LT8331 datasheet(11/30 Pages) LINER

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LT8331

8331fa

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the minimum and/or maximum allowed duty cycles

for the LT8331, 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 LT8331 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 LT8331 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.

Figure 3. Soft-Start Waveforms

FAULT PROTECTION

An inductor overcurrent fault (> 1.15A) 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 LT8331 will soft-start VC and hence induc-

tor peak current.

FREQUENCY FOLDBACK

Duringstart-uporfaultconditionsinwhichVOUTisverylow,

extremely small duty cycles may be required to maintain

control of inductor peak current. The minimum on-time

limitationofthepowerswitchmightpreventtheselowduty

cycles from being achievable. In this scenario inductor

current rise will exceed inductor current fall during each

cycle, causing inductor current to “walk up” beyond the

switch current limit. The LT8331 provides protection from

thisbyfoldingbackswitchingfrequencywheneverFBXpin

APPLICATIONS INFORMATION

2ms/DIV

VOUT

20V/DIV

IL1+IL2

100mA/DIV

8331 F03

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