S-8337AAAA-P8T1G Datasheet, PDF (17/51 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-UP, 1.2 MHz HIGH-FREQUENCY, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.4.0_01
S-8337/8338 Series
 External Parts Selection
1. Inductor
The inductance has a strong influence on the maximum output current (IOUT) and efficiency (η).
The peak current (IPK) increases by decreasing L and the stability of the circuit improves and IOUT
increases. If L is decreased further, the efficiency falls, and IOUT decreases if the current drive
capability of the external transistor is insufficient.
The loss of IPK by the switching transistor decreases by increasing L and the efficiency becomes
maximum at a certain L value. Further increasing L decrease the efficiency due to the loss of the DC
resistance of the inductor. IOUT also decreases.
If the oscillation frequency is higher, a smaller L value can be chosen, making the inductor smaller. In
the S-8337/8338 Series, the oscillation frequency can be varied within the range of 286 kHz to 1.133
MHz by the external resistor, so select an L value best suited to the frequency. The recommended
value is between 2.2 μH and 22 μH.
When selecting an inductor, note the allowable current of the inductor. If a current exceeding this
allowable current flows through the inductor, magnetic saturation occurs, substantially lowering the
efficiency and increasing the current, which results in damage to the IC.
Therefore, select an inductor so that IPK does not exceed the allowable current. IPK is expressed by
the following equations in the discontinuous mode and continuous mode.
IPK =
2 IOUT(VOUT + VD − VIN) _{( discontinuous mode ) ..................................................................(17)}
fosc • L
IPK =
VOUT +
VIN
VD
•
IOUT
+
(VOUT +
2 • (VOUT
VD − VIN) • VIN
+ VD) • fosc • L
(continuous mode)
................................................................ (18)
fOSC = Oscillation frequency, VD ≅ 0.4 V.
2. Diode
Use an external diode that meets the following requirements.
• Low forward voltage
• High switching speed
• Reverse breakdown voltage: VOUT + [Spike voltage] or more
• Rated current: IPK or more
3. Capacitors (CIN, CL)
The capacitor on the input side (CIN) can lower the supply impedance and level the input current for
better efficiency. Select CIN according to the impedance of the power supply to be used.
The capacitor on the output side (CL) is used to smooth the output voltage. Select an appropriate
capacitance value based on the I/O conditions and load conditions. A capacitance of 10 μF or more is
recommended.
By adjusting the phase compensation of the feedback loop using the external resistor (RZ) and
capacitor (CZ), a ceramic capacitor can be used as the capacitor on the output side. If a capacitor
whose equivalent series resistance is between 30 mΩ and 500 mΩ is used as the output capacitor, the
adjustable range of the phase compensation is wider; however, note that other characteristics may be
affected by ripple voltage or other conditions at this time. The optimal capacitor differs depending on
the L value, capacitance value, wiring, and application (output load), so select the capacitor after
performing sufficient evaluation under the actual usage conditions.
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