LT8646SEVPBF datasheet(24/30 Pages) AD

24 / 30 page

LT8645S/LT8646S

Rev. B

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APPLICATIONS INFORMATION

of the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency. A

phase lead capacitor (Cpl) across the feedback divider can

be used to improve the transient response and is required

to cancel the parasitic pole caused by the feedback node

to ground capacitance.

Output Voltage Tracking and Soft-Start

The LT8645S/LT8646S allows the user to program its out-

putvoltagerampratebymeansoftheTR/SSpin.Aninternal

1.9μApullsuptheTR/SSpintoINTVCC.Puttinganexternal

capacitor on TR/SS enables soft starting the output to

prevent current surge on the input supply. During the soft-

start ramp the output voltage will proportionally track the

TR/SS pin voltage.

For output tracking applications, TR/SS can be externally

driven by another voltage source. For the LT8645S, from

0V to 0.97V, the TR/SS voltage will override the internal

0.97Vreferenceinputtotheerroramplifier,thusregulating

the FB pin voltage to that of TR/SS pin. When TR/SS is

above 0.97V, tracking is disabled and the feedback volt-

age will regulate to the internal reference voltage. For the

LT8646S, from 0V to 1.6V, the TR/SS voltage will override

the internal 0.97V reference input to the error amplifier,

thus regulating the FB pin voltage to a function of the TR/

SSpin.SeeplotintheTypicalPerformanceCharacteristics

section. When TR/SS is above 1.6V, tracking is disabled

and the feedback voltage will regulate to the internal ref-

erence voltage. The TR/SS pin may be left floating if the

function is not needed.

An active pull-down circuit is connected to the TR/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 EN/UV pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Output Power Good

WhentheLT8645S/LT8646S’soutputvoltageiswithinthe

±8% window of the regulation point, the output voltage

is considered good and the open-drain PG pin goes high

impedance and is typically pulled high with an external

resistor. Otherwise, the internal pull-down device will pull

the PG pin low. To prevent glitching both the upper and

lower thresholds include 0.4% of hysteresis. PG is valid

when VIN is above 3.4V

The PG pin is also actively pulled low during several fault

conditions: EN/UV pin is below 1V, INTVCC has fallen too

low, VIN is too low, or thermal shutdown.

Paralleling (LT8646S Only)

Toincreasethepossibleoutputcurrent,twoLT8646Sscan

be connected in parallel to the same output. To do this, the

VCandFBpinsareconnectedtogether,andeachLT8646S’s

SW node is connected to the common output through its

own inductor. The CLKOUT pin of one LT8646S should be

connected to the SYNC/MODE pin of the second LT8646S

to have both devices operate in the same mode. During

pulse-skipping, Spread Spectrum, and Synchronization

modes, both devices will operate at the same frequency.

Figure 5 shows an application where two LT8646S are

paralleled to get one output capable of up to 16A.

Figure 5. Paralleling Two LT8646S

LT8646S

8645S F05

CLKOUT

LT8646S

SYNC/MODE

COUT

VOUT

16A

Shorted and Reversed Input Protection

The LT8645S/LT8646S will tolerate a shorted output.

Several features are used for protection during output

short-circuit and brownout conditions. The first is the

switching frequency will be folded back while the output

is lower than the set point to maintain inductor current

control. Second, the bottom switch current is monitored

such that if inductor current is beyond safe levels switch-

ing of the top switch will be delayed until such time as the

inductor current falls to safe levels.

LT8646SEVPBF 数据表(PDF) 24 Page - Analog Devices

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