LT3471 Просмотр технического описания (PDF) - Linear Technology

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производитель

LT3471

Dual 1.3A, 1.2MHz Boost/Inverter in 3mm × 3mm DFN

Linear Technology 

LT3471

BLOCK DIAGRAM

2 FB1P

1 FB1N

VIN

8

+

A1

–

RC

CC

1.00V

REFERENCE

VREF

3

–

A2

+

∑

9 SHDN/SS1

LEVEL

SHIFTER

RAMP

GENERATOR

R

Q

S

DRIVER

10 SW1

Q1

0.01Ω

11 GND

4 FB2P

+

A3

FB2N

5

–

7 SHDN/SS2

LEVEL

SHIFTER

–

RC

A4

CC

+

∑

RAMP

GENERATOR

R

Q

S

DRIVER

6 SW2

Q2

0.01Ω

GND

1.2MHz

OSCILLATOR

3471 F01

Figure 1. Block Diagram

OPERATION

The LT3471 uses a constant frequency, current mode

control scheme to provide excellent line and load regu-

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

oscillator cycle, the SR latch is set, which turns on the

power switch, Q1 (Q2). 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 compara-

tor A2 (A4). When this voltage exceeds the level at the

negative input of A2 (A4), the SR latch is reset, turning

off the power switch Q1 (Q2). The level at the negative

input of A2 (A4) is set by the error amplifier A1 (A3) and

is simply an amplified version of the difference between

the negative feedback voltage and the positive feedback

voltage, usually tied to the reference voltage VREG. 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. Each switcher functions independently but

they share the same oscillator and thus the switchers are

always in phase. Enabling the part is done by taking either

SHDN/SS pin above 1.8V. Disabling the part is done by

grounding both SHDN/SS pins. The soft-start feature of

the LT3471 allows for clean start-up conditions by limiting

the amount of voltage rise at the output of comparator A1

and A2, which in turn limits the peak switching current.

The soft-start feature for each switcher is enabled by

slowly ramping that switcher’s SHDN/SS pin, using an

RC network, for example. Typical resistor and capacitor

values are 0.33μF and 4.7k, allowing for a start-up time

on the order of milliseconds. The LT3471 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 1.6A). If the switch

3471fb

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