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LT8390A

8390afa

For more information www.linear.com/LT8390A

APPLICATIONS INFORMATION

∆VCAP(BOOST) =

IOUT(MAX) • VOUT − VIN(MIN)

(

)

COUT • VOUT • f

∆VCAP(BUCK) =

VOUT • 1−

VOUT

VIN(MAX)













8 •L • f2 • COUT

The maximum steady ripple due to the voltage drop across

the ESR is given by:

∆VESR(BOOST) =

VOUT •IOUT(MAX)

VIN(MIN)

•ESR

∆VESR(BUCK) =

VOUT • 1−

VOUT

VIN(MAX)













L • f

•ESR

INTVCC Regulator

An internal P-channel low dropout regulator produces

5V at the INTVCC pin from the VIN supply pin. The INTVCC

powers internal circuitry and gate drivers in the LT8390A.

The INTVCC regulator can supply a peak current of 145mA

and must be bypassed to ground with a minimum of

4.7µF ceramic capacitor. Good local bypass is necessary

to supply the high transient current required by MOSFET

gate drivers.

Higher input voltage applications with large MOSFETs be-

ing driven at higher switching frequencies may cause the

maximum junction temperature rating for the LT8390A

to be exceeded. The system supply current is normally

dominated by the gate charge current. Additional external

loading of the INTVCC also needs to be taken into account

for the power dissipation calculation. The total LT8390A

power dissipation in this case is VIN • IINTVCC, and overall

efficiency is lowered. The junction temperature can be

estimated by using the equation:

TJ = TA + PD •

θJA

where

θJA (in °C/W) is the package thermal resistance.

To prevent maximum junction temperature from being

exceeded, the input supply current must be checked op-

erating in continuous mode at maximum VIN.

Figure 12. VIN Undervoltage Lockout (UVLO)

Top Gate MOSFET Driver Supply (CBST1, CBST2)

ThetopMOSFETdrivers,TG1andTG2,aredrivenbetween

their respective SW and BST pin voltages. The boost volt-

ages are biased from floating bootstrap capacitors CBST1

and CBST2,whicharenormallyrechargedthroughboththe

externalandinternalbootstrapdiodeswhentherespective

top MOSFET is turned off. External bootstrap diodes are

recommended because the internal bootstrap diodes are

notalwaysstrongenoughtorefreshtopMOSFETsat 2MHz.

Both capacitors are charged to the same voltage as the

INTVCCvoltage.ThebootstrapcapacitorsCBST1andCBST2,

need to store about 100 times the gate charge required by

the top switches A and D. In most applications, a 0.1µF

to 0.47µF, X5R or X7R dielectric capacitor is adequate.

Programming VIN UVLO

A resistor divider from VINtotheEN/UVLOpinimplements

VIN undervoltage lockout (UVLO). The EN/UVLO enable

falling threshold is set at 1.220V with 13mV hysteresis. In

addition, the EN/UVLO pin sinks 2.5µA when the voltage

on the pin is below 1.220V. This current provides user

programmable hysteresis based on the value of R1. The

programmable UVLO thresholds are:

VIN(UVLO+) = 1.233V •

R1+R2

+ 2.5µA •R1

VIN(UVLO−) = 1.220V •

R1+R2

Figure12showstheimplementationofexternalshut-down

control while still using the UVLO function. The NMOS

grounds the EN/UVLO pin when turned on, and puts the

LT8390Ainshutdownwithquiescentcurrentlessthan2µA.

LT8390A

VIN

RUN/STOP

CONTROL

(OPTIONAL)

8390a F12

EN/UVLO

GND

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