s-8551 Seiko Instruments Inc., s-8551 Datasheet - Page 11

• Manufacturer Part Number: s-8551
• Description: Step-down, Built-in Fet, Synchronous Rectification, Pwm Control Switching Regulators
• Manufacturer: Seiko Instruments Inc.

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS
Rev.2.1
2. Backflow current
−113.5 mA
113.5 mA
The S-8550/8551 Series performs PWM synchronous rectification even if I
current is generated in V
Use the following equation to calculate the maximum backflow current value, which should be taken into
consideration when designing.
The current value waveform of the inductor is a triangular wave, of which the maximum value is I
minimum value is I
Figure 9) backflows when no load is applied (see Figure 9).
If about 113.5 mA of I
made 0 mA and no backflow current flows.
When an input capacitor (C
current to flow in the power supply. Be sure to connect an input capacitor to reduce backflow current to the
power supply (see Figure 10).
The above presents the conditions required to prevent backflow current from flowing, which is only a guideline.
Perform sufficient confirmation using an actual application.
_01
I
OUT
0 mA
Duty (I
∆I
I
L
L
max. = ∆I
I
L
= ∆V / L × t
Inductor current with no load
Figure 9 Example of Conditions to Prevent Backflow Current from Flowing
OUT
= 0) = V
L
/ 2 = 113.5 mA, I
on
L
= (V
min. (negative value), and the negative value (the portion marked by diagonal lines in
V
OUT
OUT
IN
IN
IN
/ V
Example : V
Example : V
flows in the above conditions, the minimum value (I
and the backflow current becomes maximum when no load is applied (see Figure 9).
IN
− V
) is connected, the backflow current is absorbed by C
C
IN
IN
OUT
∆I
VIN
L
L
Backflow
current
I
) × Duty / (L × f
L
min. = −∆I
I
Figure 10 Backflow Current
L
min.
max.
Seiko Instruments Inc.
IN
IN
= 3.6 V, V
= 3.6 V, V
L
/ 2 = −113.5 mA
OSC
OUT
OUT
)
113.5 mA
I
CONT
OUT
= 1.8 V …… Duty = 50%
= 1.8 V, f
227 mA
0 mA
Backflow current
Inductor
current I
I
L
Inductor current when load is a
current of 113.5 mA
OSC
= 1.2 MHz, L = 3.3 µH …… ∆I
L
V
L
OUT
min. is less than 0 A, so a backflow
Backflow current = 0 mA
L
min.) of the triangular wave is
IN
, thus reducing the backflow
S-8550/8551 Series
∆I
L
I
I
L
L
min.
max.
L
L
= 227 mA
max. and the
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