give R2:

R2 = RCOMBINED - R3

(16)

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