LT3489

5

3489f

OPERATIO

The LT3489 uses a constant frequency, current mode con-

trol scheme to provide excellent line and load regulation.

Please refer to Figure 1 for the following description of the

part’s operation. At the start of the oscillator cycle, the SR

latch is set, turning on the power switch, Q1. The switch

current flows through the internal current sense resistor

generating a voltage. This voltage 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

is simply an amplified version of the difference between

the feedback voltage and the reference voltage of 1.235V.

In this manner, the error amplifier sets the correct peak

current level to keep the output in regulation.

A soft-start function is provided to enable a clean start-up

for the LT3489. When the part is brought out of shut-

down, 10µA of current is sourced out of the SS pin. By

connecting an external capacitor to the SS pin, the rate

of voltage rise on the pin can be set. Typical values for

the soft-start capacitor range from 10nF to 200nF. The SS

in turn limits the peak switch current. Current limit is

not shown in Figure 1. The switch current is constantly

monitored and not allowed to exceed the nominal value

of 2.5A. If the switch current reaches 2.5A, the SR latch

is reset regardless of the output of comparator A2. This

current limit helps protect the power switch as well as the

external components connected to the LT3489.