S8355 datasheet(28/72 Pages) SII

28 / 72 page

STEP-UP, SUPER-SMALL PACKAGE, 600 kHz, PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

S-8355/56/57/58 Series

Rev.8.1_01

1. Inductor

The inductance value (L value) has a strong influence on the maximum output current (IOUT) and efficiency (

η).

Figure 16

shows the relationship between the IOUT and

η dependency on L of S-8355/56/57/58 series.

F, G, H, J, L and M types

CDRH6D28, VOUT = 5.0 V, VIN = 3.0 V

Recommended range

IOUT

4.7

L value (

μH)

Efficiency

decreases

IOUT decreases

IPK increases

Coil size :

smaller

Efficiency

decreases

IOUT decreases

IPK decreases

Coil size : bigger

Figure 16 L Value vs. IOUT Characteristics, L Value vs.

η Characteristics

The peak current (IPK) increases by decreasing L value and the stability of the circuit improves and IOUT increases. If L

value is decreased, the efficiency falls causing a decline in the current drive capacity for the switching transistor, and

IOUT decreases.

The loss of IPK by the switching transistor decreases by increasing L value and the efficiency becomes maximum at a

certain L value. Further increasing L value decreases the efficiency due to the loss of the direct current resistance of

the coil. IOUT also decreases.

A higher oscillation frequency allows selection of a lower L value, making the coil smaller.

The recommended inductances are a 22 to 100

μH for B, E, and K types, a 4.7 to 47 μH for F, G, H, J, L, and M

types, 3.0 to 22

μH for N, P and Q types.

Be careful of the allowable inductor current when choosing an inductor. Exceeding the allowable current of the

inductor causes magnetic saturation, much lower efficiency and destruction of the IC chip due to a large current.

Choose an inductor so that IPK does not exceed the allowable current. IPK in discontinuous mode is calculated by the

following equation:

)

(

OSC

OUT

•

−

(A)

(17)

fosc = oscillation frequency, VD

≅ 0.4 V.

2. Diode

Use an external diode that meets the following requirements :

• Low forward voltage : VF

< 0.3 V

• High switching speed : 500 ns max.

• Reverse voltage :

VOUT

+ VF or more

• Current rate :

IPK or more

S8355 Datasheet(PDF) 28 Page - Seiko Instruments Inc

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