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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

APPLICATIONS INFORMATION

is the minimum top switch on-time at the loading condi-

tion as shown in Figure 5. Figure 6 shows the relation-

ship between the maximum input voltage vs the switching

frequency. If a smaller RT is selected, to ensure that the

regulator is switching at the higher frequency as illus-

trated in Figure 4, the maximum input supply voltage has

to be lowered; and it needs to be further reduced if the

load is decreased or removed.

For forced continuous mode, if there is a momentarily VIN

voltage surge higher than the voltage shown in Figure 6,

resulting in minimum on-time operation, an overvoltage

comparator guards against transient overshoots as well

as other more serious conditions that may overvoltage the

output. When the VFB voltage rises by more than 3.75%

above its nominal value, the top MOSFET is turned off

and the bottom MOSFET is turned on. At this moment,

the output voltage continues to increase until the inductor

current reverses. The actual peak output voltage will be

higher than 3.75%, depending on external components

value, loading condition and output voltage setting. The

bottom MOSFET remains on continuously until the induc-

tor current exceeds the bottom MOSFET reverse current

or overvoltage condition is cleared. With high reverse cur-

rent, both top and bottom MOSFETs shut off till the next

clock cycle.

Low Supply Operation

The LT8619 is designed to remain operational during

short line transients when the input voltage may briefly

dip below 3.0V. Below this voltage, the INTVCC voltage

might drop to a point that is not able to provide adequate

gate drive voltage to turn on the MOSFET. The LT8619 has

two circuits to detect this undervoltage condition. A UVLO

comparator monitors the INTVCC voltage to ensure that it

is above 2.8V during startup; once in regulation, the chip

continues to operate as long as INTVCC stays above 2.65V.

If this UVLO comparator trips, the chip is shut down until

INTVCC recovers. Another comparator monitors the VIN

supply voltage, add a resistor divider from VIN to EN/UV

to turn off the regulator if VIN dips below the undesirable

voltage.

The LT8619 is capable of a maximum duty cycle of greater

than 99%, and the VIN-to-VOUT dropout is limited by the

RDS(ON) of the top switch. In deep dropout, the loop

attempt to turn on the top switch continuously. However,

the top switch gate drive is biased from the floating boot-

strap capacitor CB, which normally recharges during each

off cycle; in dropout, this capacitor loses its refresh cycle

and charge depleted. A comparator detects the drop in

boot-strap capacitor voltage, forces the top switch off and

recharges the capacitor.

Figure 5. Minimum On-Time vs Load Current

Figure 6. Forced Continuous Mode Maximum Input

Voltage vs Switching Frequency

VOUT = 3.3V

fSW = 2MHz

L = 3.3µH

FORCED CONTINUOUS MODE

LOAD CURRENT (A)

0.2

0.4

0.6

0.8

1.0

1.2

8619 F05

VOUT = 3.3V

L = 10µH

FORCED CONTINUOUS MODE

0.2A LOAD

fSW (MHz)

0.2

0.6

1.0

1.4

1.8

2.2

8619 F06

0.5A

LOAD

NO LOAD

High Supply Operation

For Burst Mode operation or pulse-skipping mode, VIN

voltage may go as high as the absolute maximum rating

of 60V regardless of the frequency setting; however, the

LT8619 will reduce the switching frequency as necessary

to regulate the output voltage.

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