LTC7802 datasheet(28/34 Pages) AD

28 / 34 page

LTC7802-3.3

Rev. 0

For more information www.analog.com

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

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 violated,

the LTC7802-3.3 skips pulses at high input voltage,

resulting in lower frequency operation and higher

inductor current ripple than desired. If undesirable, this

behavior can be avoided by decreasing the frequency

(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 cal-

culated based on the minimum value for the maximum

current sense threshold (45mV):

RSENSE �

45mV

23A

� 2m�

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 sufficient mar-

gin 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 resis-

tor in series with the SENSE+ pin and 1nF capacitor

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. For channel 1, the

feedback resistor network is implemented internally.

Directly connect the VOUT1 pin to the output.

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 LTC7802-

3.3 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 transition 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 subsequently 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). Minimize

the inductance of the TG and BG gate drive traces and

their respective return paths to the controller IC (SW

and GND) by using wide traces and multiple parallel

vias.

7. Select the input and output capacitors. CIN is chosen

for an RMS current rating of at least 10A (IOUT/2, with

margin) at temperature assuming only this channel

is on. COUT is chosen with an ESR of 3mΩ for low

output ripple. Multiple capacitors connected in parallel

may be required to reduce the ESR to this level. The

output ripple in continuous mode will be highest at the

maximum input voltage. The output voltage ripple due

to ESR is approximately:

VORIPPLE = ESR • ΔIL = 3mΩ • 6A = 18mVP-P

On the 3.3V output, this is equal to 0.55% of peak to

peak voltage ripple.

APPLICATIONS INFORMATION

LTC7802 Datasheet(PDF) 28 Page - Analog Devices

LTC7802 Datasheet(HTML) 28 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL