LT8335 datasheet(13/20 Pages) AD

13 / 20 page

LT8338

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

prevent damage to external components or the load.

As shown in the Block Diagram, the soft-start function

controls the ramp of the power switch current by con-

trolling the ramp of VC through Q1. This allows the output

capacitor to be charged gradually toward its final value

while limiting the start-up peak currents. The typical

start-up waveforms are shown in the Typical Performance

Characteristics section.

Hot Plug

If the LT8338 boost converter is plugged into a live sup-

ply, VOUT can ring to twice the VIN voltage due to the

resonant circuit composed by L, C2, and the body diode

of MTOP (refer to the Block Diagram). If such overshoot

exceeds the VOUT rating, it needs to be limited to protect

the load and the converter. In these conditions, a small

diode (Schottky diode or silicon diode) can be connected

between VIN and VOUT to deactivate the resonant circuit

and limit VOUT overshoot as shown in Figure 3. With the

diode connected, the LT8338 boost is also more robust

against output fault conditions such as output short cir-

cuit or overload, due to the diode's ability to divert a great

amount of output current from the LT8338. The diode

can be rated for about one-half to one-fifth the full load

current since it only conducts current during start-up or

output fault conditions.

Fault Protection

INTVCC undervoltage (INTVCC < 2.2V), or thermal lockout

(TJ > 170°C) will immediately stop the converter from

switching, pull down VC and reset soft-start. Faults are

removed when INTVCC > 2.3V, and the die temperature

has dropped down to 165°C or lower. Once all faults are

removed, the LT8338 will resume switching with a soft-

started VC inductor peak current limiting.

In addition, converter will stop switching immediately

when VOUT overvoltage (VOUT > 41.4V) happens, and will

resume switching once VOUT is lower than 40.6V.

Duty Cycle Consideration

The LT8338 minimum on-time, minimum off-time and

switching frequency define the allowable minimum and

maximum duty cycles of the converter (see Minimum

On-Time, Minimum Off-Time, and Switching Frequency

in the Electrical Characteristics table) as:

Minimum Allowable Duty Cycle =

Minimum On-Time(MAX) • fOSC(MAX)

Maximum Allowable Duty Cycle =

1 – Minimum Off-Time(MAX) • fOSC(MAX)

The required switch duty cycle range for a boost converter

operating in continuous conduction mode (CCM) can be

calculated using Equation 3.

DMIN =

VOUT – VIN(MAX)

VOUT

DMAX =

VOUT – VIN(MIN)

VOUT

(3)

If the above duty cycle calculations for a given appli-

cation violate the minimum and/or maximum allowed

duty cycles, operation in discontinuous conduction

mode (DCM) may provide a solution. For the same VIN and

VOUT levels, operation in DCM does not demand as low a

duty cycle as in CCM. DCM also allows higher duty cycle

operation than CCM. The additional advantage of DCM is

the removal of the limitations to inductor value and duty

cycle required to avoid sub-harmonic oscillations and the

right half plane zero (RHPZ). While DCM provides these

Figure 3. A Simplified LT8338 Power Stage with a

Diode Added Between VIN and VOUT

VOUT

VIN

VOUT

GND

LT8338

8338 F03

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