s-8337aaca-t8t1g Seiko Instruments Inc., s-8337aaca-t8t1g Datasheet - Page 16

• Manufacturer Part Number: s-8337aaca-t8t1g
• Description: Step-up, 1.2 Mhz High-frequency, Pwm Control Switching Regulator Controller
• Manufacturer: Seiko Instruments Inc.

16
STEP-UP, 1.2 MHz HIGH-FREQUENCY, PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8337/8338 Series
External Parts Selection
1. Inductor
2. Diode
3. Capacitors (C
The inductance has a strong influence on the maximum output current (I
The peak current (I
increases. If L is decreased further, the efficiency falls, and I
capability of the external transistor is insufficient.
The loss of I
maximum at a certain L value. Further increasing L decrease the efficiency due to the loss of the DC
resistance of the inductor. I
If the oscillation frequency is higher, a smaller L value can be chosen, making the inductor smaller. In
the S-8337/8338 Series, the oscillation frequency can be varied within the range of 286 kHz to 1.133
MHz by the external resistor, so select an L value best suited to the frequency. The recommended
value is between 2.2 μH and 22 μH.
When selecting an inductor, note the allowable current of the inductor. If a current exceeding this
allowable current flows through the inductor, magnetic saturation occurs, substantially lowering the
efficiency and increasing the current, which results in damage to the IC.
Therefore, select an inductor so that I
the following equations in the discontinuous mode and continuous mode.
f
Use an external diode that meets the following requirements.
The capacitor on the input side (C
better efficiency. Select C
The capacitor on the output side (C
capacitance value based on the I/O conditions and load conditions. A capacitance of 10 μF or more is
recommended.
By adjusting the phase compensation of the feedback loop using the external resistor (RZ) and
capacitor (CZ), a ceramic capacitor can be used as the capacitor on the output side. If a capacitor
whose equivalent series resistance is between 30 mΩ and 500 mΩ is used as the output capacitor, the
adjustable range of the phase compensation is wider; however, note that other characteristics may be
affected by ripple voltage or other conditions at this time. The optimal capacitor differs depending on
the L value, capacitance value, wiring, and application (output load), so select the capacitor after
performing sufficient evaluation under the actual usage conditions.
I
I
OSC
PK
PK
=
=
= Oscillation frequency, V
• Low forward voltage
• High switching speed
• Reverse breakdown voltage: V
• Rated current: I
V
OUT
I 2
V
IN
+
OUT
V
D
PK
(V
•
IN
fosc
I
OUT
OUT
, C
by the switching transistor decreases by increasing L and the efficiency becomes
L
+
+
)
•
PK
2
L
(V
V
•
) increases by decreasing L and the stability of the circuit improves and I
D
(V
OUT
PK
−
OUT
V
or more
+
IN
IN
V
+
OUT
D
)
V
according to the impedance of the power supply to be used.
−
D
(
)
V
D
Seiko Instruments Inc.
discontinu
•
also decreases.
IN
≅ 0.4 V.
fosc
)
•
IN
V
) can lower the supply impedance and level the input current for
•
IN
L
) is used to smooth the output voltage. Select an appropriate
L
PK
OUT
(continuou
does not exceed the allowable current. I
ous
+ [Spike voltage] or more
mode
s
mode)
)
..................................................................(17)
................................................................(18)
OUT
decreases if the current drive
OUT
) and efficiency (η).
PK
is expressed by
Rev.3.1
OUT
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