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S8353 Datasheet(PDF) 20 Page - Seiko Instruments Inc |
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S8353 Datasheet(HTML) 20 Page - Seiko Instruments Inc |
20 / 49 page STEP-UP, PWM CONTROL or PWM / PFM SWITCHABLE BUILT-IN TRANSISTOR SWITCHING REGULATOR S-8353/8354 Series Rev.3.0_01 20 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 ? For smaller ripple voltage ? At PFM control At PWM control 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: L f ) V V V ( I 2 I OSC IN D OUT OUT PK • − + = (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 : 50 ns max. • Reverse voltage : VOUT + VF or more • Current rate : IPK or more |
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