LM2717_15 Datasheet, PDF (9/17 Pages) Texas Instruments – LM2717 Dual Step-Down DC/DC Converter

LM2717
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SNVS253D – MAY 2005 – REVISED MARCH 2013
INDUCTOR SELECTION
The most critical parameters for the inductor 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):
(4)
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:
(5)
The ESR term usually plays the dominant role in determining the voltage ripple. Low ESR ceramic, aluminum
electrolytic, or tantalum capacitors (such as Taiyo Yuden MLCC, Nichicon PL series, Sanyo OS-CON, Sprague
593D, 594D, AVX TPS, and CDE polymer aluminum) is recommended. An electrolytic capacitor is not
recommended for temperatures below −25°C since its ESR rises dramatically at cold temperature. Ceramic or
tantalum capacitors have much better ESR specifications at cold temperature 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 recommended, generally 0.1µF to 1µF to
ensure plenty of gate drive for the internal switches and a consistently low R_DS(ON).
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_SS) charges
the external soft-start capacitor, C_SS. The soft-start time can be estimated as:
T_SS = C_SS*0.6V/I_SS
(6)
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 D₁ and D₂ is preferred to be 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_OUT
however the peak current rating should be higher than the maximum load current.
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