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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

APPLICATIONS INFORMATION

threshold can be adjusted by setting the values R3 and

R4 such that they satisfy the following equation:

VIN(EN/UV) = 1+

⎛

⎝⎜

⎞

⎠⎟

• 1V

where the LT8619 will remain off until VIN is above

VIN(EN/UV). Due to the comparator’s hysteresis, switching

will not stop until the input falls slightly below VIN(EN/UV).

When in Burst Mode operation for light load currents,

the current through the VIN(EN/UV) resistor network can

easily be greater than the supply current consumed by

the LT8619. Therefore, the VIN(EN/UV) resistors should

be large enough to minimize their impact on efficiency

at low loads.

INTVCC Regulator

An internal low dropout (LDO) regulator produces the 3.3V

supply from VIN that powers the drivers and the internal

bias circuitry. The INTVCC can supply enough current for

the LT8619’s circuitry and must be bypassed to ground

with at least a 1μF ceramic capacitor. Good bypassing is

necessary to supply the high transient currents required

by the power MOSFET gate drivers. To improve efficiency

the internal LDO can draw current from the BIAS pin when

the BIAS pin is at 3.1V or higher. Typically the BIAS pin can

be tied to the output of the switching regulator, or can be

tied to an external supply which must also be at 3.3V or

above. If BIAS is connected to a supply other than VOUT,

be sure to bypass with a local ceramic capacitor. If the

BIAS pin is below 3.0V, the internal LDO will consume

current from VIN. Applications with high input voltage and

high switching frequency where the internal LDO pulls

current from VIN will increase die temperature because

of the higher power dissipation across the LDO. Do not

connect an external load to the INTVCC pin.

Output Power Good

When the LT8619’s output voltage is within the ±7.5%

window of the regulation point, the open-drain PG pin

goes high impedance and is typically pulled high with an

external resistor. Otherwise, the internal open-drain tran-

sistor will pull the PG pin low. The PG pin is also actively

pulled low during several fault conditions: EN/UV pin is

below 1V, INTVCC drops below its UVLO threshold, VIN is

too low, or thermal shutdown.

Synchronization

Synchronizing the LT8619 oscillator to an external fre-

quency can be done by connecting a square wave (with

20% to 80% duty cycle) to the SYNC pin. The square wave

amplitude should have valleys that are below 0.4V and

peaks above 2V (up to 6V). During frequency synchroni-

zation, the part operates in forced continuous mode with

the SW rising edge synchronized to the SYNC positive

edge. The LT8619 may be synchronized over a 300kHz

to 2.2MHz range. The RT resistor must be chosen to set

the LT8619 switching frequency equal or below the lowest

synchronization input. For example, if the synchroniza-

tion signal will be 500kHz and higher, the RT should be

selected for 500kHz.

Start-Up Inrush Current, Short-Circuit Protection

Upon start-up, the internal soft-start action regulates

the VOUT slew rate; the LT8619 provides the maximum

rated output current to charge up the output capacitor

as quickly as possible. During start-up, if the output is

overloaded, the regulator continues to provide the maxi-

mum sourcing current to overcome the output load, but

at the same time, the bottom switch current is monitored

such that if the inductor current is beyond the safe levels,

switching of the top switch will be delay until such time

as the inductor current falls to safe levels.

Once the soft-start period has expired and the FB voltage

is higher than 0.74V, the LT8619 switching frequency will

be folded back if the external load pulls down the output.

At the same time, the bottom switch current will continue

to be monitored to limit the short-circuit current. Figure 8

shows the frequency foldback transfer curve and Figure 9

shows the short circuit waveform. During this overcurrent

condition, if the SYNC pin is connected to a clock source,

the LT8619 will get out from the synchronization mode.

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