STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.2.1_00
S-8460
Seiko Instruments Inc. 11
ON/OFF Pin
CR Oscillation
Circuit
All EXT Pin
Voltage
Output Voltage
“H” Active − Set value
“L” Non-active V
SS
Open
ON/OFF
VIN
VSS
Figure 12 ON/OFF Pin Structure
4. Overload Protection Circuit
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
PRO
), the overload protection circuit will hold the pins EXT1 to EXT3 at “L”
to protect the switching transistors and the inductor. When the overload protection circuit works, the output
voltage (V
OUT
) rises slowly since a soft start is carried out in the reference voltage circuit in the IC to raise the
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
PRO
, the overload protection circuit will work again. Repeat of this process leads to an operation
of intermittent mode. If the overload state is eliminated, the S-8460 goes back to the normal operation.
t
PRO
is determined by the external capacitor (C
PRO
). The approximate value of t
PRO
, which is from the beginning of
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.
t
PRO
[ms] = 0.0011 × C
PRO
[pF]
External Parts Selection
1. Inductor
The inductance value (L value) greatly affects the maximum output current (I
OUT
) and the efficiency (η).
As the L value is reduced gradually, the peak current (I
PK
) increases, the stability of the circuit is improved, and the
output current I
OUT
increases. As the L value is made even smaller, I
OUT
decreases since the current driveability of
the switching transistor is insufficient.
As the L value is increased, the dissipation in the switching transistor due to I
PK
decreases, and the efficiency
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
OUT
also decreases.
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
PK
to surpass its allowable current. I
PK
is
represented by the following equations in step-up operation and in step-down operation. Comparing each
calculation result for step-up and step-down, larger value should be taken as the I
PK
. Adding some margin to the
obtained result, an inductor with the allowable current can be thus chosen.
