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S-85S1PC28-I8T1U Datasheet(PDF) 10 Page - Seiko Instruments Inc |
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S-85S1PC28-I8T1U Datasheet(HTML) 10 Page - Seiko Instruments Inc |
10 / 38 page POWER MONITORING OUTPUT, 5.5 V INPUT, 200 mA SYNCHRONOUS STEP-DOWN SWITCHING REGULATOR WITH 260 nA QUIESCENT CURRENT S-85S1P Series Rev.1.3_00 10 1. 5 Under voltage lockout function (UVLO) The S-85S1P Series has a built-in UVLO circuit to prevent the IC from malfunctioning due to a transient status at power-on or a momentary drop in the supply voltage. When UVLO status is detected, the high side power MOS FET and low side power MOS FET will turn off, and the SW pin will change to "High-Z". For this reason, switching operation will stop. The soft-start function is reset if UVLO status is detected once, and is restarted by releasing the UVLO status. Note that the other internal circuits operate normally and the status is different from the disabled status. Also, there is a hysteresis width for avoiding malfunctions due to generation of noise etc. in the input voltage. 1. 6 Thermal shutdown function The S-85S1P Series has a built-in thermal shutdown circuit to limit overheating. When the junction temperature increases to 135°C typ., the thermal shutdown circuit becomes the detection status, and the switching operation is stopped. When the junction temperature decreases to 115°C typ., the thermal shutdown circuit becomes the release status, and the switching operation is restarted. If the thermal shutdown circuit becomes the detection status due to self-heating, the switching operation is stopped and output voltage (VOUT) decreases. For this reason, the self-heating is limited and the temperature of the IC decreases. The thermal shutdown circuit becomes release status when the temperature of the IC decreases, and the switching operation is restarted, thus the self-heating is generated again. Repeating this procedure makes the waveform of VOUT into a pulse-like form. Switching operation stopping and starting can be stopped by either setting the EN pin to "L", lowering the output current (IOUT) to reduce internal power consumption, or decreasing the ambient temperature. Table 9 Thermal Shutdown Circuit VOUT Pin Voltage Release: 115 °C typ. *1 Set value Detection: 135 °C typ. *1 "High-Z" *1. Junction temperature 1. 7 Overcurrent protection function The S-85S1P Series has a built-in current limit circuit. The overcurrent protection circuit monitors the current that flows through the low side power MOS FET and limits current to prevent thermal destruction of the IC due to an overload, magnetic saturation in the inductor, etc. When a current exceeding the current limit (ILIM) flows through the low side power MOS FET, the current limit circuit operates and prohibits turning on the high side power MOS FET until the current falls below the low side current limit (ILIMDET). If the value of the current that flows through the low side power MOS FET falls to the ILIMDET or lower, the S-85S1P Series returns to normal operation. ILIMDET is fixed at 270 mA typ. in the IC, and ILIM will vary depending on the external parts to be used. The relation between ILIM, the inductor value (L), the input voltage (VIN), and the output voltage (VOUT) are shown in the following expression. ILIM = ILIMDET + 1 2 × L × fSW × (VIN − VOUT) × VOUT VIN |
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