LTC4355 datasheet(9/24 Pages) LINEAR

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LTC4420

4420f

For more information www.linear.com/LTC4420

applicaTions inForMaTion

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

LTC4420 draws its internal quiescent current ICC and the

supply to which OUT is connected after input supplies

have been qualified.

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

a low self discharge 7.6V Li-Thionyl Chloride (Li-SOCl2)

hold-up battery which is completely discharged when its

voltage drops to 6V. Li-SOCL2 battery life is maximized

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

following subsections.

Figure 1. The LTC4420 Protecting 2-Cell Lithium Battery Packs

on V1 and V2 from Discharge Below Their Safe Minimum Voltage

Setting Switchover and V2 Undervoltage Thresholds

Several factors affect switchover voltage and should be

taken into account when calculating resistor values. 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:

VSW1=

VTHA

+R2

• R1

+R2+R3

(

)

(1)

Table 1. OUT and LTC4420 ICC Power

INPUT VOLTAGES

V1 > 1.55V

ADJ > 1.097V

V2 > 1.55V

V2DIS > 0.9V

V2UV > 0.397*

ICC SOURCE

OUT CONNECTION

Y†

Hi-Z

VMAX**

Hi-Z

*Note: Refers to V2UV voltage at the end of the V2 monitoring period.

**Note: VMAX = higher of V1 and V2.

† For 64ms.

ADJ

CMP1

V2UV

V1UV

V2UV

GNDSW

4420 F01

V1UV: V1 < 6V (V1 FALLING)

V2UV: V2 < 6V (V2 FALLING)

COUT

10µF

150k

78.7k

OUT

V2TEST

V2OK

CMPOUT1

V2DIS

LTC4420

GND

4.02M

SWITCHOVER

THRESHOLD: V1 < 5.6V (V1 FALLING)

+ 2-CELL

Li-Ion

7.4V

2-CELL

Li-SOCl2

7.4V

280k

4.7µF

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