LT8331 datasheet(12/30 Pages) AD

12 / 30 page

LT8365

Rev. A

For more information www.analog.com

DUTY CYCLE CONSIDERATION

The LT8365 minimum on-time, minimum off-time and

switching frequency (fOSC) 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).

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 as:

DMIN = 1 –

VIN(MAX)

VOUT + VD

DMAX = 1 –

VIN(MIN)

VOUT + VD

where VD is the diode forward voltage drop. If the above

duty cycle calculations for a given application violate

the minimum and/or maximum allowed duty cycles

for the LT8365, operation in discontinuous conduction

mode (DCM) might 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

anddutycyclerequiredtoavoidsub-harmonicoscillations

and the right half plane zero (RHPZ). While DCM provides

these benefits, the trade-off is higher inductor peak cur-

rent, lower available output power and reduced efficiency.

SETTING THE OUTPUT VOLTAGE

The output voltage is programmed with a resistor divider

from the output to the FBX pin. Choose the resistor values

for a positive output voltage according to:

R1 = R2 •

VOUT

1.60V

–1

⎛

⎝

⎜

⎞

⎠

⎟

Choose the resistor values for a negative output voltage

according to:

R1 = R2 •

|VOUT|

0.80V

–1

⎛

⎝

⎜

⎞

⎠

⎟

The locations of R1 and R2 are shown in the Block Dia-

gram. 1% resistors are recommended to maintain output

voltage accuracy.

Higher-value FBX divider resistors result in the lowest

input quiescent current and highest light-load efficiency.

FBX divider resistors R1 and R2 are usually in the range

from 25k to 1M.

SOFT-START

The LT8365 contains several features to limit peak switch

currents and output voltage (VOUT) overshoot during

start-up or recovery from a fault condition. The primary

purpose of these features is to prevent damage to external

components or the load.

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

chargetheoutputcapacitorasquicklyaspossible,resulting

in large peak currents. A large surge current may cause

inductor saturation or power switch failure.

The LT8365 addresses this mechanism with a program-

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

thesoft-startfunctioncontrolstherampofthepowerswitch

current by controlling the ramp of VC through Q1. This

allowstheoutputcapacitortobechargedgraduallytoward

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

Figure 3 shows the output voltage and supply current for

the first page Typical Application. It can be seen that both

the output voltage and supply current come up gradually.

FAULT PROTECTION

An inductor overcurrent fault (> 3A) and/or INTVCC

undervoltage (INTVCC < 2.5V) and/or thermal lockout

(TJ > 170°C) will immediately prevent switching, will

reset the SS pin and will pull down VC. Once all faults are

removed, the LT8365 will soft-start VC and hence inductor

peak current.

APPLICATIONS INFORMATION

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