S-8353 Seiko Instruments, S-8353 Datasheet - Page 19

• Manufacturer Part Number: S-8353
• Description: (S-8353 / S-8354) PWM Control Switchable Built-in Transistor Switching Regulator
• Manufacturer: Seiko Instruments

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Rev.1.4
STEP-UP, PWM CONTROL or PWM / PFM SWITCHABLE BUILT-IN TRANSISTOR SWITCHING REGULATOR
External Parts Selection
The relationship between the major characteristics of the step-up circuit and the characteristic parameters of the external
parts is shown in Figure 12 .
1. Inductor
2. Diode
The inductance value (L value) has a strong influence on the maximum output current (I
The peak current (I
value is decreased, the efficiency falls causing a decline in the current drive capacity for the switching transistor, and
I
The loss of I
certain L value. Further increasing L value decreases the efficiency due to the loss of the direct current resistance of
the coil. I
A higher oscillation frequency allows selection of a lower L value, making the coil smaller.
The recommended inductances are a 47 to 220 µH for A, C, and D types, a 10 to 47 µH for H and J types.
Be careful of the allowable inductor current when choosing an inductor. Exceeding the allowable current of the
inductor causes magnetic saturation, much lower efficiency and destruction of the IC chip due to a large current.
Choose an inductor so that I
following equation:
f
Use an external diode that meets the following requirements :
I
OUT
osc
PK
Larger output capacitance
For larger output current ?
• Low forward voltage : V
• High switching speed : 50 ns max.
• Reverse voltage :
• Current rate :
= oscillation frequency, V
=
decreases.
_00
Figure 12 Relationship between Major Characteristics of Step-up Circuit and External Parts
I 2
OUT
OUT
(
PK
V
also decreases.
f
OUT
OSC
by the switching transistor decreases by increasing L and the efficiency becomes maximum at a
Smaller inductance
PK
+
Smaller direct current resistance of inductor
•
L
V
) increases by decreasing L value and the stability of the circuit improves and I
D
−
V
IN
PK
V
I
)
D
PK
F
OUT
≅
does not exceed the allowable current. I
(A)
< 0.3 V
or more
0.4 V.
+ V
F
At PFM control
or more
Seiko Instruments Inc.
For higher efficiency ?
At PWM control
PK
in discontinuous mode is calculated by the
Larger inductance
For smaller ripple voltage ?
Larger output capacitance
OUT
S-8353/8354 Series
) and efficiency (η).
OUT
increases. If L
(17)
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