LT8335 datasheet(12/20 Pages) AD

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LT8338

Rev. 0

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TYPICAL APPLICATIONS

The input synchronization clock signal can be square

wave, triangle wave, or sinusoidal wave. The input signal

should have valleys that are below 1V and peaks above 2V.

The minimum duration time that the input signal ampli-

tude stays higher than the 2V threshold and lower than

the 0.8V threshold, should be no less than 30ns.

Frequency Spread Spectrum Modulation

The LT8338 features spread spectrum operation to further

reduce EMI/EMC emissions. The user can select frequency

spectrum modulation with Burst Mode operation by con-

necting the SYNC/MODE pin to ground through a 100k

resistor, or frequency spread modulation with pulse-skip-

ping operation by connecting the SYNC/MODE pin to

INTVCC. When frequency spectrum modulation is selected

and the converter operates at heavy load, the triangu-

lar frequency modulation varies the switching frequency

between the value programmed by RT to approximately

14% higher than that value. The modulation frequency

is approximately 0.42% of the switching frequency. For

example, when the LT8338 is programmed to 2MHz, the

frequency will vary from 2MHz to 2.3MHz at a 9kHz rate.

When operating at light load, frequency spread spectrum

modulation is more effective in pulse-skipping mode than

in Burst Mode operation, due to the fact that pulse-skip-

ping operation maintains the switching frequency with

spread spectrum down to a much lower load current com-

pared to Burst Mode operation.

CTRL Resistor Network

The output voltage is internally set as shown in Equation 2.

VOUT = VCTRL • 18

(2)

Typically, the CTRL pin voltage is programmed with a

resistor divider between the INTVCC and ground (refer to

the Block Diagram).

1% resistors are recommended to maintain output voltage

accuracy. The current flowing in the divider acts as a load

current of the internal LDO and will increase the no-load

input current to the converter. If low input quiescent cur-

rent and good light-load efficiency are desired, use large

resistor values for the CTRL resistor divider.

VIN to VOUT PassThru Mode Operation

When VIN rises above the regulated VOUT voltage pro-

grammed by the CTRL pin voltage, the LT8338 boost con-

verter enters PassThru operation, where the synchronous

power switch MTOP (refer to the Block Diagram) is kept

on continuously and power switch MBOT is kept off con-

tinuously. An internal charge pump circuit is activated to

deliver sufficient current to the boost capacitor (CBST)

to maintain the MTOP’s gate drive voltage. In PassThru

mode VOUT is essentially shorted to VIN by the inductor

and power switch. When VIN falls below VOUT voltage, or

the inductor conducts more than 300mA current from

VOUT to VIN, LT8338 exits PassThru mode operation. If

VOUT is lower than the desired voltage, the normal boost

switching operation resumes.

Switching Frequency Foldback when VIN

Approaches VOUT

In some boost applications, VIN may rise to a voltage very

close to VOUT. When this occurs, the switching regulator

must operate at very low duty cycle to keep VOUT in reg-

ulation. However, the minimum on-time limitation may

prevent the switcher from attaining a sufficiently low duty

cycle at the programmed switching frequency; as a result

a typical boost converter may experience a large output

ripple. LT8338 addresses this issue by adopting a switch-

ing frequency foldback function to smoothly decrease the

switching frequency when its minimum on-time starts

to limit the switcher from attaining a sufficiently low

duty cycle. The typical switching waveforms when VIN

approaches VOUT are shown in the Typical Performance

Characteristics section.

Soft-Start

High peak switch currents during start-up may occur

in switching regulators. Since VOUT is far from its final

value, the feedback loop is saturated and the regulator

tries to charge the output capacitor as quickly as possible,

resulting in large peak currents. A large surge current may

cause inductor saturation or power switch failure.

The LT8338 utilizes a soft-start function to limit peak

switch currents and output voltage (VOUT) overshoot

during start-up or recovery from a fault condition to

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