Data Sheet

LT7200S

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Rev 0

17 of 35

APPLICATIONS INFORMATION

The first page of the data sheet shows a general LT7200S application circuit. External component selection is largely

driven by the target current ripple, load requirement, and switching frequency. Component selection typically begins

with the selection of the inductor L and resistor RRT. Once the inductor is chosen, select the input capacitor, CIN, and

the output capacitor, COUT. Next, select the feedback resistors to set the desired output voltage. Finally, select the

remaining optional external components for functions such as external loop compensation, tracking/soft-start, input

UVLO, and PGOOD.

Programming Switching Frequency

Selection of the switching frequency is a trade-off between efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low

output ripple voltage.

Connecting a resistor, RRT, from the RT pin to SGND programs the switching frequency, f, from 400kHz to 3MHz

according to the following formula:

f(Hz) =

1e11

RRT(Ω)

The internal PLL has a synchronization range of ±30% around its programmed frequency. Therefore, during external

clock synchronization be sure that the external clock frequency is within this ±30% range of the RRT programmed

frequency.

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

f x L

x (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), which is around 5A per channel.

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

f x ∆IL(MAX)

(1 −

VOUT

VIN(MAX)

)

Once the value for L is known, select the type of inductor. 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.