LTM8024 datasheet(18/28 Pages) AD

18 / 28 page

LTM8080

Rev. 0

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section gives an example of both channels of the LTM8080

configured for load sharing. When load sharing among

n channels and using a single RSET resistor, the value of

the resistor is shown in Equation 1.

RSET =

VOUT

n •100µA

(1)

CIN

VOUT

(MAX)

RSET

COUT1

COUT

20m

COUT2

20m

CBUS

VIN

RUN

SYNC

VIN

GND

VBUS

LDO1

LDO2

VOUT1

VSNS1

VOUT2

VSNS2

PGFB1

SET1

PGFB2

SET2

STEP-DOWN

SWITCHING

REGULATOR

CSET

CSS

LTM8080

8080 F01

Figure 1. Paralleling Two Channels of LTM8080

Burst-Mode Operation

To enhance efficiency at light loads, the LTM8080 can be

configured to Burst Mode operation, which keeps the VBUS

capacitor charged to the proper voltage while minimizing

the quiescent input current. During Burst Mode opera-

tion, the LTM8080 delivers single-cycle bursts of current

to the VBUS capacitor, followed by sleep periods where

most of the internal circuitry is powered off, and energy is

provided to the LDOs by the VBUS capacitor. During sleep

time, VIN quiescent current is significantly reduced, so as

the load current decreases towards a no-load condition,

the percentage of time that the LTM8080 operates in sleep

mode increases, and the average input current are sig-

nificantly reduced, resulting in higher light load efficiency.

Burst Mode operation is enabled by tying SYNC to GND.

Minimum Input Voltage

The LTM8080’s front end is a step-down converter, so

a minimum headroom is required to keep the output in

regulation. Keep the input above 3.5V to ensure proper

operation. Voltage transients or ripple valleys that cause

the input to fall below 3.5V may turn off the LTM8080.

VBUS Soft-Start

The LTM8080 allows the user to program its VBUS volt-

age ramp rate using the SS pin. An internal 2μA pulls up

the SS pin to about 3.1V. Putting an external capacitor

on SS enables soft-starting the VBUS to reduce current

surges on the input supply. The VBUS voltage will pro-

portionally track the SS pin voltage during the soft-start

ramp. When the SS voltage exceeds 0.778V, the internal

feedback voltage will take over and regulate the internal

reference voltage. The SS pin may be left floating if the

function is not needed.

An active pull-down circuit is connected to the SS pin,

which will discharge the external soft-start capacitor in

the case of fault conditions and restart the ramp when the

faults are cleared. Fault conditions that clear the soft-start

capacitor are the RUN pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Refer to Set PIN (Bypass) Capacitance: Noise, PSRR,

Transient Response, and VOUT Soft-Start section for the

output soft-start function.

Prebiased Output

As the LTM8080 regulates the VOUT to VSET, if the

LTM8080 output is higher than the target output volt-

age, the LTM8080 cannot regulate the output to target

output voltage.

VBUS can be used as an auxiliary supply, so care should

be taken if the VBUS voltage is higher than the target

VBUS voltage. LTM8080 front-end switching regulator

will attempt to regulate the VBUS voltage to the target

VBUS voltage by returning a small amount of energy back

to the input supply. If nothing is loading the input supply,

its voltage may rise. Take care that it stays high enough

that the input voltage is at most the absolute maximum

rating of the LTM8080.

Frequency Foldback

The LTM8080 is equipped with frequency foldback, which

reduces the thermal and energy stress on the internal

power elements during a short-circuit or output overload

condition. During the start-up time, frequency foldback is

also active to limit the energy delivered to the potentially

APPLICATIONS INFORMATION

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