S-8550AA-M5T1X Datasheet, PDF (11/31 Pages) Seiko Instruments Inc – RECTIFICATION, PWM CONTROL SWITCHING REGULATORS

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS
Rev.5.0_01
S-8550 Series
2. Backflow current
The S-8550 Series performs PWM synchronous rectification even if IL min. is less than 0 A, so a backflow
current is generated in VIN and the backflow current becomes maximum when no load is applied (Refer to
Figure 11). Use the following equation to calculate the maximum backflow current value, which should be
taken into consideration when designing.
Duty (IOUT = 0) = VOUT / VIN
Example : VIN = 3.6 V, VOUT = 1.8 V …… Duty = 50%
ΔIL = ΔV / L × ton = (VIN − VOUT) × Duty / (L × fOSC)
Example : VIN = 3.6 V, VOUT = 1.8 V, fOSC = 1.2 MHz, L = 3.3 μH …… ΔIL = 227 mA
IL max. = ΔIL / 2 = 113.5 mA, IL min. = −ΔIL / 2 = −113.5 mA
The current value waveform of the inductor is a triangular wave, of which the maximum value is IL max. and the
minimum value is IL min. (negative value), and the negative value (the portion marked by diagonal lines in
Figure 11) backflows when no load is applied (Refer to Figure 11).
If about 113.5 mA of IOUT flows in the above conditions, the minimum value (IL min.) of the triangular wave is
made 0 mA and no backflow current flows.
When an input capacitor (CIN) is connected, the backflow current is absorbed by CIN, 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 (Refer to Figure 12).
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.
ΔI_L
Backflow
current
I_L min.
113.5 mA
I_OUT
0 mA
ΔI_L
I_L min.
Backflow current = 0 mA
Figure 11 Example of Conditions to Prevent Backflow Current from Flowing
VIN
V_IN C_IN
Backflow current
CONT
V_OUT
Inductor
current I_L
Figure 12 Backflow Current
11