LM2717-ADJ_08 NSC [National Semiconductor], LM2717-ADJ_08 Datasheet - Page 10

• Manufacturer Part Number: LM2717-ADJ_08
• Description: Dual Step-Down DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

www.national.com
be lowered and/or a pre-load added to the output to keep the
device full PWM operation. Note that the OVP function mon-
itors the FB pin so it will not function if the feedback resistor
is disconnected from the output. Due to slight differences be-
tween the two converters it is recommended that Buck 1 be
used for the lower of the two output voltages for best opera-
tion.
DESIGN PROCEDURE
This section presents guidelines for selecting external com-
ponents.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the feedback pin and a resistor
divider connected to the output as shown in Figure 4. The
feedback pin voltage (V
back resistors sets the output voltage according to the follow-
ing equation:
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is need-
ed between the input pin and power ground. This capacitor
prevents large voltage transients from appearing at the input.
The capacitor is selected based on the RMS current and volt-
age requirements. The RMS current is given by:
The RMS current reaches its maximum (I
V
regulators and added to give a total RMS current rating. For
an aluminum or ceramic capacitor, the voltage rating should
be at least 25% higher than the maximum input voltage. If a
tantalum capacitor is used, the voltage rating required is
about twice the maximum input voltage. The tantalum capac-
itor should be surge current tested by the manufacturer to
prevent being shorted by the inrush current. The minimum
capacitor value should be 47µF for lower output load current
applications and less dynamic (quickly changing) load condi-
tions. For higher output current applications or dynamic load
conditions a 68µF to 100µF low ESR capacitor is recom-
mended. It is also recommended to put a small ceramic
capacitor (0.1µF to 4.7µF) between the input pins and ground
to reduce high frequency spikes.
INDUCTOR SELECTION
The most critical parameter for the inductor in a current mode
switcher is the minimum value required for stable operation.
To prevent subharmonic oscillations and achieve good phase
margin a target minimum value for the inductor is:
Where V
maximum switch ON resistance. For best stability the inductor
should be in the range of 0.5L
2L
cause subharmonic oscillations. The inductor should meet
IN
MIN
equals 2V
. Using an inductor with a value less than 0.5L
IN
is the minimum input voltage and R
OUT
. This value should be calculated for both
FB
) is 1.258V, so the ratio of the feed-
MIN
(absolute minimum) and
OUT
DSON
/2) when
MIN
is the
can
10
this minimum requirement at the peak inductor current ex-
pected for the application regardless of what the inductor
ripple current and output ripple voltage requirements are. A
value larger than 2L
ments of the application require it but it may reduce the phase
margin and increase the difficulty in compensating the circuit.
The most important parameters for the inductor from an ap-
plications standpoint are the inductance, peak current and the
DC resistance. The inductance is related to the peak-to-peak
inductor ripple current, the input and the output voltages (for
300kHz operation):
A higher value of ripple current reduces inductance, but in-
creases the conductance loss, core loss, and current stress
for the inductor and switch devices. It also requires a bigger
output capacitor for the same output voltage ripple require-
ment. A reasonable value is setting the ripple current to be
30% of the DC output current. Since the ripple current in-
creases with the input voltage, the maximum input voltage is
always used to determine the inductance. The DC resistance
of the inductor is a key parameter for the efficiency. Lower DC
resistance is available with a bigger winding area. A good
tradeoff between the efficiency and the core size is letting the
inductor copper loss equal 2% of the output power.
OUTPUT CAPACITOR
The selection of C
output voltage ripple. The output ripple in the constant fre-
quency, PWM mode is approximated by:
The ESR term usually plays the dominant role in determining
the voltage ripple. Low ESR ceramic, aluminum electrolytic,
or tantalum capacitors (such as MuRata MLCC, Taiyo Yuden
MLCC, Nichicon PL series, Sanyo OS-CON, Sprague 593D,
594D, AVX TPS, and CDE polymer aluminum) is recom-
mended. An aluminum electrolytic capacitor is not recom-
mended for temperatures below −25°C since its ESR rises
dramatically at cold temperatures. Ceramic or tantalum ca-
pacitors have much better ESR specifications at cold tem-
perature and is preferred for low temperature applications.
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 rec-
ommended, 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-ADJ contains circuitry that can be used to limit
the inrush current on start-up of the DC/DC switching regula-
tors. 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
start capacitor, C
When programming the soft-start time use the equation given
in the Soft-Start Capacitor section above. The soft-start func-
tion is used simply to limit inrush current to the device that
SS
OUT
. The soft-start time can be estimated as:
T
MIN
SS
is driven by the maximum allowable
= C
is acceptable if the ripple require-
SS
*0.6V/I
SS
) charges the external soft-
SS
DSON
.