S-8340A25AFT-T2-X Datasheet, PDF (22/55 Pages) Seiko Instruments Inc – SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, 600 kHz, PWM CONTROL OR PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8340/8341 Series
Rev.4.0_01
2. Inductor
The inductance value (L value) greatly affects the maximum output current (IOUT) and the efficiency (η).
As the L value is reduced gradually, the peak current (IPK) increases and IOUT increases. As the L value is made
even smaller, IOUT decreases since the efficiency degrades and the current driveability is insufficient.
As the L value is increased, the dissipation in the switching transistor due to IPK decreases, and the efficiency
reaches the maximum at a certain L value. As the L value is made even larger, the efficiency degrades since the
dissipation due to the series resistance of the inductor increases. IOUT also decreases.
In the S-8340/8341 Series, as the L value is increased, the output voltage may be unstable depending on the
conditions of the input voltage, output voltage, and load current. Select the L value after performing a thorough
valuation under actual use conditions. The guidelines for the L range are from 2.2 to 22 μH for the A and B types,
and 4.7 to 47 μH for the C and D types.
The recommended L value is 5 to 10 μH for the A and B types, and 10 to 22 μH for the C and D types.
When choosing an inductor, attention to its allowable current should be paid since the current exceeding the
allowable value will cause magnetic saturation in the inductor, leading to a marked decline in efficiency and a
breakdown of the IC due to large current.
An inductor should therefore be selected so that IPK does not surpass its allowable current. IPK is represented by
the following equations in non-continuous operation mode.
IPK = ^{2 × IOUT × (VOUT + VF − VIN)}
fosc × L
Where fOSC is the oscillation frequency, L is the inductance value of the inductor, and VF is the forward voltage of
the diode. VF should be appropriately 0.4 V.
For example, if a power supply with the input voltage (VIN) = 3 V, output voltage (VOUT) = 5 V, and load current (IOUT)
= 30 mA is used, fOSC = 600 kHz when the S-8340A50AFT is used. When 10 μH is selected for the L value, IPK =
155 mA from the above formula. Therefore, in this case, an inductor with a permissible current of 155 mA or
higher for the L value of 10 μH should be selected.
3. Diode
Use an external diode that meets the following requirements :
 Low forward voltage (Schottky barrier diode is recommended.)
 High switching speed (50 ns max.)
 The reverse-direction withstand voltage is VOUT + VF or higher.
 The current rating is IPK or larger.
4. Capacitors (CIN, CL)
A capacitor inserted on the input side (CIN) improves the efficiency by reducing the power impedance and stabilizing
the input current. Select a CIN value according to the impedance of the power supply used. Approximately 47 to
100 μF is recommended for a capacitance depending on the impedance of the power source and load current
value.
For the output side capacitor (CL), select a large capacitance with low ESR (Equivalent Series Resistance) for
smoothing the ripple voltage. When the input voltage is extremely high or the load current is extremely large, the
output voltage may become unstable. In this case the unstable area will become narrow by selecting a large
capacitance for an output capacitor. A tantalum electrolyte capacitor is recommended since the unstable area
widens when a capacitor with a large ESR, such as an aluminum electrolyte capacitor, or a capacitor with a small
ESR, such as a ceramic capacitor, is chosen.
It is recommended that a capacitor of which the capacitance is 47 to 200 μF and ESR is 40 to 270 mΩ be selected.
Fully evaluate input and output capacitors under actual operating conditions, then select them.
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