Data Sheet

LT8349

analog.com

Rev B

20 of 29

When using large FB resistors, a 4.7pF to 22pF phase-lead capacitor should be connected from VOUT to FB, and a

careful evaluation of system stability should be made to ensure adequate design margin.

Overvoltage Lockout

The VOUT pin voltage is constantly monitored by the LT8349. An overvoltage condition occurs when VOUT pin voltage

exceeds approximately 9V. Switching is stopped at such condition. Normal switching is resumed when the VOUT pin

voltage drops back to 8.9V or lower.

Start-Up

To limit the peak switch current and VOUTovershoot during start-up, the LT8349 contains internal circuitry to provide

soft-start operation (refer to the error amplifier EA in Block Diagram). During start-up, the internal soft-start circuity

slowly ramps the internal SS signal from zero to 1V. When the SS voltage falls between the FB initial voltage and 1V,

the LT8349 regulates the FB pin voltage to the SS voltage instead of 1V. In this way the output capacitor is charged

gradually towards its final value while limiting the start-up peak switch currents.

Referring to Figure 29, the start-up time TSTART_UP is the time period from EN/UVLO transitioning high to VOUT having

reached 90% of its regulation voltage programmed by FB resistor network. TSTART_UP is approximately given by

Equation:

TSTART_UP ≈ 0.15ms +

3.6V

VIN − 0.1V

•

2100

fSW

The LT8349 selects fixed frequency operaton with no spread spectrum frequency modulation during startup, and the

SYNC/MODE pin configuration is ignored.

Figure 29.Typical Start-Up Waveforms

If the LT8349 boost converter is plugged into a live supply, the VOUT could ring to twice the voltage of VIN, due to the

resonant circuit composed by L1, L2, COUT, and the body diode of M2 and M4 (refer to Block Diagram). If such over-

shoot exceeds the VOUT rating, it must be limited to protect the load and the converter. For these situations, a small

Schottky diode or silicon diode can be connected between VIN and VOUT to deactivate the resonant circuit and limit

the VOUT over-shoot as shown in Figure 30. With the diode connected, the boost is also more robust against output

fault conditions such as output short circuit or overload, due to the fact that the diode diverts a great amount of

output current from the IC. D should be rated for short circuit current of VIN source, example - the current limit of the

input side battery or voltage source.

VIN = 3.7V

tSTART_UP

500µs/DIV

EN/UVLO

2V/DIV

VOUT

2V/DIV