S-8352 Seiko Instruments, S-8352 Datasheet - Page 19

• Manufacturer Part Number: S-8352
• Description: (S-8351 / S-8352) SMALL PACKAGE PFM CONTROL STEP-UP SWITCHING REGULATOR
• Manufacturer: Seiko Instruments

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Rev.2.0
External Parts Selection
1. Inductor
2. Diode
To minimize the loss due to inductor direct current resistance, select an inductor with the smallest possible direct
current resistance (less than 1 Ω). Set the inductance value (L value) to around 22 µH to 1 mH.
To make the average value of the output voltage (V
to the output current (I
the inductor can supply energy only during t
Operation ” in the “ Operation ” with 0 → t
When the oscillation duty ratio of OSC is 75%, I
into transistor (M1).
The S-8351 Series includes a switching current controller which monitors the current flowing into the CONT pin by
the voltage (CONT control voltage) and controls the current. This controller prevents destruction of the IC due to
excess current.
If an inductor with a large L value is selected, both I
equal to
value. As a result, stepping up at a low voltage becomes difficult and the minimum operating input voltage becomes
high. However, the direct current resistance loss of L value and the M1 transistor decreases by the amount I
decreased, and the inductance efficiency improves.
On the other hand, if an inductor with a smaller L value is selected, both I
minimum operating input voltage becomes low but the inductance efficiency deteriorates.
Caution An excessively large I
*1. Level of current that causes magnetic saturation.
Use an external diode that meets the following requirements :
STEP-UP, SUPER-SMALL PACKAGE, PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER
• Low forward voltage : V
• High switching speed : 500 ns max.
• Reverse voltage :
• Current rate :
∴
I
_00
PK
2
I
PK
•
t
=
OFF
2
1
destruction of the IC. Use a core with material that satisfies I
2
L
•
•
=
t
I (
ON
I
OUT
PK
t
OFF
+
)
2
×
t
, the energy decreases because I
OFF
t (
ON
OUT
•
+
I
) from the inductor. The amount of charge required for I
OUT
t
OFF
V
I
PK
F
OUT
)
< 0.3 V
or more
+ V
PK
F
may cause magnetic saturation for some core materials, leading to the
or more
Seiko Instruments Inc.
OFF
, namely,
OFF
PK
= 8 • I
, the charge is obtained by integrating equation (7) in the “ 3.
OUT
PK
PK
and I
) constant, it is necessary to supply the energy corresponding
I
OUT
PK
2
decreases in steps of squares offsetting the increase of L
•
. Therefore, an I
OUT
t
OFF
decrease. Since the energy stored in the inductor is
. Thus,
sat
PK
PK
*1
current which is eight times I
and I
> I
OUT
PK
OUT
is I
S-8351/8352 Series
increase. Accordingly, the
OUT
× (t
ON
(17)
(18)
+ t
OFF
). Because
OUT
flows
19
PK