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S-8337ACDB-P8T1G Datasheet(PDF) 11 Page - Seiko Instruments Inc |
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S-8337ACDB-P8T1G Datasheet(HTML) 11 Page - Seiko Instruments Inc |
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11 / 51 page  STEP-UP, 1.2 MHz HIGH-FREQUENCY, PWM CONTROL SWITCHING REGULATOR CONTROLLER Rev.4.0_01 S-8337/8338 Series 11 Operation 1. Switching control method PWM control (S-8337/8338 Series) The S-8337/8338 Series is a DC-DC converter using a pulse width modulation method (PWM). The pulse width of the S-8337/8338 Series varies from 0% to the maximum duty set by RDuty depending on the load current (the pulse width of the S-8338 Series is fixed to 88%), but its switching frequency does not change. Consequently, the ripple voltage generated from switching can be removed easily via a filter. 2. Soft-start function For this IC, the built-in soft-start circuit controls the rush current and overshoot of the output voltage when powering on or when the OFF ON/ pin is switched to the “H” level. A reference voltage adjustment method is adopted as the soft-start method. The following describes the soft-start function. The raising of the output voltage is controlled by slowly raising the reference voltage of the error amplifier input from 0 V at power on as shown in Figure 7. The soft-start function is realized by controlling the voltage of the FB pin so that it is the same potential as the reference voltage that is slowly raised. A Rail-to-Rail amplifier is adopted as the error amplifier, which means that the voltage is loop controlled so that it can be the same as the reference voltage. The following explains the operation at power on (refer to Figure 8). When VIN is raised from 0 V to 3.3 V, the VOUT voltage rises to a value close to VIN via the inductor L and diode SD. This raises the voltage of the FB pin (VFB) by approximately 0.35 V (when RFB1 = 8.2 k Ω, RFB2 = 1.0 kΩ). Because the reference voltage rises from 0 V, the VFB voltage is higher than the reference voltage while the voltage rises from 0 V to 0.35 V. During this period, the EXT output is low. The EXT output is in the stepped-up status between high and low after the reference voltage reaches 0.35 V and VOUT is slowly raised in accordance with the rising of the reference voltage. Once the reference voltage rises, the voltage cannot be reset (the reference voltage is 0 V) unless the power supply voltage is the UVLO detection voltage or lower or the shutdown pin is the “L” level. Conversely, when the power supply voltage rises up to the reset voltage after it is lowered to the UVLO detection voltage or lower, the output voltage is stepped up by the soft-start function. PWM Comparator VOUT RFB2 RFB1 FB SD L VIN EXT CC RZ CZ M1 + – CL Error amplifier Vref 0.5 V 0 V + – Error amplifier reference voltage Figure 7 |
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