LTC1735CF datasheet(14/32 Pages) LINER

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LTC1735

1735fc

CIN Selection

In continuous mode, the source current of the top

N-channel MOSFET is a square wave of duty cycle VOUT/

VIN. To prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

RMS

O MAX

OUT

≅









()

–1

This formula has a maximum at VIN = 2VOUT, where

IRMS = IO(MAX)/2. This simple worst case condition is com-

monly used for design because even significant deviations do

not offer much relief. Note that capacitor manufacturers’

ripple current ratings are often based on only 2000 hours of

life. This makes it advisable to further derate the capacitor or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

size or height requirements in the design. Always consult the

manufacturer if there is any question.

COUT Selection

The selection of COUT is primarily determined by the

effective series resistance (ESR) to minimize voltage

ripple. The output ripple (

∆VOUT) in continuous mode is

determined by:

∆∆

I ESR

OUT

≈+









Where f = operating frequency, COUT = output capaci-

tance and

∆IL = ripple current in the inductor. The output

ripple is highest at maximum input voltage since

∆IL

increases with input voltage. Typically, once the ESR

requirement for COUT has been met, the RMS current

rating generally far exceeds the IRIPPLE(P–P) requirement.

With

∆IL= 0.3IOUT(MAX) and allowing 2/3 of the ripple due

to ESR the output ripple will be less than 50mV at max VIN

assuming:

COUT required ESR < 2.2 RSENSE

COUT > 1/(8fRSENSE)

APPLICATIO S I FOR ATIO

The first condition relates to the ripple current into the ESR

of the output capacitance while the second term guaran-

tees that the output capacitance does not significantly

discharge during the operating frequency period due to

ripple current. The choice of using smaller output capaci-

tance increases the ripple voltage due to the discharging

term but can be compensated for by using capacitors of

very low ESR to maintain the ripple voltage at or below

50mV. The ITH pin OPTI-LOOP compensation compo-

nents can be optimized to provide stable, high perfor-

mance transient response regardless of the output capaci-

tors selected.

The selection of output capacitors for CPU or other appli-

cations with large load current transients is primarily

determined by the voltage tolerance specifications of the

load. The resistive component of the capacitor, ESR,

multiplied by the load current change plus any output

voltage ripple must be within the voltage tolerance of the

load (CPU).

The required ESR due to a load current step is:

RESR < ∆V/∆I

where

∆I is the change in current from full load to zero load

(or minimum load) and

∆V is the allowed voltage deviation

(not including any droop due to finite capacitance).

The amount of capacitance needed is determined by the

maximum energy stored in the inductor. The capacitance

must be sufficient to absorb the change in inductor current

when a high current to low current transition occurs. The

opposite load current transition is generally determined by

the control loop OPTI-LOOP components, so make sure

not to over compensate and slow down the response. The

minimum capacitance to assure the inductors’ energy is

adequately absorbed is:

OUT

()

∆

where

∆I is the change in load current.

Manufacturers such as Nichicon, United Chemi-Con and

Sanyo can be considered for high performance through-

hole capacitors. The OS-CON semiconductor electrolyte

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