LT8580 datasheet(7/32 Pages) LINER

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LT8580

8580fa

For more information www.linear.com/LT8580

operaTion

Figure 1. SEPIC Topology Allows for the Input to Span

the Output Voltage. Coupled or Uncoupled Inductors

Can Be Used. Follow Noted Phasing if Coupled

Figure 2. Dual Inductor Inverting Topology Results in

Low Output Ripple. Coupled or Uncoupled Inductors

Can Be Used. Follow Noted Phasing if Coupled

SHUTDOWN

VIN > VOUT

VIN = VOUT

VIN < VOUT

VOUT

VIN

8580 F01

LT8580

SHDN

GND

FBX

SYNC

CSS

•

SHUTDOWN

VIN

VOUT

VIN

8580 F02

LT8580

SHDN

GND

FBX

SYNC

CSS

•

TheLT8580usesaconstant-frequency,currentmodecon-

trol scheme to provide excellent line and load regulation.

Refer to the Block Diagram for the following description

of the part’s operation. At the start of each oscillator cycle,

the SR latch (SR1) is set, which turns on the power switch,

Q1. The switch current flows through the internal current

sense resistor, generating a voltage proportional to the

switch current. This voltage (amplified by A4) is added

to a stabilizing ramp and the resulting sum is fed into the

positive terminal of the PWM comparator A3. When this

voltage exceeds the level at the negative input of A3, the

SR latch is reset, turning off the power switch. The level

at the negative input of A3 (VC pin) is set by the error

amplifier A1 (or A2) and is simply an amplified version of

the difference between the feedback voltage (FBX pin) and

the reference voltage (1.204V or 3mV, depending on the

configuration). In this manner, the error amplifier sets the

correct peak current level to keep the output in regulation.

The LT8580 has an FBX pin architecture that can be used

for either noninverting or inverting configurations. When

configured as a noninverting converter, the FBX pin is

pulled up to the internal bias voltage of 1.204V by the

RFBX resistor connected from VOUT to FBX. Amplifier A2

becomes inactive and amplifier A1 performs the invert-

ing amplification from FBX to VC. When the LT8580 is in

an inverting configuration, the FBX pin is pulled down to

3mV by the RFBX resistor connected from VOUT to FBX.

Amplifier A1 becomes inactive and amplifier A2 performs

the noninverting amplification from FBX to VC.

SEPIC Topology

As shown in Figure 1, the LT8580 can be configured as

a SEPIC (single-ended primary inductance converter).

This topology allows for the input to be higher, equal, or

lower than the desired output voltage. Output disconnect

is inherently built into the SEPIC topology, meaning no DC

path exists between the input and output. This is useful

for applications requiring the output to be disconnected

from the input source when the circuit is in shutdown.

Inverting Topology

The LT8580 can also work in a dual inductor inverting

topology, as shown in Figure 2. The part’s unique feedback

pin allows for the inverting topology to be built by simply

changing the connection of external components. This

solution results in very low output voltage ripple due to

the inductor L2 in series with the output. Abrupt changes

in output capacitor current are eliminated because the

output inductor delivers current to the output during both

the off-time and the on-time of the LT8580 switch.

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