LT8331 datasheet(13/34 Pages) AD

13 / 34 page

LT8357

Rev. 0

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8357 F02

Figure 2. Average Conducted EMI

8357 F03

Figure 3. Peak Conducted EMI

Frequency Synchronization and Mode Selection

The switching frequency of the LT8357 can also be syn-

chronized to an external clock using the SYNC/MODE

pin. Driving the SYNC/MODE pin with a 50% duty cycle

waveform is always a good choice, otherwise maintain

the duty cycle between 10% and 90%. The high level

of the external clock must be higher than 2.5V, and the

low level must be lower than 0.4V. The frequency range

must be between 100kHz and 2MHz. The RT resistor

is still required in this case, and the resistance should

correspond to the frequency of the external clock. If the

APPLICATIONS INFORMATION

external clock ever stops, the LT8357 will rely on the RT

resistor to set the frequency. When synchronizing to an

external clock, the LT8357 will forbid entering Burst Mode

operation at light load conditions, but instead operate in

the pulse-skipping mode to maintain regulation.

In addition to the frequency synchronization, the LT8357

also provides four operation mode options with the

switching frequency set by the internal oscillator, which

can be selected by programming the SYNC/MODE pin. At

light load conditions, the user can choose either pulse-

skipping mode or Burst Mode operation. The spread

spectrum frequency modulation can also be enabled or

disabled depending on the EMI performance requirement

of the applications. Please refer to the Pin Functions sec-

tion for more details on mode selection.

Programming VIN UVLO

A resistor divider from VIN to the EN/UVLO pin imple-

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

enable rising threshold is set at 1.220V (typical) and fall-

ing threshold is set at 1.178V (typical) with 42mV hyster-

esis. The programmable UVLO thresholds are:

VIN(UVLO+) =1.220V•

R1+R2

VIN(UVLO−) =1.178V•

R1+R2

Figure 4 shows the implementation of external shut-down

control while still using the UVLO function. The NMOS

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

LT8357 in shutdown with quiescent current less than 3μA.

EN/UVLO

LT8357

GND

VIN

RUN/STOP

CONTROL

(OPTIONAL)

8357 F04

Figure 4. VIN Undervoltage Lockout (UVLO)

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