S-8540 Datasheet, PDF (15/36 Pages) Seiko Instruments Inc – STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.3.2_00
S-8540/8541 Series
2. Inductor
The inductance value (L) greatly affects the maximum output current (I_OUT) and the efficiency (η).
The peak current (I_PK) increases by decreasing L and the stability of the circuit improves and I_OUT
increases. If L is made even smaller, the efficiency falls causing a decline in the current drive capacity
for the switching transistor, and I_OUT decreases.
The loss of I_PK by the switching transistor decreases by increasing L and the efficiency becomes
maximum at a certain L value. Increasing L further decreases the efficiency due to the loss of coil DC
resistance. I_OUT also decreases.
When the inductance is large in an S-8540/8541 series product, the output voltage may grow 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.
The recommended inductances are 10 μH for A, B types and 22 μH for C, D types.
When choosing an inductor, attention to its allowable current should be paid since the current over the
allowable value will cause magnetic saturation in the inductor, leading to a marked decline in efficiency.
An inductor should therefore be selected so as not I_PK to surpass its allowable current. The peak current
(I_PK) is represented by the following equation in non-continuous operation mode:
IPK = IOUT + ^{VOUT × (VIN − VOUT)}
2 × fOSC × L × VIN
Where f_OSC is the oscillation frequency.
3. Diode
The diode to be externally coupled to the IC should be a type that meets the following conditions:
• The forward voltage is low (Schottky barrier diode recommended).
• The switching speed is high (50 ns max.).
• The reverse direction voltage is higher than V_IN.
• The current rating is larger than I_PK.
4. Capacitors
4. 1 Capacitors (C_IN, C_OUT)
The capacitor inserted in the input side (C_IN) serves to reduce the power impedance and to average
the input current for better efficiency. The C_IN value should be selected according to the impedance of
the power supply. It should be 47 to 100 μF, although the actual value depends on the impedance of
the power source used and load current value.
For the output side capacitor (C_OUT), select a large capacitance with low ESR (Equivalent Series
Resistance) to smoothen 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 side capacitor. A tantalum
electrolytic capacitor is recommended since the unstable area widens when a capacitor with a large
ESR, such as an aluminum electrolytic capacitor, or a capacitor with a small ESR, such as a ceramic
capacitor, is chosen. The range of the capacitance should generally be 47 to 100 μF.
4. 2 Internal power source stabilization capacitor (C_VL)
The main circuits of the IC work on an internal power source connected to the CVREF pin. The C_VL
is a bypass capacitor for stabilizing the internal Power source. C_VL should be a 1 μF ceramic
capacitor and wired in a short distance and at a low impedance.
Seiko Instruments Inc.
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