BOOTSTRAP CAPACITOR

A 4.7nF ceramic capacitor or larger is recommended for the

bootstrap capacitor. For applications where the input voltage

is less than twice the output voltage a larger capacitor is

recommended, generally 0.1µF to 1µF to ensure plenty of

gate drive for the internal switches and a consistently low

R

SOFT-START CAPACITOR (BOTH REGULATORS)

The LM2717 does not contain internal soft-start which allows

for fast startup time but also causes high inrush current.

Therefore for applications that need reduced inrush current

the LM2717 has circuitry that is used to limit the inrush

current on start-up of the DC/DC switching regulators. This

inrush current limiting circuitry serves as a soft-start. The

external SS pins are used to tailor the soft-start for a specific

application. A current (I

capacitor, C

T

When programming the softstart time simply use the equa-

tion given in the Soft-Start Capacitor section above.

SCHOTTKY DIODE

The breakdown voltage rating of D

25% higher than the maximum input voltage. The current

rating for the diode should be equal to the maximum output

current for best reliability in most applications. In cases

where the input voltage is much greater than the output

voltage the average diode current is lower. In this case it is

possible to use a diode with a lower average current rating,

approximately (1-D)*I

should be higher than the maximum load current.

LAYOUT CONSIDERATIONS

The LM2717 uses two separate ground connections, PGND

for the drivers and boost NMOS power device and AGND for

the sensitive analog control circuitry. The AGND and PGND

pins should be tied directly together at the package. The

feedback and compensation networks should be connected

directly to a dedicated analog ground plane and this ground

plane must connect to the AGND pin. If no analog ground

plane is available then the ground connections of the feed-

back and compensation networks must tie directly to the

AGND pin. Connecting these networks to the PGND can

inject noise into the system and effect performance.

The input bypass capacitor C

be placed close to the IC. This will reduce copper trace

resistance which effects input voltage ripple of the IC. For

additional input voltage filtering, a 0.1µF to 4.7µF bypass

capacitors can be placed in parallel with C

pins to shunt any high frequency noise to ground. The output

capacitors, C

the IC. Any copper trace connections for the C

tors can increase the series resistance, which directly effects

output voltage ripple. The feedback network, resistors R

FB1

and R

the inductor to minimize copper trace connections that can

inject noise into the system. Trace connections made to the

inductors and schottky diodes should be minimized to re-

duce power dissipation and increase overall efficiency. For

more detail on switching power supply layout considerations

see Application Note AN-1149: Layout Guidelines for Switch-

ing Power Supplies.

Application Information

Some recommended Inductors (others may be used)

Manufacturer

Inductor

Contact Information

Coilcraft

DO3316 and DO5022 series

www.coilcraft.com

Coiltronics

DRQ73 and CD1 series

www.cooperet.com

Pulse

P0751 and P0762 series

www.pulseeng.com

Sumida

CDRH8D28 and CDRH8D43 series

www.sumida.com

Some recommended Input and Output Capacitors (others may be used)

Manufacturer

Capacitor

Contact Information

Vishay Sprague

293D, 592D, and 595D series tantalum

www.vishay.com

Taiyo Yuden

High capacitance MLCC ceramic

www.t-yuden.com

Cornell Dubilier

ESRD seriec Polymer Aluminum Electrolytic

SPV and AFK series V-chip series

www.cde.com

Panasonic

High capacitance MLCC ceramic

EEJ-L series tantalum

www.panasonic.com

www.national.com

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