LT3477IUF-TRPBF データシートの表示（PDF） - Linear Technology

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LT3477IUF-TRPBF

3A, DC/DC Converter with Dual Rail-to-Rail Current Sense

Linear Technology 

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OPERATIO

The LT3477 uses a fixed frequency, current mode control

scheme to provide excellent line and load regulation.

Operation 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 regulation. Ampli-

fier 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 constant-

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

LT3477

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 current.

The switch current is constantly monitored and not al-

lowed 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.

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