S-8520A20MC-AVFT2X Datasheet, PDF (15/44 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.9.0_01
S-8520/8521 Series
4. Overload protection circuit (A, C, E types)
The A, C and E types of the S-8520/8521 Series have a built-in overload protection circuit.
If the output voltage drops under the overload status, the maximum duty status (100%) continues. If this 100% duty
status lasts longer than the overload detection time (tpro), the circuit keeps the EXT pin at “H” to protect the
switching transistor and inductor. When the overload protection circuit is functioning, the soft start works in the IC
for the reference voltage, and the reference voltage rises slowly from 0 V. The reference voltage and the feedback
voltage obtained by dividing the output voltage are compared to each other. While the reference voltage is low, the
EXT pin keeps “H” and oscillation stops. If the reference voltage rises and exceeds the feedback voltage, oscillation
restarts.
After oscillation is restarted, if a load is heavy and the EXT pin keeps the “L” level longer than the overload
detection time (tpro), the circuit operates again, and the IC enters the intermittent operation mode by repeating the
action described above. Once the overload state is off, the IC restarts the normal operation.
Waveforms
at EXT pin
Overload detection time (t_pro) Protection circuit ON
[t_ss × 0.3]
Figure 10 Waveforms at EXT pin when the overload protection circuit operates
5. 100% duty cycle
The S-8520/8521 Series operates with a maximum duty cycle of 100%. When using the B, D and F types products,
which do not have the overload protection circuit, the switching transistor keeps ON continually to supply current to
the load, even in cases where the input voltage drops to the set value of output voltage or less. The output voltage
in this case is; the voltage subtracting both of the inductance’s DC resistance and the voltage drop by the switching
resistor’s ON-resistance, from the input voltage.
The A, C and E types that have the overload protection circuit cannot be used for continually supplying current to
the load, as described above, because these types enter the intermittent operation mode by the overload protection
circuit’s operation, when 100% duty lasts longer than the overload detection time (tpro).
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