LTC4413 datasheet(16/32 Pages) AD

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LTC4421

Rev. 0

For more information www.analog.com

Selecting the Output Capacitor

When switching connection to the output between the

two input supplies, the LTC4421 utilizes break-before-

make circuitry to ensure the first channel has completely

disconnected from the output before the second turns

on. This prevents current from flowing from one input to

the other via the output, a phenomenon known as cross-

conduction. As a result, there is a dead time during swi-

tchover when neither supply is powering the output.

Users must choose an output capacitance COUT to support

the output load current and minimize the output voltage

step and droop during switchover. When the first channel

disconnects, a voltage step occurs at the output due to

the load current flowing through COUT’s equivalent series

resistance RESR. The magnitude of the voltage step is

given by Equation 8.

STEP = (ILOAD

• R

ESR )

(8)

For the duration of the dead time, the output voltage

droops as the load current discharges COUT. The maxi-

mum magnitude of the droop is given by Equation 9.

DROOP =

LOAD(MAX) • tG(SWITCH),MAX

(

)

OUT

(9)

Set COUT to optimize the trade-off between minimizing

output voltage droop and minimizing the time required to

fully charge the output from 0V. Set VDROOP(MAX) as high

as possible; usually, VDROOP(MAX) ≤ 0.1 • VOUT is accept-

able. Typically, using 10µF to 50µF of output capacitance

per Ampere of maximum load current achieves a reason-

able trade-off.

APPLICATIONS INFORMATION

Figure 4 shows an output voltage waveform during swi-

tchover for a system having 5A output load current and

a 220µF output capacitor with 100mΩ RESR. When the

first channel is turned off, the 5A load is provided by the

220µF capacitor. With 5A flowing through the 100mΩ

RESR, VSTEP = 500mV. Following the ESR step, the output

discharges at a rate dV/dt = 5A/220µF until the second

channel is switched in.

Because of the high output currents, it is imperative

to choose capacitors having very low ESR to minimize

VSTEP. Also, consult the capacitor vendor’s curves of

capacitance versus DC bias voltage and capacitance ver-

sus temperature, and account for temperature and voltage

coefficients of COUT.

Determining the Maximum Time to Charge the Output

Voltage

Whenever the output is being charged from a lower

voltage to a higher voltage, it charges in current limit.

As a result, the overcurrent fault timer is running dur-

ing charging. It is imperative to determine the maximum

time t(CHG,MAX) required to charge the output and set the

overcurrent fault time tTMR,FLT > t(CHG,MAX). The maximum

charge time is given by Equation 10.

(CHG,MAX) =

OUT • VIN,MAX

(

)

LIM– ILOAD,CHG

(

)

(10)

where VIN,MAX is the highest input voltage and ILOAD,CHG

is the maximum DC load current present when COUT is

being charged. The worst case occurs when ILOAD,CHG

= ILOAD,MAX. If possible, disable the output load current

Figure 4. Output Voltage ESR Step and Linear Discharge During Channel Switchover

ESR STEP

0.5/DIV

10µs/DIV

VOUT

DROOP DUE TO

SWITCHOVER DELAY

4421 F04

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