LTC7803 datasheet(34/44 Pages) AD

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LTC7817

Rev 0

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Buck Design Example

As a design example for one of the buck channels,

assume VIN(NOMINAL) = 12V, VIN(MAX) = 22V, VOUT = 3.3V,

IOUT = 20A, and fSW = 1MHz.

1. Set the operating frequency. The frequency is not one

of the internal preset values, so a resistor from the

FREQ pin to GND is required, with a value of:

RFREQ(in k� )=

37MHz

1MHz

= 37k�

2. Determine the inductor value. Initially select a value

based on an inductor ripple current of 30%. The

inductor value can then be calculated from the fol-

lowing equation:

VOUT

fSW � IL

( )

1–

VOUT

VIN(NOM)

�

��

�

÷÷=0.4µH

The highest value of ripple current occurs at the maxi-

mum input voltage. In this case the ripple at VIN = 22V

is 35%

3. Verify that the minimum on-time of 40ns is not vio-

lated. The minimum on-time occurs at VIN(MAX):

tON(MIN) =

VOUT

VIN(MAX)(fSW )

= 150ns

This is more than sufficient to satisfy the minimum

on time requirement. If the minimum on time is vio-

lated, the LTC7817 skips pulses at high input volt-

age, resulting in lower frequency operation and higher

inductor current ripple than desired. If undesirable,

this behavior can be avoided by decreasing the fre-

quency (with the inductor value accordingly adjusted)

to avoid operation near the minimum on-time.

4. Select the RSENSE resistor value. The peak inductor

current is the maximum DC output current plus half of

the inductor ripple current. Or 20A • (1+0.30/2) = 23A

in this case. The RSENSE resistor value can then be

calculated based on the minimum value for the maxi-

mum current sense threshold (45mV):

RSENSE �

45mV

23A

� 2m�

APPLICATIONS INFORMATION

To allow for additional margin, a lower value RSENSE

may be used (for example, 1.8mΩ); however, be

sure that the inductor saturation current has suffi-

cient margin above VSENSE(MAX)/RSENSE, where the

maximum value of 55mV is used for VSENSE(MAX).

For this low inductor value and high current applica-

tion, an RC filter into the sense pins should be used

to compensate for the parasitic inductance (ESL) of

the sense resistor. Assuming an RSENSE geometry of

1225 with a parasitic inductance of 0.2nH, the RC filter

time constant should be RC = ESL/RSENSE = 0.2nH /

2mΩ = 100ns, which may be implemented with 100Ω

resistor in series with the SENSE+ pin and 1nF capaci-

tor between SENSE+ and SENSE−.

5. Select the feedback resistors. If very light load effi-

ciency is required, high value feedback resistors may

be used to minimize the current due to the feedback

divider. However, in most applications a feedback

divider current in the range of 10μA to 100μA or more

is acceptable. For a 50μA feedback divider current,

RA = 0.8V/50μA = 16kΩ. RB can then be calculated

as RB = RA(3.3V/0.8V – 1) = 50kΩ.

6. Select the MOSFETs. The best way to evaluate

MOSFET performance in a particular application is

to build and test the circuit on the bench, facilitated

by an LTC7817 demo board. However, an educated

guess about the application is helpful to initially select

MOSFETs. Since this is a high current, low voltage

application, I2R losses will likely dominate over tran-

sition losses for the top MOSFET. Therefore, choose

a MOSFET with lower RDS(ON) as opposed to lower

gate charge to minimize the combined loss terms.

The bottom MOSFET does not experience transition

losses, and its power loss is generally dominated by

I2R losses. For this reason, the bottom MOSFET is

typically chosen to be of lower RDS(ON) and subse-

quently higher gate charge than the top MOSFET.

Due to the high current in this application, two

MOSFETs may needed in parallel to more evenly bal-

ance the dissipated power and to lower the RDS(ON).

Be sure to select logic-level threshold MOSFETs, since

the gate drive voltage is limited to 5.1V (INTVCC).

LTC7803 Datasheet(PDF) 34 Page - Analog Devices

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