LTC3129 datasheet(19/30 Pages) LINER

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LTC3129

3129fc

For more information www.linear.com/LTC3129

applicaTions inForMaTion

Examining the previous equations reveals that the output

voltage ripple increases with load current and is gener-

ally higher in boost mode than in buck mode. Note that

these equations only take into account the voltage ripple

that occurs from the inductor current to the output being

discontinuous. They provide a good approximation to the

ripple at any significant load current but underestimate the

output voltage ripple at very light loads where the output

voltage ripple is dominated by the inductor current ripple.

In addition to the output voltage ripple generated across

the output capacitance, there is also output voltage ripple

produced across the internal resistance of the output

capacitor. The ESR-generated output voltage ripple is

proportionaltotheseriesresistanceoftheoutputcapacitor

and is given by the following expressions where RESR is

the series resistance of the output capacitor and all other

terms as previously defined.

ΔVP−P(BUCK) =

ILOADRESR

1– tLOWf

≅ ILOADRESR V

ΔVP−P(BOOST) =

ILOADRESRVOUT

VIN 1– tLOWf

(

)

≅ ILOADRESR

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟ V

InmostLTC3129applications,anoutputcapacitorbetween

10µF and 22µF will work well. To minimize output ripple

in Burst Mode operation, or transients incurred by large

step loads, values of 22µF or larger are recommended.

Input Capacitor Selection

The VIN pin carries the full inductor current and provides

power to internal control 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 4.7µF

should be located as close to the VIN pin as possible. The

traces connecting this capacitor to VIN and the ground

plane should be made as short as possible.

Burst Mode operation and improve transient response. In

addition, due to the wide VIN and VOUT operating range

of the LTC3129 and its fixed internal loop compensation,

some applications may require the use of a feedforward

capacitor to assure light-load stability (less than ~15mA)

when operating in PWM mode (PWM pin pulled high).

Therefore, to reduce Burst Mode ripple and improve

phase margin at light load when PWM mode operation is

selected, a feedforward capacitor is recommended for all

applications. The recommended feedforward capacitor

value can be calculated by:

CFF = 66/R1

Where R1 is the top feedback divider resistor value in MΩ

and CFF is the recommended feedforward capacitor value

in picofarads (use the nearest standard value). Refer to

the application circuits for examples.

Output Capacitor Selection

A low effective series resistance (ESR) output capacitor

of 4.7µF minimum should be connected at the output of

the buck-boost converter in order to minimize output volt-

age ripple. Multilayer ceramic capacitors are an excellent

option as they have low ESR and are available in small

footprints. The capacitor value should be chosen large

enough to reduce the output voltage ripple to acceptable

levels. Neglecting the capacitor's ESR and ESL (effec-

tive series inductance), the peak-to-peak output voltage

ripple in PWM mode can be calculated by the following

formula, where f is the frequency in MHz (1.2MHz), COUT

is the capacitance in µF, tLOW is the switch pin minimum

low time in µs (0.09µs typical) and ILOAD is the output

current in amperes.

ΔVP−P(BUCK) =

ILOADtLOW

COUT

ΔVP−P(BOOST) =

ILOAD

fCOUT

VOUT – VIN + tLOWfVIN

VOUT

⎛

⎝

⎜

⎞

⎠

⎟ V

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