Datasheet

(H)L
MIN
=
(D-0.5+2/S)(V
IN
-V
OUT
)R
DSON
(1-D)(0.164*F
SW
)
LM2717-ADJ
SNVS407C –DECEMBER 2005–REVISED MARCH 2013 www.ti.com
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is needed 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 voltage requirements. The RMS current is given by:
(3)
The RMS current reaches its maximum (I
OUT
/2) when V
IN
equals 2V
OUT
. This value should be calculated for both
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 capacitor 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 conditions. For higher output
current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is recommended. 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:
(4)
Where V
IN
is the minimum input voltage and R
DSON
is the maximum switch ON resistance. For best stability the
inductor should be in the range of 0.5L
MIN
(absolute minimum) and 2L
MIN
. Using an inductor with a value less
than 0.5L
MIN
can cause subharmonic oscillations. The inductor should meet this minimum requirement at the
peak inductor current expected for the application regardless of what the inductor ripple current and output ripple
voltage requirements are. A value larger than 2L
MIN
is acceptable if the ripple requirements 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 applications 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):
(5)
A higher value of ripple current reduces inductance, but increases 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 requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the
ripple current increases 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
OUT
is driven by the maximum allowable output voltage ripple. The output ripple in the constant
frequency, PWM mode is approximated by:
(6)
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 recommended. An aluminum electrolytic
capacitor is not recommended for temperatures below −25°C since its ESR rises dramatically at cold
temperatures. Ceramic or tantalum capacitors have much better ESR specifications at cold temperature and is
preferred for low temperature applications.
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