LTC3129 datasheet(17/30 Pages) LINER

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LTC3129

3129fc

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applicaTions inForMaTion

A standard application circuit for the LTC3129 is shown on

the front page of this data sheet. The appropriate selection

of external components is dependent upon the required

performance of the IC in each particular application given

considerations and trade-offs such as PCB area, input

and output voltage range, output voltage ripple, transient

response, required efficiency, thermal considerations and

cost. This section of the data sheet provides some basic

guidelines and considerations to aid in the selection of

external components and the design of the applications

circuit, as well as more application circuit examples.

Programming VOUT

The output voltage of the LTC3129 is set by connecting

the FB pin to an external resistor divider from VOUT to

ground, as shown in Figure 5, according to the equation:

VOUT = 1.175V • (1+ R1/R2)

sible. VCC is the regulator output and is also the internal

supply pin for the LTC3129 control circuitry as well as the

gate drivers and boost rail charging diodes. The VCC pin is

not intended to supply current to other external circuitry.

Inductor Selection

ThechoiceofinductorusedinLTC3129applicationcircuits

influences the maximum deliverable output current, the

converterbandwidth,themagnitudeoftheinductorcurrent

ripple and the overall converter efficiency. The inductor

must have a low DC series resistance, when compared to

the internal switch resistance, or output current capabil-

ity and efficiency will be compromised. Larger inductor

values reduce inductor current ripple but may not increase

output current capability as is the case with peak current

mode control as described in the Maximum Output Cur-

rent section. Larger value inductors also tend to have a

higher DC series resistance for a given case size, which

will have a negative impact on efficiency. Larger values

of inductance will also lower the right half plane (RHP)

zero frequency when operating in boost mode, which can

compromise loop stability. Nearly all LTC3129 application

circuitsdeliverthebestperformancewithaninductorvalue

between 3.3µH and 10µH. Buck mode-only applications

can use the larger inductor values as they are unaffected

bytheRHPzero,whilemostlyboostapplicationsgenerally

require inductance on the low end of this range depending

on how large the step-up ratio is.

Regardless of inductor value, the saturation current rating

shouldbeselectedsuchthatitisgreaterthantheworst-case

averageinductorcurrentplushalfoftheripplecurrent.The

peak-to-peak inductor current ripple for each operational

mode can be calculated from the following formula, where

f is the switching frequency (1.2MHz), L is the inductance

in µH and tLOW is the switch pin minimum low time in

µs. The switch pin minimum low time is typically 0.09µs.

ΔIL(P−P)(BUCK) =

VOUT

VIN – VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

– tLOW

⎛

⎝

⎜

⎞

⎠

⎟ A

ΔIL(P−P)(BOOST) =

VIN

VOUT – VIN

VOUT

⎛

⎝

⎜

⎞

⎠

⎟

– tLOW

⎛

⎝

⎜

⎞

⎠

⎟ A

Figure 5. VOUT Feedback Divider

Asmallfeedforwardcapacitorcanbeaddedinparallelwith

R1 (in Figure 5) to reduce Burst Mode ripple and improve

transient response. Details on selecting a feedforward

capacitor are provided later in this data sheet.

VCC Capacitor Selection

The VCC output of the LTC3129 is generated from VIN by a

low dropout linear regulator. The VCC regulator has been

designed for stable operation with a wide range of output

capacitors. For most applications, a low ESR capacitor of

at least 2.2µF should be used. The capacitor should be

located as close to the VCC pin as possible and connected

to the VCCpinandgroundthroughtheshortesttracespos-

3129 F05

LTC3129

VOUT

COUT

CFF

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