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LT3471

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OPERATION

APPLICATIONS INFORMATION

Duty Cycle

The typical maximum duty cycle of the LT3471 is 94%.

The duty cycle for a given application is given by:

DC =

|VOUT |+|VD|–|VIN|

|VOUT |+|VD|–|VCESAT |

Where VD is the diode forward voltage drop and VCESAT

is in the worst case 330mV (at 1.3A)

The LT3471 can be used at higher duty cycles, but it must

be operated in the discontinuous conduction mode so that

the actual duty cycle is reduced.

Setting Output Voltage

Setting the output voltage depends on the topology used.

For normal noninverting boost regulator topologies:

VOUT = VFBP 1+

where VFBN is connected between R1 and R2 (see the

Typical Applications section for examples).

Select values of R1 and R2 according to the following

equation:

R1= R2

VOUT

VREF

–1

A good value for R2 is 15k which sets the current in the

resistor divider chain to 1.00V/15k = 67μA.

VFBP is usually just tied to VREF = 1.00V, but VFBP can also

be tied to a divided down version of VREF or some other

voltage as long as the absolute maximum ratings for the

feedback pins are not exceeded (see Absolute Maximum

Ratings).

For inverting topologies, VFBN is tied to ground and VFBP

is connected between R1 and R2. R2 is between VFBP

and VREF and R1 is between VFBP and VOUT (see the Ap-

plications section for examples). In this case:

VOUT = VREF

Select values of R1 and R2 according to the following

equation:

R1=R2

VOUT

VREF

A good value for R2 is 15k, which sets the current in the

resistor divider chain to 1.00V/15k = 67μA.

Switching Frequency and Inductor Selection

The LT3471 switches at 1.2 MHz, allowing for small valued

inductors to be used. 4.7μH or 10μH will usually sufﬁce.

Choose an inductor that can handle at least 1.4A without

saturating, and ensure that the inductor has a low DCR

(copper-wire resistance) to minimize I2R power losses.

Note that in some applications, the current handling

requirements of the inductor can be lower, such as in the

SEPIC topology where each inductor only carries one half

of the total switch current. For better efﬁciency, use similar

valued inductors with a larger volume. Many different sizes

and shapes are available from various manufacturers.

Choose a core material that has low losses at 1.2 MHz,

such as ferrite core.

Table 1. Inductor Manufacturers

Sumida

(847) 956-0666

www.sumida.com

TDK

(847) 803-6100

www.tdk.com

Murata

(714) 852-2001

www.murata.com

current reaches this value, the SR latch is reset regardless

of the state of the comparator A2 (A4). Also not shown

in the Block Diagram is the thermal shutdown circuit. If

the temperature of the part exceeds approximately 160°C,

both latches are reset regardless of the state of compara-

tors A2 and A4. The current limit and thermal shutdown

circuits protect the power switch as well as the external

components connected to the LT3471.

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