S-8520A20MC-AVFT2X Datasheet, PDF (18/44 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8520/8521 Series
Rev.9.0_01
2. Inductor
The inductance value (L value) has a strong influence on the maximum output current (IOUT) and efficiency (η).
The peak current (IPK) increases by decreasing L value and the stability of the circuit improves and IOUT increases. If
L value is decreased, the efficiency falls causing a decline in the current drive capacity for the switching transistor,
and IOUT decreases.
The loss of IPK by the switching transistor decreases by increasing L and the efficiency becomes the maximum at a
certain L value. Further increasing L value decreases the efficiency due to the loss of the direct current resistance
of the coil. IOUT also decreases.
For the S-8520/8521 Series, increasing the inductance value, the output voltage may be unstable in some cases,
depending on the conditions of the input voltage, output voltage, and the load current. Perform sufficient evaluation
under the actual condition and decide an optimum inductance value.
The recommended inductances are 47 μH for A, B, C, D types and 22 μH for E, F types.
Be careful of the allowable inductor current when choosing an inductor. Exceeding the allowable current of the
inductor causes magnetic saturation, much lower efficiency and destruction of the IC chip due to a large current.
Choose an inductor so that IPK does not exceed the allowable current. IPK in continuous mode is calculated by the
following equation:
IPK = IOUT +
(VOUT + VF^*2 ) × (VIN − VOUT)
2 × fosc^*1 × L × (VIN + VF^*2)
*1. fosc: Oscillation frequency
*2. VF: Forward voltage of the diode
3. Diode
Use an external diode that meets the following requirements :
• Its forward voltage is low (Schottky barrier diode is recommended).
• Its switching speed is high (50 ns max.).
• Its reverse direction voltage is higher than VIN.
• Its current rating is higher than IPK.
4. Capacitors (CIN, COUT)
A capacitor for the input (CIN) improves efficiency by reducing power impedance and stabilizing the input current.
Select the CIN value according to impedance of the power supply to be used. Approximately 47 to 100 μF is
recommended for the capacitor depending on impedance of the power source and load current value.
For a capacitor for output (CL), select a large capacitance with low ESR (Equivalent Series Resistance) for
smoothing the ripple voltage. However, capacitor with extremely small ESR such as ceramic capacitor (about 0.3
Ω or less) may destabilize the output voltage, according to the conditions of input and output voltages. A
tantalum electrolyte capacitor is recommended. 47 μF to 100 μF is recommended for the capacitor.
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