LT3467 datasheet(6/18 Pages) LINER

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LT3467/LT3467A

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APPLICATIONS INFORMATION

OPERATION

The LT3467 uses a constant frequency, current-mode con-

trol scheme to provide excellent line and load regulation.

Refer to the Block Diagram. At the start of each oscillator

cycle, the SR latch is set which turns on the power switch

Q1. A voltage proportional to the switch current is added

to a stabilizing ramp and the resulting sum is fed into the

positive terminal of the PWM comparator A2. When this

voltage exceeds the level at the negative input of A2, the

SR latch is reset, turning off the power switch. The level

at the negative input of A2 is set by the error ampliﬁer A1,

and is simply an ampliﬁed version of the difference between

the feedback voltage and the reference voltage of 1.255V.

In this manner, the error ampliﬁer sets the correct peak

current level to keep the output in regulation. If the error

ampliﬁer’s output increases, more current is delivered to

the output. Similarly, if the error decreases, less current

is delivered. The soft-start feature of the LT3467 allows

for clean start-up conditions by limiting the rate of voltage

rise at the output of comparator A1 which, in turn, limits

the peak switch current. The soft-start pin is connected

to a reference voltage of 1.255V through a 250k resistor,

providing 4μA of current to charge the soft-start capacitor.

Typical values for the soft-start capacitor range from 10nF

to 200nF. The LT3467 has a current limit circuit not shown

in the Block Diagram. The switch current is constantly

monitored and not allowed to exceed the maximum switch

current (typically 1.4A). If the switch current reaches

this value, the SR latch is reset regardless of the state

of comparator A2. This current limit protects the power

switch as well as the external components connected to

the LT3467.

The Block Diagram for the LT3467A (not shown) is identical

except that the oscillator frequency is 2.1MHz.

Duty Cycle

The typical maximum duty cycle of the LT3467 is 94%

(88% for the LT3467A). The duty cycle for a given ap-

plication is given by:

|VOUT |+|VD |–|VIN |

|VOUT |+|VD |–|VCESAT |

where VD is the diode forward voltage drop and VCESAT is

in the worst case 330mV (at 1.1A)

The LT3467 and LT3467A can be used at higher duty cycles,

but must be operated in the discontinuous conduction

mode so that the actual duty cycle is reduced.

Setting Output Voltage

R1 and R2 determine the output voltage.

VOUT = 1.255V (1+ R1/R2)

Switching Frequency and Inductor Selection

The LT3467 switches at 1.3 MHz, allowing for small valued

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

The LT3467A switches at 2.1MHz, allowing for even smaller

valued inductors to be used. 0.9μH to 6.8μH will usually

sufﬁce. Choose an inductor that can handle at least 1.2A

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 1.3MHz,

(2.1MHz for the LT3467A) such as ferrite core.

LT3467 Datasheet(PDF) 6 Page - Linear Technology

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