Datasheet

V
UVL
=
1.17V x (R1 + R2 + R3)
R2 + R3
V
UVH
= 1.17V + [R1 x (20 PA +
(R2 + R3)
1.17V
)]
R2 =
1.17V x 50 k:
(7V ± 1.17V)
- 4.64 k: = 5.39 k:
R3 =
1.16V x 50 k: x 7V
15V x (7V - 1.17V)
= 4.64 k:
R1 =
8V - 7V
20 PA
=
1V
= 50 k:
20 PA
V
OVL
= [(R1 + R2) x ((1.16V) - 20 PA)] + 1.16V
R3
R2 =
1.17V x R1
V
UVL
- 1.17V
- R3
R3 =
1.16V x R1 x V
UVL
V
OVH
x (V
UVL
± 1.17V)
R1 =
V
UVH
- V
UVL
20 PA
=
V
UV(HYS)
20 PA
VIN
UVLO
OVLO
GND
R1
R2
R3
TIMER AND GATE
LOGIC CONTROL
LM25069
V
SYS
20 PA
1.17V
1.16V
20 PA
LM25069
SNVS607E –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
Figure 31. UVLO and OVLO Thresholds Set By R1-R3
The procedure to calculate the resistor values is as follows:
• Choose the upper UVLO threshold (V
UVH
), and the lower UVLO threshold (V
UVL
).
• Choose the upper OVLO threshold (V
OVH
).
• The lower OVLO threshold (V
OVL
) cannot be chosen in advance in this case, but is determined after the
values for R1-R3 are determined. If V
OVL
must be accurately defined in addition to the other three thresholds,
see Option B below.
The resistors are calculated as follows:
(11)
(12)
(13)
The lower OVLO threshold is calculated from:
(14)
As an example, assume the application requires the following thresholds: V
UVH
= 8V, V
UVL
= 7V, V
OVH
= 15V.
(15)
(16)
(17)
The lower OVLO threshold calculates to 13.9V, and the OVLO hysteresis is 1.1V. Note that the OVLO hysteresis
is always slightly greater than the UVLO hysteresis in this configuration. When the R1-R3 resistor values are
known, the threshold voltages and hysteresis are calculated from the following:
(18)
(19)
V
UV(HYS)
= R1 x 20 µA (20)
18 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM25069