STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

S-8540/8541 Series

15

2. Inductor

η).

increases. If L is made even smaller, the efficiency falls causing a decline in the current drive capacity

maximum at a certain L value. Increasing L further decreases the efficiency due to the loss of coil DC

When the inductance is large in an S-8540/8541 series product, the output voltage may grow unstable

in some cases, depending on the conditions of the input voltage, output voltage, and the load current.

Perform sufficient evaluation under the actual condition and decide an optimum inductance.

The recommended inductances are 10

μH for A, B types and 22 μH for C, D types.

When choosing an inductor, attention to its allowable current should be paid since the current over the

allowable value will cause magnetic saturation in the inductor, leading to a marked decline in efficiency.

IN

OSC

OUT

IN

OUT

OUT

PK

V

L

f

2

)

V

(V

V

I

I

×

×

×

−

×

+

=

3. Diode

The diode to be externally coupled to the IC should be a type that meets the following conditions:

• The forward voltage is low (Schottky barrier diode recommended).

• The switching speed is high (50 ns max.).

4. Capacitors

the power supply. It should be 47 to 100

μF, although the actual value depends on the impedance of

the power source used and load current value.

Resistance) to smoothen the ripple voltage. When the input voltage is extremely high or the load

current is extremely large, the output voltage may become unstable. In this case the unstable area

will become narrow by selecting a large capacitance for an output side capacitor. A tantalum

electrolytic capacitor is recommended since the unstable area widens when a capacitor with a large

ESR, such as an aluminum electrolytic capacitor, or a capacitor with a small ESR, such as a ceramic

capacitor, is chosen. The range of the capacitance should generally be 47 to 100

μF.

μF ceramic

capacitor and wired in a short distance and at a low impedance.