S-8550 Datasheet, PDF (11/26 Pages) Seiko Instruments Inc – STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECIFICATION, PWM CONTROL SWITCHING REGULATORS

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS
Rev.2.1_01
S-8550/8551 Series
2. Backflow current
The S-8550/8551 Series performs PWM synchronous rectification even if I_L min. is less than 0 A, so a backflow
current is generated in V_IN and the backflow current becomes maximum when no load is applied (see Figure 9).
Use the following equation to calculate the maximum backflow current value, which should be taken into
consideration when designing.
Duty (I_OUT = 0) = V_OUT / V_IN
Example : V_IN = 3.6 V, V_OUT = 1.8 V …… Duty = 50%
∆I_L = ∆V / L × t_on = (V_IN − V_OUT) × Duty / (L × f_OSC)
Example : V_IN = 3.6 V, V_OUT = 1.8 V, f_OSC = 1.2 MHz, L = 3.3 µH …… ∆I_L = 227 mA
I_L max. = ∆I_L / 2 = 113.5 mA, I_L min. = −∆I_L / 2 = −113.5 mA
The current value waveform of the inductor is a triangular wave, of which the maximum value is I_L max. and the
minimum value is I_L min. (negative value), and the negative value (the portion marked by diagonal lines in
Figure 9) backflows when no load is applied (see Figure 9).
If about 113.5 mA of I_OUT flows in the above conditions, the minimum value (I_L min.) of the triangular wave is
made 0 mA and no backflow current flows.
When an input capacitor (C_IN) is connected, the backflow current is absorbed by C_IN, thus reducing the backflow
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.
Inductor current with no load
I_L
Inductor current when load is a
current of 113.5 mA
I_L
227 mA
I_L max.
113.5 mA
0 mA
I_OUT
−113.5 mA
I_L max.
∆I_L
Backflow
current
I_L min.
113.5 mA
I_OUT
0 mA
∆I_L
I_L min.
Backflow current = 0 mA
Figure 9 Example of Conditions to Prevent Backflow Current from Flowing
VIN
V_IN C_IN
Backflow current
CONT
V_OUT
Inductor
current I_L
Figure 10 Backflow Current
Seiko Instruments Inc.
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