LM2717_08 NSC [National Semiconductor], LM2717_08 Datasheet - Page 10

• Manufacturer Part Number: LM2717_08
• Description: Dual Step-Down DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

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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 applica-
tion. A current (I
C
When programming the softstart time simply use the equation
given in the Soft-Start Capacitor section above.
SHUTDOWN OPERATION (BOTH REGULATORS)
The shutdown pins of the LM2717 are designed so that they
may be controlled using 1.8V or higher logic signals. If the
shutdown function is not to be used the pin may be left open.
The maximum voltage to the shutdown pin should not exceed
7.5V. If the use of a higher voltage is desired due to system
or other constraints it may be used, however a 100k or larger
resistor is recommended between the applied voltage and the
shutdown pin to protect the device.
SCHOTTKY DIODE
The breakdown voltage rating of D
25% higher than the maximum input voltage. The current rat-
ing 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, approximate-
ly (1-D)*I
than the maximum load current.
Application Information
SS
. The soft-start time can be estimated as:
Vishay Sprague
Cornell Dubilier
Manufacturer
Manufacturer
OUT
Taiyo Yuden
Coiltronics
Panasonic
Coilcraft
Sumida
however the peak current rating should be higher
Pulse
SS
) charges the external soft-start capacitor,
T
SS
= C
Some Recommended Input And Output Capacitors (Others May Be Used)
SS
*0.6V/I
1
Some Recommended Inductors (Others May Be Used)
and D
SS
2
ESRD seriec Polymer Aluminum Electrolytic
is preferred to be
293D, 592D, and 595D series tantalum
CDRH8D28 and CDRH8D43 series
SPV and AFK series V-chip series
High capacitance MLCC ceramic
High capacitance MLCC ceramic
DO3316 and DO5022 series
P0751 and P0762 series
DRQ73 and CD1 series
EEJ-L series tantalum
Capacitor
Inductor
10
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 feed-
back and compensation networks should be connected di-
rectly 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 in-
ject noise into the system and effect performance.
The input bypass capacitor C
be placed close to the IC. This will reduce copper trace re-
sistance which effects input voltage ripple of the IC. For
additional input voltage filtering, a 0.1µF to 4.7µF bypass ca-
pacitors 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
can increase the series resistance, which directly effects out-
put voltage ripple. The feedback network, resistors R
R
inductor to minimize copper trace connections that can inject
noise into the system. Trace connections made to the induc-
tors and schottky diodes should be minimized to reduce pow-
er dissipation and increase overall efficiency. For more detail
on switching power supply layout considerations see Appli-
cation Note AN-1149: Layout Guidelines for Switching Power
Supplies.
FB2
, should be kept close to the FB pin, and away from the
OUT1
and C
OUT2
, should also be placed close to
IN
, as shown in Figure 1, must
Contact Information
Contact Information
www.panasonic.com
www.pulseeng.com
www.cooperet.com
www.coilcraft.com
www.t-yuden.com
www.sumida.com
www.vishay.com
www.cde.com
IN
, close to the V
OUTX
capacitors
FB1
and
IN