LT8331 datasheet(11/30 Pages) AD

11 / 30 page

LT8331

Rev. C

For more information www.analog.com

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

and duty cycle required to avoid sub-harmonic oscillations

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

Diagram. 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 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 LT8331 addresses this mechanism with a programma-

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

soft-start function controls the ramp of the power switch

current by controlling 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.

Figure 3 shows the output voltage and supply current for

the first page Typical Applications. 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

inductor peak current.

FREQUENCY FOLDBACK

During start-up or fault conditions in which VOUT is very

low, extremely small duty cycles may be required to main-

tain control of inductor peak current. The minimum on-

time limitation of the power switch might prevent these

low duty 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 this by folding back switching frequency

whenever FBX pin is close to GND (low VOUT levels). This

APPLICATIONS INFORMATION

2ms/DIV

VOUT

20V/DIV

IL1+IL2

100mA/DIV

8331 F03

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