LTC4420
applicaTions inForMaTion
OUT
The LTC4420 is a low quiescent current 2-channel priori-
tizer that powers both its internal circuitry and its output
OUT from a prioritized valid input supply. Unlike an ideal
diode-OR, the LTC4420 does not necessarily draw current
from the highest supply as long as one supply is greater
than 1.8V. Table 1 lists the input supply from which the
V1
OUT
C
R3
2-CELL
Li-Ion
7.4V
R6
R7
1M
R8
OUT
+
+
10µF
1M
1M
1M
C1
ADJ
V2TEST
4.7µF
R2
150k
CMPOUT1
V1UV
V2UV
CMP1
V2OK
R1
78.7k
SWITCHOVER
LTC4420
THRESHOLD: V1 < 5.6V (V1 FALLING)
V1UV: V1 < 6V (V1 FALLING)
V2
V2DIS
V2UV: V2 < 6V (V2 FALLING)
2-CELL
Li-SOCl
7.4V
R5
4.02M
LTC4420 draws its internal quiescent current I and the
CC
2
C2
4.7µF
V2UV
GNDSW
supply to which OUT is connected after input supplies
have been qualified.
R4
280k
GND
4420 F01
A typical battery backup application is shown in Figure 1.
V1 is powered by a 2-cell Li-Ion battery pack whose safe
discharge limit is between 5.6V and 6V. V2 is powered by
Figure 1. The LTC4420 Protecting 2-Cell Lithium Battery Packs
on V1 and V2 from Discharge Below Their Safe Minimum Voltage
a low self discharge 7.6V Li-Thionyl Chloride (Li-SOCl )
2
hold-up battery which is completely discharged when its
voltage drops to 6V. Li-SOCL battery life is maximized
2
Setting Switchover and V2 Undervoltage Thresholds
as very little current is drawn from V2 during normal
operation due to the low duty cycle of V2 monitoring and
the LTC4420’s low V2 standby current. To protect the
2-cell Li-Ion battery on V1, switchover threshold is set
to be ~5.6V. After switchover to V2, the Li-Ion battery
primarily supplies only divider R1-R3’s current as the
LTC4420 draws only a small standby current from V1.
Monitor CMP1 is configured to provide V1 power failure
warning by driving V1UV low when V1 falls below 6V.
Monitor input V2UV is configured to set V2’s UV threshold
to 6V and V2DIS is tied low to disconnect the V2 to OUT
power path when V2 falls below 6V. V2TEST is tied high
to monitor V2 once every 132s. Relevant equations used
to calculate these component values are discussed in the
Several factors affect switchover voltage and should be
takenintoaccountwhencalculatingresistorvalues. These
includeresistortolerance,1.5%ADJcomparatorthreshold
error, divider impedance and worst-case ADJ pin leakage.
These factors also apply to resistive dividers connected
to monitor inputs CMP1 and V2UV. Referring to Figure
1 and the Electrical Characteristics table, the typical V1
switchover threshold:
VTHA
VSW1
=
• R1+ R2+ R3
(
(1)
)
R1+ R2
following subsections.
Table 1. OUT and LTC4420 ICC Power
INPUT VOLTAGES
V1 > 1.55V
ADJ > 1.097V
V2 > 1.55V
V2DIS > 0.9V
V2UV > 0.397*
I
CC
SOURCE
V1
OUT CONNECTION
†
†
Y
Y
X
Y
Y
Y
N
Y
Y
Y
N
X
Y
N
N
X
N
N
Y
X
X
X
Y
N
X
N
Y
X
X
V1
V2
Y
Y
Y
Y
N
N
N
N
N
N
N
N
X
X
X
X
V2
V2
V2
V1
Hi-Z
Hi-Z
Hi-Z
V2
V1
V2
V2
V2
V2
V **
MAX
Hi-Z
*Note: Refers to V2UV voltage at the end of the V2 monitoring period.
**Note: V
= higher of V1 and V2.
MAX
†
For 64ms.
4420f
9
For more information www.linear.com/LTC4420
Linear Systems [ Linear Systems ]
