LT3477 datasheet(7/20 Pages) LINER

7 / 20 page

LT3477

3477fc

OPERATION

The LT3477 uses a ﬁxed frequency, current mode control

scheme to provide excellent line and load regulation. Op-

eration can be best understood by referring to the Block

Diagram in Figure 1. The start of each oscillator cycle sets

the SR latch and turns on power switch Q1. The signal

at the noninverting input of the PWM comparator (A4

SLOPE) is proportional to the sum of the switch current

and oscillator ramp. When SLOPE exceeds VC (the output

of the feedback ampliﬁer), the PWM comparator resets

the latch and turns off the power switch. In this manner,

the feedback ampliﬁer and PWM comparators set the

correct peak current level to keep the output in regula-

tion. Ampliﬁer A3 drives A4 inverting input. A3 has three

inputs, one from the voltage feedback loop and the other

two from the current feedback loop. Whichever feedback

input is higher takes precedence, forcing the converter into

either a constant-current or a constant-voltage mode. The

LT3477 is designed to transition cleanly between the two

modes of operation. Current sense ampliﬁer IA1 senses

the voltage between the ISP1 and ISN1 pins and provides

a pre-gain to ampliﬁer A1. When the voltage between ISP1

and ISN1 reaches 100mV, the output of IA1 provides VADJ

to the inverting input of A1 and the converter is in con-

stant-current mode. If the current sense voltage exceeds

100mV, the output of IA1 will increase causing the output

of A3 to decrease, thus reducing the amount of current

delivered to the output. In this manner the current sense

voltage is regulated to 100mV. The current sense level is

also pin adjustable by IADJ1. Forcing IADJ1 to less than

625mV will overwrite VADJ voltage that’s set internally,

thus providing current level control. The second current

sense ampliﬁer, IA2, works the same as the ﬁrst current

sense ampliﬁer IA1. Both current sense ampliﬁers provide

rail-to-rail current sense operation. Similarly, for positive

output voltage operation where FBP is tied to VREF, if

the FBN pin increases above VREF, the output of A3 will

decrease to reduce the peak current level and regulate

the output (constant-voltage mode). For negative output

voltage operation where FBN is tied to GND, if the FBP pin

decreases below GND level, the output of A3 will decrease

to reduce the peak current level and regulate the output

(constant-voltage mode).

The LT3477 also features a soft-start function. During

start-up, 9μA of current charges the external soft-start

capacitor. The SS pin directly limits the rate of voltage rise

on the VC pin, which in turn limits the peak switch cur-

rent. The switch current is constantly monitored and not

allowed to exceed the nominal value of 3A. If the switch

current reaches 3A, the SR latch is reset regardless of the

output of the PWM comparator. Current limit protects the

power switch and external components.

LT3477 Datasheet(PDF) 7 Page - Linear Technology

LT3477 Datasheet(HTML) 7 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL