S-8540 SII [Seiko Instruments Inc], S-8540 Datasheet - Page 15

• Manufacturer Part Number: S-8540
• Description: STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
• Manufacturer: SII [Seiko Instruments Inc]

Rev.3.2
3. Diode
4. Capacitors
2. Inductor
STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
The diode to be externally coupled to the IC should be a type that meets the following conditions:
4. 1 Capacitors (C
4. 2 Internal power source stabilization capacitor (C
The inductance value (L) greatly affects the maximum output current (I
The peak current (I
increases. If L is made even smaller, the efficiency falls causing a decline in the current drive capacity
for the switching transistor, and I
The loss of I
maximum at a certain L value. Increasing L further decreases the efficiency due to the loss of coil DC
resistance. I
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 μ H for A, B types and 22 μ H 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
(I
Where f
PK
• 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
• The current rating is larger than I
The main circuits of the IC work on an internal power source connected to the CVREF pin. The C
is a bypass capacitor for stabilizing the internal Power source. C
capacitor and wired in a short distance and at a low impedance.
the input current for better efficiency. The C
the power supply. It should be 47 to 100 μ F, although the actual value depends on the impedance of
the power source used and load current value.
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 μ F.
_00
The capacitor inserted in the input side (C
For the output side capacitor (C
) is represented by the following equation in non-continuous operation mode:
I
PK
=
OSC
I
OUT
is the oscillation frequency.
+
OUT
PK
V
2
OUT
by the switching transistor decreases by increasing L and the efficiency becomes
×
also decreases.
f
×
IN
OSC
PK
(V
, C
×
) increases by decreasing L and the stability of the circuit improves and I
IN
OUT
L
−
×
V
)
V
OUT
IN
)
OUT
PK
decreases.
OUT
.
Seiko Instruments Inc.
), select a large capacitance with low ESR (Equivalent Series
IN
IN
) serves to reduce the power impedance and to average
value should be selected according to the impedance of
IN
.
PK
to surpass its allowable current. The peak current
VL
)
OUT
VL
) and the efficiency ( η ).
should be a 1 μ F ceramic
S-8540/8541 Series
15
OUT
VL