LTM8083 datasheet(11/24 Pages) AD

11 / 24 page

LTM8083

Rev. 0

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Figure 2. CISPR 25 Average Conducted EMI

Figure 3. CISPR 25 Average Radiated EMI

APPLICATIONS INFORMATION

To suppress high frequency switching spikes, ceramic

capacitors of at least 1µF should be placed from VIN to

GND and VOUT to GND as close to the LTM8083 pins as

possible. Due to their excellent low ESR characteristics,

ceramic capacitors can significantly reduce input ripple

voltage and help reduce power loss in the higher ESR

bulk capacitors. X5R or X7R capacitors of dielectrics

are preferred, as these materials retain their capacitance

over wide voltage and temperature ranges. Other types,

including Y5V and Z5U have very large temperature and

voltage coefficients of capacitance. Many ceramic capac-

itors, particularly 0805 or 0603 case sizes, have greatly

reduced capacitance at the desired operating voltage. In

an application circuit, they may have only a small fraction

of their nominal capacitance resulting in much higher out-

put voltage ripple than expected.

Input Capacitance CIN:

Discontinuous input current is highest in the buck region

due to the top switch M1 toggling on and off. A 10nF

low-ESL three-terminal capacitor is integrated inside the

module to decouple high-frequency input noise. Make

sure that the CIN capacitor network has low enough ESR

and is sized to handle the maximum RMS current. In buck

region, the input RMS current is given by:

IRMS = IoMAX •

VOUT

VIN

VOUT

– 1

The formula has a maximum at VIN = 2VOUT, where IRMS =

IoMAX/2. This simple worst-case condition is commonly

used for design.

Output Capacitance COUT:

Discontinuous current shifts from the input to the output

in the boost region. Make sure that the COUT capacitor

network is capable of reducing the output voltage rip-

ple. The effects of ESR and the bulk capacitance must be

considered when choosing the right capacitor for a given

and the internal oscillator frequency. The rising edge of

the synchronization clock represents the beginning of a

switching cycle.

CIN and COUT Selection

Input and output capacitors are necessary to suppress

voltage ripple caused by discontinuous current moving in

or out of the regulator. A parallel combination of capaci-

tors is typically used to achieve high capacitance and low

equivalent series resistance (ESR). Dry tantalum, special

polymer, aluminum electrolytic and ceramic capacitors are

all available in surface mount packages. Capacitors with

low ESR and high ripple current ratings, such as OS-CON

and POSCAP are also available.

FREQUENCY (Hz)

150k

–10

–20

8083 F01

108M

10M

CLASS 5 LIMITS

MAX EMISSIONS

FREQUENCY (Hz)

150k

–10

–20

100M

8083 F02

10M

CLASS 5 LIMITS

MAX EMISSIONS

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