s-8521b51mc-aukt2g Seiko Instruments Inc., s-8521b51mc-aukt2g Datasheet - Page 18

• Manufacturer Part Number: s-8521b51mc-aukt2g
• Description: Step-down, Pwm Control Or Pwm / Pfm Switchable Switching Regulator Controller
• Manufacturer: Seiko Instruments Inc.

18
STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLOR
S-8520 / 8521 Series
2. Inductor
3. Diode
4. Capacitors (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-8520 / 8521 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 47 µ H for A, B, C, D types and 22 µ H for E, F 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
The diode to be externally coupled to the IC should be a type that meets the following conditions:
The capacitor inserted on the input side (C
input current for better efficiency.
supplied. As a rough rule of thumb, you should use a value of 47 to 100 µ F, although the actual value
will depend on the impedance of the power in use and the load current value.
For the output side capacitor (C
Resistance) for smoothing the ripple voltage. However, notice that a capacitor with extremely low ESR
(say, below 0.3 Ω ), such as a ceramic capacitor, could make the output voltage unstable, depending on
the input voltage and load current conditions. Instead, a tantalum electrolytic capacitor is recommended.
A capacitance value from 47 to 100 µ F can serve as a rough yardstick for this selection.
I
PK
PK
• Its forward voltage is low (Schottky barrier diode recommended).
• Its switching speed is high (50 ns max.).
• Its reverse direction voltage is higher than V
• Its current rating is higher than I
) is represented by the following equation in non-continuous operation mode:
=
I
OUT
osc
+
is the oscillation frequency, and V
(
V
OUT
2
OUT
PK
×
IN
f
, C
by the switching transistor decreases by increasing L and the efficiency becomes
also decreases.
osc
+
V
OUT
×
PK
F
L
)
) increases by decreasing L and the stability of the circuit improves and I
)
×
×
(
(
V
V
IN
IN
−
+
V
V
OUT
OUT
F
OUT
)
)
decreases.
), select one of large capacitance with low ESR (Equivalent Series
PK
Select the C
.
IN
F
Seiko Instruments Inc.
) serves to lower the power impedance and to average the
the forward voltage of the diode.
IN
.
IN
PK
value according to the impedance of the power
to surpass its allowable current. The peak current
OUT
) and the efficiency ( η ).
Rev.8.3
_00
OUT