s-8521b51mc-aukt2g Seiko Instruments Inc., s-8521b51mc-aukt2g Datasheet - Page 15

• Manufacturer Part Number: s-8521b51mc-aukt2g
• Description: Step-down, Pwm Control Or Pwm / Pfm Switchable Switching Regulator Controller
• Manufacturer: Seiko Instruments Inc.

Rev.8.3
5. 100% duty cycle
4. Overload protection circuit (A, C, E types)
The A, C and E types of the S-8520 / 8521 Series come with a built-in overload protection circuit.
If the output voltage falls because of an overload, the maximum duty state (100%) will continue. If this
100% duty state lasts longer than the prescribed overload detection time (t
circuit will hold the EXT pin at "H," thereby protecting the switching transistor and inductor. When the
overload protection circuit is functioning, the reference voltage circuit will be activated by means of a soft
start in the IC, and the reference voltage will rise slowly from 0 V. The reference voltage and the
feedback voltage obtained by dividing the output voltage are compared to each other. So long as the
reference voltage is lower, the EXT pin will be held at "H" to keep the oscillation inactive.
reference voltage keeps rising and exceeds the feedback voltage, the oscillation will resume.
If the load is heavy when the oscillation is restarted, and the EXT pin holds the "L" level longer than the
specified overload detection time (t
enter intermittent operation mode, in which it repeats the actions described above. Once the overload
state is eliminated, the IC resumes normal operation.
The S-8520 / 8521 Series operates with a maximum duty cycle of 100%. When a B, D and F types
products not provided with an overload protection circuit is used, the switching transistor can be kept ON
to supply current to the load continually, even in cases where the input voltage falls below the preset
output voltage value. The output voltage delivered under these circumstances is one that results from
subtracting, from the input voltage, the voltage drop caused by the DC resistance of the inductance and
the on-resistance of the switching transistor.
If an A, C and E types products provided with an overload protection circuit is used, this protection circuit
will function when the 100% duty state has lasted longer than the preset overload detection time (t
causing the IC to enter intermittent operation mode. Under these conditions, the IC will not be able to
supply current to the load continually, unlike the case described in the preceding paragraph.
Waveforms
STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLOR
at EXT pin
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Figure 10 Waveforms appearing at EXT pin as the overload protection circuit operates
Overload detection time (t
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), the overload protection circuit will operate again, and the IC will
Seiko Instruments Inc.
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Protection circuit ON
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S-8520 / 8521 Series
), the overload protection
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