LTC3454 Просмотр технического описания (PDF) - Linear Technology

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LTC3454

1A Synchronous Buck-Boost High Current LED Driver

Linear Technology 

LTC3454

APPLICATIONS INFORMATION

Boost Mode (VIN < VOUT)

In boost mode, switch A is always on and switch B is always

off. Referring to Figure 2, when the control voltage VC is

above voltage V3, switches C and D will alternate conduct-

ing similar to a typical synchronous boost regulator. The

maximum duty cycle of the converter is limited to 88%

typical and is reached when VC is above V4.

Forward Current Limit

If the current delivered from VIN through PMOS switch A

exceeds 3.4A (typical), switch A is shut off immediately.

Switches B and D are turned on for the remainder of the

cycle in order to safely discharge the forward inductor

current at the maximum rate possible.

Reverse Current Limit

If the current delivered from VOUT backwards through

PMOS switch D exceeds 275mA (typical), switch D is

shut off immediately. Switches A and C are turned on for

the remainder of the cycle in order to safely discharge the

reverse inductor current at the maximum rate possible.

Undervoltage Lockout

To prevent operation of the power switches at high RDS(ON),

an undervoltage lockout is incorporated on the LTC3454.

When the input supply voltage drops below approximately

1.90V, the four power switches and all control circuitry are

turned off except for the undervoltage block, which draws

a few microamperes.

Overtemperature Protection

If the junction temperature of the LTC3454 exceeds 130°C

for any reason, all four switches are shut off immediately.

The overtemperature protection circuit has a typical hys-

teresis of 11°C.

limiting the rate of duty cycle change as VC transitions

from the buck region through the buck-boost region into

the boost region. Once the soft-start times out, it can only

be reset by entering shutdown, or by an undervoltage or

overtemperature condition.

Autozero Error Amp

The error amplifier is an autozeroing transconductance

amp with source and sink capability. The output of this

amplifier drives a capacitor to GND at the VC pin. This

capacitor sets the dominant pole for the regulation loop.

(See the Applications Information section for selecting

the capacitor value). The feedback signal to the error

amp is developed across a resistor through which LED

current flows.

Safety Error Amp

The safety error amplifier is a transconductance amplifier

with sink only capability. In normal operation, it has no

effect on the loop regulation. However, if the LED pin open-

circuits, the output voltage will keep rising, and the safety

error amp will eventually take over control of the regulation

loop to prevent VOUT runaway. The VOUT threshold at which

this occurs is approximately 5.15V.

LED Current Programming and Enable Circuit

Two enable pins work in conjunction with dual external

resistors to program LED current to one of three nonzero

settings. The table below explains how the current can

be set.

EN1

EN2

ILOAD (A)

GND

GND 0 (SHUTDOWN)

VIN

GND

3850 • 0.8V/RISET1

GND

VIN

3850 • 0.8V/RISET2

VIN

VIN

3850 • (0.8V/RISET1 + 0.8V/RISET2)

Soft-Start

The LTC3454 includes an internally fixed soft-start which

is active when powering up or coming out of shutdown.

The soft-start works by clamping the voltage on the VC

node and gradually releasing it such that it requires 200μs

to linearly slew from 0.9V to 2.1V. This has the effect of

With either enable pin pulled high, the buck-boost will

regulate the output voltage at the current programmed

by RISET1 and/or RISET2.

With both enable pins pulled to GND, the LTC3454 is in

shutdown and draws zero current. The enable pins are

high impedance inputs and should not be floated.

3454fa

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