Data Sheet

LT7153SP

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Rev 0

19 of 31

Soft-Start and Output Voltage TRACK

An internal 10µA pulls up the TRACK pin to INTVcc. Putting an external capacitor, CSS, from TRACK to ground enables

soft starting the output to prevent a current surge on the input supply. The relationship between output rise time,

TSS, and soft-start capacitance, CSS, is given by:

TSS = 0.5e5 • CSS

Upon start-up time, the LT7153SP operates in discontinuous mode until track voltage is higher than 0.5V. The

regulator then operates in forced continuous mode until output is above the UV threshold (VFB > 0.475V). Once the

output reaches this voltage, the operating mode of the regulator switches to the mode selected by the MODE pin as

described above.

The LT7153SP allows the user to program its output voltage ramp rate through the TRACK pin. From 0V to 0.5V, the

TRACK voltage overrides the internal 0.5V reference input to the error amplifier, thus regulating the feedback voltage

to that of the TRACK pin. When TRACK is above 0.5V, tracking is disabled, and the feedback voltage regulates to the

internal reference voltage.

Inductor Selection

For a given input voltage, VIN, output voltage, VOUT, the inductor value, L, and operating frequency, f, determine the

ripple current:

∆IL =

VOUT

fSW • L

• (1 −

VOUT

VIN

)

The inductor current ripple decreases with higher inductor value and higher operating frequency. Lower ripple

current reduces core losses in the inductor, ESR losses in the output capacitors, and output voltage ripple. Highest

efficiency operation is obtained at low frequency with small ripple current. However, achieving this requires a large

inductor. There is a trade-off between component size, efficiency, and operating frequency.

A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX). Exceeding 60% of IOUT(MAX) is not

recommended. Note that the largest ripple current occurs at the highest VIN. To guarantee that ripple current does

not exceed a specified maximum, the inductance should be chosen according to:

L =

VOUT

fSW • ∆L(MAX)

• (1 −

VOUT

VIN(MAX)

)

Once the value for L is known, the type of inductor must be selected. Actual core loss is independent of core size for

a fixed inductor value, but is very dependent on the inductance selected. As the inductance or frequency increases,

core losses decrease. Unfortunately, increased inductance requires more turns of wire leading to increased DCR and

copper losses.

Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation. Ferrite core material saturates hard, which means that

inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate.

To avoid overheating and poor efficiency, choose an inductor with an RMS current rating that is greater than the

maximum expected output load of the application. In addition, the saturation current (typically labeled ISAT) rating

of the inductor must be higher than the load current plus 1/2 of inductor ripple current:

IL(PEAK) = IOUT(MAX) +

∆IL

2

Different core materials and shapes change the size/current and price/current relationship of an inductor. Toroid or

shielded pot cores in ferrite or permalloy materials are small and do not radiate much energy, but generally cost