LTC3453

7

3453fa

OPERATIO

switch region begins is given by:

ends is given by

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

always off. Referring to Figure 2, when the control voltage

conducting similar to a typical synchronous boost regula-

tor. The maximum duty cycle of the converter is limited to

Forward Current Limit

exceeds 1612mA (typical), switch A is shut off immedi-

ately. 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

PMOS switch D exceeds 200mA (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

an undervoltage lockout is incorporated on the LTC3453.

When the input supply voltage drops below approximately

1.9V, the four power switches and all control circuitry are

turned off except for the undervoltage block, which draws

only several microamperes.

Overtemperature Protection

If the junction temperature of the LTC3453 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.

Soft-Start

The LTC3453 includes an internally fixed soft-start which

is active when powering up or coming out of shutdown.

node and gradually releasing it such that it requires

0.65ms to linearly slew from 0.9V to 2.1V. This has the

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.

Main Error Amp

The main error amplifier is a transconductance amplifier

with source and sink capability. The output of the main

capacitor sets the dominant pole for the regulation loop.

(See the Applications Information section for selecting the

capacitor value.) The error amp gets its feedback signal

from a proprietary circuit which monitors all 4 LED current

sources to determine which LED to close the regulation

loop on.

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 any of the LED

pins open-circuits, the output voltage will keep rising, and

safety error amp will eventually take over control of the

old at which this occurs is approximately 4.5V.

LED Current Setting Amplifiers and Enable Circuit

The LTC3453 includes two LED current setting amplifiers

that work in conjunction with dual external current setting

resistors and dual enable pins to program LED current to

one of four levels (including shutdown). All four LED

current source outputs are programmed to the same level.

When both enable inputs are logic low, the LTC3453 is in

shutdown, the buck-boost is disabled and all LED currents

are zero. In shutdown, the input supply current is typically

6

µA. If either enable input is logic high, the buck-boost will

regulate the output voltage such that the LEDs are biased