S-8365AAABA-M5T1Y2 Datasheet, PDF (14/63 Pages) Seiko Instruments Inc – PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
S-8365/8366 Series
Rev.2.1_01
 Operation
1. Switching control method
1.1 PWM control (S-8365 Series)
The S-8365 Series is a switching regulator controller that uses a pulse width modulation method (PWM).
In conventional PFM control switching regulators, pulses are skipped when the output load current is small,
causing a fluctuation in the ripple frequency of the output voltage, resulting in increased ripple voltage.
For the S-8365 Series, although the pulse width changes from 0% to 90% in accordance with each load current
(or 0% to 85% for 1.2 MHz products), since the switching frequency does not change, the ripple voltage
generated due to switching can be eliminated by filtering. The ripple voltage can thus be lowered in the wide
input voltage and load current ranges.
1.2 PWM / PFM switching control (S-8366 Series)
The S-8366 Series switching regulator controller automatically switches between the pulse width modulation
method (PWM) and pulse frequency modulation method (PFM) according to the load current.
A low ripple power can be supplied by operating on PWM control for which the pulse width changes from 28% to
90% (or 28% to 85% for 1.2 MHz products) in the range where the output load current is large.
The S-8366 Series operates on PFM control when the output load current is small and the fixed pulses which
have the width of 28% are skipped according to the load current amount. Therefore, the oscillation circuit
intermittently oscillates, reducing the self-current consumption. This avoids decreased efficiency when the
output load current is small. The point at which PWM control switches to PFM control varies depending on the
external element (inductor, diode, etc.), input voltage value, and output voltage value, and this method achieves
high efficiency in the output load current of about 100 μA.
2. Soft-start function
The S-8365/8366 Series has a soft-start circuit. The output voltage (VOUT) gradually rises after power-on or startup
when the ON/OFF pin is set to high, suppressing rush current and overshooting the output voltage. The soft-start
time (tSS) for the S-8365/8366 Series is defined as the time from startup until VOUT reaches 90% of the output set
voltage value (VOUT(S)). A reference voltage adjustment method is used as the soft-start method and the reference
voltage gradually rises from 0 V after soft-start.
A soft-start performs by controlling the FB pin voltage so that it follows the rise of the reference voltage. After the
reference voltage rises once, it is reset to 0 if the ON/OFF pin voltage drops to low, the power supply voltage drops
to the UVLO detection voltage, or the S-8365/8366 Series enters the short-circuit protection latch status. A
soft-start is performed regardless of conditions when resuming step-up operation.
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