LTC3129 datasheet(10/24 Pages) LINEAR

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LTC4420

4420f

For more information www.linear.com/LTC4420

applicaTions inForMaTion

Typical V1 undervoltage threshold is:

VV1UV =

VTHC

• R1

+R2+R3

(

)

(2)

Worst-case VOL due to current flow into the GNDSW pin

must be taken into account while calculating values for

the V2 undervoltage resistive divider:

VV2UV =

VTHC

VOL

100µA

• R4

+R5+

VOL

100µA









(3)

Equations 1-3 assume ADJ and CMP1 pin leakages are

negligible. To account for pin leakage, equations 1-3 must

be modified by an ILEAK • REQ term where equivalent

resistance REQ must be calculated on a case-by-case ba-

sis. Worst-case component values and reference voltage

tolerances must be used to calculate the maximum and

minimum threshold voltages. For example, to calculate

minimum falling switchover threshold voltage VSW1(MIN),

use VTHA(MIN), (R2+R1)(MAX), R3(MIN) in equation 1.

Figure 2. ADJ Comparator Propagation Delay as a

Function of Slew Rate; tPDA vs dVADJ/dt

where IOUT is the current supplied by COUT during non-

overlap or dead time, tNOV. Choosing:

COUT ≥

tNOV •IOUT

∆VOUT

(5)

limits output droop to less than

∆VOUT.

In order to estimate tNOVandIOUT,firstconsiderascenario

where power supplies are present on V1 and V2, and their

voltages are changing slowly compared to the ADJ com-

parator propagation delay tPDA. For such cases, IOUT is

ILOAD and tNOV is tSWITCH. COUT can be sized according to

equation 5 with IOUT = ILOAD(MAX) and tNOV = tSWITCH(MAX)

tolimitmaximumoutputdroopwhenswitchingtoahigher

supply. When switching to a lower supply, switchover is

initiated only after OUT falls VREV below the supply that

is being switched in. In such cases, total output droop is

∆VOUT + VREV.

Next consider a scenario where the input power source

powering OUT is unplugged. OUT backfeeds circuitry

connected to the input supply pin. Both input and output

droop at the same rate. Referring to Figure 1, assume

the battery on V1 is unplugged when OUT is connected

to V1. IOUT is the sum of ILOAD and the back fed current

IBACK, which in this example is IR3. As OUT and V1, since

the two are connected, droop below the ADJ threshold,

switchover occurs to V2 with a dead time

tNOV = tPDA + tSWITCH

(6)

where tPDA is an overdrive dependent ADJ comparator

delay. As an approximation, use tPDA from the Electrical

Characteristics table to estimate tNOV. Use this tNOV and:

IOUT = (IBACK + ILOAD)

(7)

in equation 5 to size COUT:

COUT ≥

tPDA + tSWITCH

(

)•IOUT

∆VOUT

(8)

Refer to Figure 2 for a more accurate estimate of tPDA vs

dVOUT/dt.IfADJisfilteredwithcapacitorCADJ,itsdischarge

time via divider R1 – R3 increases tPDA. This results in a

higher output droop than estimated by equation (8).

Selecting Output Capacitor COUT

COUTcanbeselectedtocontroleitheroutputvoltagedroop

during switchover or output rising slew rate during initial

power-up or when switching to a higher supply.

In general, output droop, ΔVOUT, can be calculated by:

VOUT =

tNOV •IOUT

COUT

(4)

dVADJ /dt (V/s)

100

10k

100k

100

125

4420 F02

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