S-8353A20MC-IQFT2X Datasheet, PDF (20/49 Pages) Seiko Instruments Inc – BUILT-IN TRANSISTOR SWITCHING REGULATOR

STEP-UP, PWM CONTROL or PWM / PFM SWITCHABLE BUILT-IN TRANSISTOR SWITCHING REGULATOR
S-8353/8354 Series
Rev.3.0_01
 External Parts Selection
The relationship between the major characteristics of the step-up circuit and the characteristic parameters of the external
parts is shown in Figure 12.
For larger output current ?
For higher efficiency ?
At PFM control
At PWM control
For smaller ripple voltage ?
Smaller inductance
Larger inductance
Smaller direct current resistance of inductor
Larger output capacitance
Larger output capacitance
Figure 12 Relationship between Major Characteristics of Step-up Circuit and External Parts
1. Inductor
The inductance value (L value) has a strong influence on the maximum output current (IOUT) and efficiency (η).
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 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 47 μH to 220 μH for A, C, and D types, a 10 μH to 47 μH for H and J 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:
I_PK =
2 I_OUT (V_OUT + V_D − V_IN ) _(A)
f_OSC • L
(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 : 50 ns max.
• Reverse voltage :
VOUT + VF or more
• Current rate :
IPK or more
20