LTC7124 datasheet(13/22 Pages) LINER

13 / 22 page

LTC7124

7124fa

For more information www.linear.com/LTC7124

error signal used by the regulator to return VOUT to its

steady-state value. During this recovery time, VOUT can

be monitored for overshoot or ringing that would indicate

a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine

phase margin. The addition of a feedforward capacitor

can improve the high frequency response, by providing

a phase lead due to the creation of a high frequency zero

with R1 in Figure 1.

The stability of the closed-loop system will determine

the output voltage settling behavior. LTpowerCAD® and

LTspice® can be used to check control loop and transient

performance.

Insomeapplications,amoreseveretransientcanbecaused

by switching in loads with large (>1µF) load capacitors.

The discharged load capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT. No regulator

can deliver enough current to prevent this problem if

the switch connecting the load has low resistance and is

driven quickly. The solution is to limit the turn-on speed of

the load switch driver. A Hot Swap controller is designed

specifically for this purpose and usually incorporates

current limiting, short-circuit protection and soft-starting.

Input Capacitor (CIN) Selection

The input capacitance, CIN, filters the square wave current

at the drain of the top power MOSFET. To prevent large

voltagetransientsfromoccurring,alowESRinputcapacitor

sized for the maximum RMS current is recommended. The

maximum RMS current is given by:

IRMS =IOUT(MAX) •

VOUT • VIN − VOUT

(

)

VIN

This formula has a maximum at VIN = 2VOUT where

IRMS =

IOUT

This simple worst-case condition is commonly used for

design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based on only 2000 hours of life,

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

sizeorheightrequirementsinthedesign.Forlowinputvolt-

ageapplications,sufficientbulkinputcapacitanceisneeded

to minimize transient effects during output load changes.

Output Capacitor (COUT) Selection

The selection of COUT is determined by the effective series

resistance(ESR)thatisrequiredtominimizevoltageripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response. The output ripple ΔVOUT is

approximated by:

ΔVOUT < ΔIL • ESR+

8 • f •COUT

⎛

⎝⎜

⎞

⎠⎟

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

RMS current handling requirement. Dry tantalum, special

polymer,aluminumelectrolytic,andceramiccapacitorsare

all available in surface mount packages. Special polymer

capacitors are very low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

highest capacitance density but it is important to only

use types that have been surge tested for use in switch-

ing power supplies. Aluminum electrolytic capacitors have

significantly higher ESR but can be used in cost sensitive

applications provided that consideration is given to ripple

currentratingsandlong-termreliability.Ceramiccapacitors

haveexcellentlowESRcharacteristicsandsmallfootprints.

When using low-ESR ceramic capacitors, the output

capacitor value should be chosen to fulfill a charge storage

requirement. During load step, the output capacitor must

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

to support the load. The time required for the feedback

loop to respond is dependent on the compensation and

the output capacitor size. Typically, 3 to 4 cycles are

required to respond to a load step, but only in the first cycle

APPLICATIONS INFORMATION

LTC7124 Datasheet(PDF) 13 Page - Linear Technology

LTC7124 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL