LTM8083 datasheet(10/22 Pages) AD

10 / 22 page

LTM4693

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

of the output capacitor need to be considered. In most

LTM4693 applications, an output capacitor between 68µF

and 220µF from VOUT to GND will work well. An additional

low value ceramic capacitor such as 4.7µF can be placed

between VOUT to GND to reduce switching noise to the

control circuitry. The LTpowerCAD® design tool is avail-

able to download online to perform ripple analysis based

on certain number and type of the capacitors.

INPUT DECOUPLING CAPACITORS

The VIN pin carries the full inductor current, provides

power to internal switches and drivers, and powers con-

trol circuits in the IC. To minimize input voltage ripple

and ensure proper operation of the IC, a low ESR bypass

capacitor with a value of at least 22µF should be located as

close to VIN as possible. The traces connecting this capac-

itor to VIN and the ground plane (GND) should be made as

short as possible. An additional low value ceramic capaci-

tor such as 4.7µF can be placed between VIN and GND to

reduce switching noise to the control circuitry.

RECOMMENDED INPUT AND OUTPUT CAPACITORS

The capacitors used to filter the input and output of the

LTM4693 must have low ESR and must be rated to handle

the large AC currents generated by the switching con-

verters. While there are many capacitor types for these

applications (including low ESR tantalum, OSCON and

POSCAP), ceramic capacitors are often utilized in switch-

ing converter applications due to their small size, low ESR

and low leakage currents. The major providers of ceramic

capacitors are AVX, Kemet, Murata, Taiyo Yuden and TDK.

Many ceramic capacitors intended for power applica-

tions experience a significant loss in capacitance from

their rated value as the DC bias voltage on the capacitor

increases. It is not uncommon for a small surface mount

capacitor to lose more than 50% of its rated capacitance

when operated near its maximum rated voltage. This

effect is generally reduced as the case size is increased

for the same nominal value capacitor. As a result, it is

often necessary to use a larger value capacitance or a

higher voltage rated capacitor than would ordinarily be

required to actually realize the intended capacitance at

SOFT-START

The soft-start circuit linearly ramps the average induc-

tor current during the soft-start period (tSS). An internal

soft-start interval of approximately 2ms can be selected

by connecting SS to VIN. For applications requiring a lon-

ger soft-start period, an external capacitor CSS on SS sets

soft-start period according to Equation 8. The total soft-

start time can be calculated as:

tSS (ms) = 0.8 • CSS (nF)

(8)

Figure 2. Circuit to Set Soft-Start Time

CSS

PROGRAMMABLE

SOFT-START

DEFAULT 2ms

SOFT-START

VIN

4693 F02

where CSS is the capacitance on the SS pin.

The soft-start circuit slowly ramps the error amplifier

output at VC. In doing so, the current command of the

IC is slowly increased, starting from zero. The soft-start

period is defined as the time it takes the SS capacitor to

ramp to 0.9V, allowing VC to command full rated current.

In most situations, VOUT comes into regulation without

needing full inductor current, resulting in power up times

at a fraction of the soft-start period. After initial power up,

soft-start can be reset by VIN UVLO asserting, thermal

shutdown, or a VOUT short-circuit.

OUTPUT CAPACITORS

A low effective series resistance (ESR) output capacitor

should be connected at the output of the buck-boost con-

verter in order to minimize output voltage ripple. Multilayer

ceramic capacitor is an excellent option as it has low ESR

and is available in small footprint. The capacitor value

should be chosen large enough to reduce the output

voltage ripple to acceptable levels. The output voltage

ripple increases with load current and is generally higher

in boost mode than in buck mode. Both output voltage

ripple generated across the output capacitance, and out-

put voltage ripple produced across the internal resistance

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