LT8331 datasheet(14/34 Pages) AD

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LT8357

Rev. 0

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INTVCC Regulator

An internal P-channel low dropout regulator produces

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

powers the internal circuitry and MOSFET gate driver in

the LT8357. The INTVCC regulator can supply a peak cur-

rent of 60mA (typical) and must be bypassed to ground

with a minimum of 2.2μF ceramic capacitor. Good local

bypass is necessary to supply the high transient current

required by the MOSFET gate driver.

Higher input voltage applications with large power

MOSFET being driven at higher switching frequencies

may cause the maximum junction temperature rating for

the LT8357 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

LT8357 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 when

operating in continuous mode at maximum VIN.

Duty Cycle Consideration

Switching duty cycle is a key variable defining converter

operation. As such, its limits must be considered. The

minimum on-time is the smallest time duration that the

LT8357 is capable of turning on the power MOSFET. In

each switching cycle, the LT8357 also keeps the power

MOSFET off for a minimum amount of time duration,

which is the minimum off-time. Both minimum on-time

and minimum off-time will increase as the switching

frequency decreases (see the Minimum/Maximum Duty

Cycle vs Oscillator Frequency in the Typical Performance

Characteristics section).

APPLICATIONS INFORMATION

The minimum on-time TMIN_ON_TIME and minimum off-

time TMIN_OFF_TIME and the switching frequency f define

the minimum and maximum switching duty cycles that

the converter is able to operate at:

Minimum Allowable Duty Cycle = TMIN_ON_TIME(MAX)• f(MAX)

MaximumAllowableDutyCycle=1−TMIN_OFF_TIME(MAX)•f(MAX)

Programming the Output Voltage and Thresholds

The LT8357 has a voltage feedback pin FB that can be

used to program its output regulation voltage. The output

voltage can be set by selecting the values of R3 and R4

(Figure 5) according to the following equation:

VOUT =1V•

R3+R4

In addition, the FB pin voltage also determines output

overvoltage threshold and output power good thresholds.

For an application with small output capacitors, the output

voltage may excessively overshoot during load transient

event. Once the FB pin hits its overvoltage threshold

1.08V (typical), the LT8357 stops switching immediately

by turning off the power MOSFET. The output overvoltage

threshold is set with the equation:

VOUT(OVP) =1.08V•

R3+R4

When the LT8357 is operating in Burst Mode, the current

flowing through the feedback resistor divider can easily

exceed the input quiescent current consumed by the part.

Therefore, high resistor values are preferred for R3 and

R4 to achieve high light-load efficiency.

LT8357

GND

VOUT

8357 F05

Figure 5. Feedback Resistor Connection

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