LTC3111 datasheet(16/32 Pages) LINER

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LTC3111

3111fa

For more information www.linear.com/LTC3111

APPLICATIONS INFORMATION

chosen large enough to reduce the output voltage ripple to

acceptable levels. The minimum output capacitor needed

for a given output voltage ripple (neglecting the capacitor

ESR and ESL) can be calculated by the following formulas:

∆VP-P BUCK

(

) =

ILOAD • tLOW

COUT

∆VP-P BOOST

(

) =

ILOAD

f •COUT

•

VOUT – VIN + tLOW • f• VIN

VOUT









where f is the frequency in Hz, COUT is the output capaci-

tance in μF, ILOAD is the output current in amps and tLOW

is the switch pin minimum low time in seconds, which is

typically 160ns.

In addition to output ripple generated across the output

capacitor, there is also output ripple produced across

the internal resistance of the output capacitor. The ESR-

generated output voltage ripple is proportional to the

series resistance of the output capacitor and is given by

the following expression:

∆VP-P BUCK

(

) =

ILOAD •RESR

1– tLOW • f

≅ILOAD •RESR

∆VP-P BOOST

(

) =

ILOAD •RESR • VOUT

VIN 1– tLOW • f

(

)

≅

ILOAD •RESR • VOUT

VIN

where RESR is the series resistor of the output capacitor

and all other terms are as previously defined.

Input Capacitor Selection

It is recommended that a low ESR ceramic capacitor with

a value of at least 10μF be located as close to the VIN pin

as possible. In addition, the return trace from the pin to

the ground plane should be made as short as possible.

It is important to minimize any stray resistance from the

converter to the battery or power source. If cabling is

required to connect the LTC3111 to the battery or power

supply, a higher ESR capacitor or a series resistor with a

low ESR capacitor in parallel with the low ESR capacitor

may be required to damp out ringing caused by the cable

inductance.

Capacitor Vendor Information

Both the input bypass capacitors and output capacitors

used with the LTC3111 must be low ESR and designed

to handle the large AC currents generated by switching

converters.Thisisimportanttomaintainproperfunctioning

of the IC and to reduce input/output ripple. Many modern

low voltage ceramic capacitors experience significant

loss in capacitance from their rated value with increased

DC bias voltages. For example, it is not uncommon for a

small surface mount ceramic capacitor to lose more than

50% of its rated capacitance when operated near its rated

voltage. As a result, it is sometimes necessary to use a

larger value capacitance or a capacitor with a larger case

size than required in order to actually realize the intended

capacitance at the full operating voltage. For details, con-

sult the capacitor vendor’s curve of capacitance versus

DC bias voltage.

The capacitors listed in Table 2 provide a sampling of small

surface mount ceramic capacitors that are well suited to

LTC3111applicationcircuits.Alllistedcapacitorsareeither

X5R or X7R dielectric in order to ensure that capacitance

loss over temperature is minimized.

Table 2. Representative Bypass and Output Capacitors

PART NUMBER

VALUE

(μF)

VOLTAGE

(V)

SIZE (mm) L

× W × H

(FOOTPRINT)

AVX

12103D226MAT2A

3.2

× 2.5 × 2.79

X5R Ceramic

Kemet

C220X226K3RACTU

A700D226M016ATE030

5.7

× 5.0 × 2.4

X7R Ceramic

7.3

× 4.3 × 2.8

Al Poly, 25mΩ

Murata

GRM32ER71E226KE15L

3.2

× 2.5 × 2.5

X7R Ceramic

Panasonic

ECJ-4YB1E226M

3.2

× 2.5 × 2.5

X5R Ceramic

Sanyo

25SVPF47M

6.6

× 6.6 × 5.9

OS-CON, 30mΩ

Vishay

94SVPD476X0035F12

10.3

× 10.3 × 12.6

OS-CON, 30mΩ

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