S-8351A20MC-J2FT2X Datasheet, PDF (20/51 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-UP, BUILT-IN / EXTERNAL FET PFM CONTROL SWITCHING REGULATOR / SWITCHING REGULATOR CONTROLLER
S-8351/8352 Series
Rev.3.0_01
 External Parts Selection
1. Inductor
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 (VOUT) constant, it is necessary to supply the energy corresponding
to the output current (IOUT) from the inductor. The amount of charge required for IOUT is IOUT × (tON + tOFF). Because
the inductor can supply energy only during tOFF, the charge is obtained by integrating equation (7) in the “3.
Operation” in the “ Operation” with 0 → tOFF, namely,
I_PK
2
• t_OFF
.
Thus,
I_PK
2
• t_OFF
= I_OUT × (t_ON
+ t_OFF )
(17)
∴ I_PK
=
2•
t_ON + t_OFF
t_OFF
• I_OUT
(18)
When the oscillation duty ratio of OSC is 75%, IPK = 8 • IOUT. Therefore, an IPK current which is eight times IOUT flows
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 IPK and IOUT decrease. Since the energy stored in the inductor is
equal to
1
2
L
•
(I_PK
)²
,
the
energy
decreases
because
IPK
decreases
in
steps
of
squares
offsetting
the
increase
of
L
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 IPK
decreased, and the inductance efficiency improves.
On the other hand, if an inductor with a smaller L value is selected, both IPK and IOUT increase. Accordingly, the
minimum operating input voltage becomes low but the inductance efficiency deteriorates.
Caution An excessively large IPK may cause magnetic saturation for some core materials, leading to the
destruction of the IC. Use a core with material that satisfies Isat ^*1 > IPK
*1. Level of current that causes magnetic saturation.
2. Diode
Use an external diode that meets the following requirements :
• Low forward voltage : VF < 0.3 V
• High switching speed : 500 ns max.
• Reverse voltage :
VOUT + VF or more
• Current rate :
IPK or more
20