s-8460 Seiko Instruments Inc., s-8460 Datasheet - Page 11

• Manufacturer Part Number: s-8460
• Description: Step-up Or Down, Single-coil, Pwm Control Switching Regulator Controller
• Manufacturer: Seiko Instruments Inc.

Rev.2.1
4. Overload Protection Circuit
External Parts Selection
1. Inductor
STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Figure 12 ON/OFF Pin Structure
The S-8460 contains a built-in overload protection circuit.
When the output voltage falls because of an overload despite the step-up operation or step-down, the S-8460
enters the step-up operation and holds the maximum duty step-up operation. If this maximum duty state lasts
longer than the overload detection time (t
to protect the switching transistors and the inductor. When the overload protection circuit works, the output
voltage (V
reference voltage slowly from 0 V. If the load is still heavy at this time and the maximum duty step-up operation
lasts longer than t
of intermittent mode. If the overload state is eliminated, the S-8460 goes back to the normal operation.
t
the maximum duty operation to the instant at which pin voltage of the EXT1 to EXT3 is held “L” to protect switching
transistors, is expressed by the following equation.
The inductance value (L value) greatly affects the maximum output current (I
As the L value is reduced gradually, the peak current (I
output current I
the switching transistor is insufficient.
As the L value is increased, the dissipation in the switching transistor due to I
reaches the maximum at a certain L value. As the L value is made even larger, the efficiency degrades since the
dissipation due to the series resistance of the coil increases. I
An inductance of 22 μH is recommended for the S-8460.
When choosing an inductor, attention to its allowable current should be paid since the current exceeding the
allowable value will cause magnetic saturation in the inductor, leading to a marked decline in efficiency and
breakdown of the IC due to large current.
An inductor should therefore be selected so as not the peak current I
represented by the following equations in step-up operation and in step-down operation.
calculation result for step-up and step-down, larger value should be taken as the I
obtained result, an inductor with the allowable current can be thus chosen.
PRO
ON/OFF
_00
is determined by the external capacitor (C
OUT
) rises slowly since a soft start is carried out in the reference voltage circuit in the IC to raise the
OUT
PRO
increases. As the L value is made even smaller, I
, the overload protection circuit will work again. Repeat of this process leads to an operation
VIN
VSS
Seiko Instruments Inc.
PRO
ON/OFF Pin
t
PRO
), the overload protection circuit will hold the pins EXT1 to EXT3 at “L”
“H”
“L”
PRO
[ms] = 0.0011 × C
). The approximate value of t
PK
) increases, the stability of the circuit is improved, and the
CR Oscillation
Non-active
Circuit
Active
OUT
PRO
also decreases.
[pF]
OUT
PK
decreases since the current driveability of
to surpass its allowable current. I
OUT
All EXT Pin
Voltage
) and the efficiency (η).
PRO
V
PK
−
PK
SS
, which is from the beginning of
decreases, and the efficiency
. Adding some margin to the
Output Voltage
Set value
Comparing each
Open
S-8460
PK
11
is