LT3095 datasheet(14/22 Pages) LINER

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LT3095

3095fa

For more information www.linear.com/LT3095

APPLICATIONS INFORMATION

Capacitance degradation under DC bias can be dramatic.

In some cases, the effective value of a capacitor may be

less than 20% of the nominal value, even at voltages well

below the nominal voltage rating of the component. Dif-

ferent dielectric materials have different sensitivities to

DC bias, with X5R and X7R offering good performance.

Package size also has a large influence on the effective

capacitanceachievedathighervoltages.Ingeneral,capaci-

tors in physically larger packages suffer less degradation

under DC bias than capacitors of the same voltage rating

in a smaller size. Figure 2 shows typical bias curves for

three X5R capacitors with nominal values of 10µF, all rated

for 25V, but in different package sizes.

The effective values of some ceramic capacitors can

also degrade significantly with temperature, as Figure 3

demonstrates. Of the common dielectric types, X5R and

X7R offer relatively stable capacitance over a wide tem-

perature range. Both are widely available in a variety of

sizes and values.

TEMPERATURE (°C)

–50

–100

–80

–25

100

3095 F03

–60

–40

Y5V

–20

125

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

X5R

Figure 3. Capacitance De-Rating with Temperature

In the case of the linear regulator output capacitor, the

regulator can show ringing or degraded ripple rejection

if the effective capacitance is more than 20% below the

recommended minimum of 2.2µF. The recommended

minimum value for CBSTOUT — the output capacitor for

the boost converter — is 10µF; however, lower effective

values may be tolerable in certain applications. At high

duty cycles, for example, the linearized transconductance

of the current control loop is reduced, and less output

capacitance is required to achieve the same unity gain

frequency. When using a boost output capacitor with ef-

fective capacitance below 8µF, verify the loop dynamics

experimentally or with an AC model to ensure adequate

loop stability in realistic operating conditions.

Programming the Switching Frequency

The LT3095’s two switching regulator channels run out of

phase to reduce input current ripple amplitude. An internal

oscillatorgeneratesapreciseclockthatcanbeprogrammed

from 450kHz to 2MHz by connecting an external resistor

from the RT pin to ground. Table 2 lists the closest 1%

resistor values for a few common frequencies. Refer to the

Typical Performance section for a plot of clock frequency

as a function of RT.

Table 2. SW Frequency vs RT Value

fOSC (MHz)

RT (kΩ)

0.45

232

0.50

210

0.75

137

1.00

100

1.50

63.4

2.00

44.2

Alternatively, an external frequency source can be used to

synchronize the switching edges to a system clock using

the SYNC pin. The voltage and duty-cycle requirements

for the logic level SYNC signal are listed in the Electrical

Characteristics table. When using the SYNC pin to set the

frequency, connect a resistor to the RT pin as if program-

ming the oscillator to the SYNC frequency. Use a resistor

with 1% tolerance to insure appropriate scaling of internal

control signals. The LT3095 will sense a pulsed signal on

the SYNC pin and override the oscillator to align the switch

edges with the external clock.

When using the internal oscillator, each channel will run

at the programmed switching frequency, fOSC, and the

power switches will turn on 180º out-of-phase. If SYNC

functionality is used, each channel will run at the SYNC

pulse frequency: one power switch will turn on at the ris-

ing edge of the SYNC input, and the other power switch

at the falling edge.

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