LT7101 datasheet(21/38 Pages) AD

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LT7101

Rev. 0

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APPLICATIONS INFORMATION

RIND Resistor Selection

The resistor on the RIND pin is used to indicate to the

LT7101 what inductance value is being used. This is

required for the internal reconstruction of the inductor

current waveform and to set the DC gain of the current

loop. Once the inductor value is selected, the RIND pin

resistor is chosen according to:

RIND =

3.3 •L

If fixed VOUT operation is selected using the VPRG1 and

VPRG2 pins, the RIND pin can be left floating, but only if

the inductance value is chosen according to Table 3. Do

not leave the RIND pin floating when adjustable VOUT mode

is selected. If the RIND pin is left floating and adjustable

VOUT mode selected (VPRG1 and VPRG2 are both floating),

the LT7101 will detect this as a fault condition and will

not operate.

The allowable current range on the RIND pin is between

8μA and 220μA, which means that:

2.5 ≤ f • L ≤ 67

In practice, the above constraint does not normally affect

the choice of inductor value.

CIN Selection

The input capacitance, CIN, is needed to filter the trap-

ezoidal current at the drain of the top power MOSFET. CIN

should be sized to do this without causing a large variation

in input voltage. In addition, the input capacitor needs

to have a very low ESR and must be rated to handle the

worst-case RMS input current of:

IRMS =

IOUT(MAX)

Note that capacitor manufacturers’ ripple current ratings

are often based on only 2000 hours of life. This makes

it advisable to further derate the capacitor, or to choose

a capacitor rated at a higher temperature than required.

Several capacitors may be paralleled to meet size or height

requirements in the design. Due to the high operating

frequency of the LT7101, ceramic capacitors can also be

used for CIN. In many applications, an X7R capacitor of

4.7μF or greater is a suitable choice. Always consult the

manufacturer if there is any question.

The input capacitor, CIN, should be placed as close as

possible to the VIN pins, with a low inductance connec-

tion to the PGND (paddle) of the IC. In addition to a larger

bulk capacitor, a smaller case-size (0603 or 0805) ceramic

decoupling capacitor can be placed closer to the VIN pins

to reduce EMI.

Using an LC Input Filter

Figure 4.

7101 F04

VIN

ZOUT

ZIN

LT7101

CBULK

OPTIONAL

Input Filter with Optional Damping Network

For high voltage applications, it can be costly to use bulk

capacitance that is rated to handle the required RMS input

current. Moreover, when using a simple capacitor to filter

the AC input current, it is difficult to determine exactly

where this AC current is flowing when a power supply is

placed into a larger system. To avoid these issues, an LC

filter can be used on the power supply input as shown in

Figure 4. This keeps the higher AC currents contained in

a relatively small and inexpensive capacitor (CF) whose

RMS current rating is known to be adequate. Choose an

LC filter such that:

2π LFCF

where f is the switching frequency. This will attenuate the

RMS input current by a factor of approximately 5X, greatly

alleviating the RMS input requirements of the larger bulk

capacitor CBULK. The filter inductor LF should have a satu-

ration current of at least:

ISAT(LF) ≥ 1.3 •

VOUT IOUT(MAX)

VIN(MIN)

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