LT8551 datasheet(23/32 Pages) AD

23 / 32 page

LT8551

Rev 0

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APPLICATIONS INFORMATION

The term (1+ δ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs Temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

Based on the power dissipation, the MOSFET junction

temperature can be obtained using the formula (1) in

the REG LDO Current Limit and External Power PMOS

Selection section to pick an adequate MOSFET that will

not overheat.

An optional Schottky diode in parallel with the top switch

conducts during the dead time between the conduction

of the main switch and the synchronous switch. This pre-

vents the body diode of the synchronous switch from turn-

ing on, storing charge and requiring a reverse recovery

period that could reduce the overall efficiency. Although

improving the efficiency, the Schottky diode also exhib-

its much higher reverse leakage current than the silicon

diode particularly at high temperature, the combination of

high reverse voltage and current can lead to self-heating

of the diode. Choose a package with lower thermal resis-

tance (θJA) to minimize self-heating of the diode.

CIN and COUT Selection

The input ripple current in a boost converter is relatively

low (compared with the output ripple current), because

this current is continuous. The input capacitor CIN volt-

age rating should comfortably exceed the maximum input

voltage. Although ceramic capacitors can be relatively

tolerant of overvoltage conditions, aluminum electrolytic

capacitors are not. Be sure to characterize the input volt-

age for any possible overvoltage transients that could

apply excess stress to the input capacitors.

The value of CIN is a function of the source impedance, and

in general, the higher the source impedance, the higher

the required input capacitance. The required amount of

input capacitance is also greatly affected by the duty cycle.

High output current applications that also experience high

duty cycles can place great demands on the input supply,

both in terms of DC current and ripple current.

In a boost converter, the output has a discontinuous cur-

rent, so COUT must be capable of reducing the output

voltage ripple. The effects of ESR (equivalent series resis-

tance) and the bulk capacitance must be considered when

choosing the right capacitor for a given output ripple

voltage. The steady ripple voltage due to charging and

discharging the bulk capacitance in a single phase boost

converter is given by:

VRIPPLE =

IOUT MAX

(

) • VOUT – VIN MIN

(

)

(

)

COUT • VOUT • f

where COUT is the output filter capacitor.

The steady ripple due to the voltage drop across the ESR

is given by:

∆VESR = IL(MAX) • ESR

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount

packages. Ceramic capacitors have excellent low ESR

characteristics but can have a high voltage coefficient.

Capacitors are now available with low ESR and high ripple

current ratings (e.g., OS-CON and POSCAP).

Topside MOSFET Driver Supply (CBX, DBX)

An external bootstrap capacitor, CBX, supplies the gate

driver voltage for the top switch. This capacitor is con-

nected between BSTx and SWx and is charged through

Schottky diode DBX from REG when the SWx pin is low.

When the top switch turns on, the SWx rises to VOUT

and the BSTx rises to VOUT + REG. The boost capacitor

needs to store about 100 times the gate charge required

by the top switch. In most applications, a 0.1μF to 0.47μF,

X5R or X7R dielectric capacitor is adequate. The bypass

capacitance from REG to GND should be at least ten times

the bootstrap capacitor value. In addition, the reverse

breakdown of the Schottky diode must greater than the

maximum power VOUT voltage.

Inductor Current Sensing

The LT8551 can be configured to sense the inductor

current through either low value series current sensing

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