LT8331 datasheet(11/24 Pages) AD

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LT8334

Rev. 0

For more information www.analog.com

DUTY CYCLE CONSIDERATION

The LT8334 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 and Equation 3).

Minimum Allowable Duty Cycle =

Minimum On-Time(MAX) • fOSC(MAX)

Maximum Allowable Duty Cycle =

1 – Minimum Off-Time(MAX) • fOSC(MAX)

(3)

The required switch duty cycle range for a boost converter

operating in continuous conduction mode (CCM) can be

calculated with Equation 4.

DMIN = 1 –

VIN(MAX)

VOUT + VD

DMAX = 1 –

VIN(MIN)

VOUT + VD

(4)

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 LT8334, 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 Equation 5

R1 = R2 •

VOUT

1.6V

–1

⎛

⎝⎜

⎞

⎠⎟

(5)

Choose the resistor values for a negative output voltage

according to Equation 6.

R1 = R2 •

|VOUT|

0.8V

–1

⎛

⎝⎜

⎞

⎠⎟

(6)

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 LT8334 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 LT8334 addresses this mechanism with a program-

mable 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 gradu-

ally 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 Application. It shows

that both the output voltage and supply current come

up gradually.

APPLICATIONS INFORMATION

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