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A p p l i c a t i o n N o t e The Oscillator Maximum Duty Cycle Externally Setting Some products support a maximum oscillator duty cycle (Maxduty) setting by an external resistor connected to the DTC pin. The Maxduty value is large in case the DTC pin voltage is large and vice versa, the Maxduty cycle value is small when DTC pin voltage is small. The soft start function is created by an external capacitor connected to the DTC pin, during start-up the DTC pin voltage is gradually increasing. The soft start initiates from a Maxduty value of 0% and the output voltage ramps up gradually, preventing an inrush current. The soft start time can be adjusted by the capacitance of the capacitor. R1215D L SD R3 C1 C2 C6 8 4 2 6 VIN DELAY EXT VFB GND AMPOUT VREFOUT DTC 1 5 7 3 C5 NMOS R4 C4 R1 R2 C3 LOAD R5 R6 C7 Products With Externally Settable Oscillator Maximum Duty Cycle R1212D R1215D R1290K R1293K The Phase Compensation Externally Setting The phase compensation of some DC/DC converters can be set with external components. Please set the phase compensation of such products by connecting an external capacitor (speed up capacitor) and resistor to the AMPOUT pin. The response of the products with internal phase compensation may not be optimal in some operating conditions, because the phase compensation settings of these products give priority to the stability of the circuit in all conditions. There are guidelines available for selecting the external parts (inductor and output capacitor). The response and the stability of the products with external phase compensation should be best during actual operating conditions such as input voltage, output voltage, output current, inductor, output capacitor and oscillator frequency. In addition, please select the capacitor and the resistor used for external phase compensation based on the recommended value and formula which is described in the product specification and carefully adjust by evaluation during operating conditions. L R1215D SD DC/DC R3 C1 C2 C6 8 4 2 6 VIN DELAY EXT VFB GND AMPOUT VREFOUT DTC 1 5 7 3 C5 NMOS R4 C4 R1 R2 C3 LOAD R5 R6 C7 Products Supporting External Phase Compensation R1211x R1212D R1213K R1215D R1290K R1293K Application Note 126
Application Note The Over Voltage Protection Circuit (OVP) The OVP circuit monitors the VOUT pin voltage and halts oscillation once it reaches the OVP detect voltage. Oscillation resumes when the VOUT pin voltage decreases below the threshold again. In case the cause of the excess VOUT pin voltage is not removed the OVP circuit will stop and resume repeatedly in order limit the VOUT pin voltage. Products Supporting Over Voltage Protection (OVP) R1205N823B OVP Operation Output Voltage Waveform (OVP detect voltage: Typ. 25.0V, OVP release voltage: Typ. 23.2V) Output Voltage VOUT (V) 27 26 25 24 23 22 21 0 10 20 30 40 50 Time t (ms) R1200x R1201N R1202x R1204x R1205x R1206N R1207N R1218x Protection Circuits (Latch Type/Reset Type/Fold back Type) Operation of over current protection by circuit type There are three kinds of current limit protection circuits used in <strong>Ricoh</strong> DC/DC converters, Latch type, Reset type and Fold back type. • A Latch type protection circuit latches the power MOSFET Latch Type Over Current Over Current Cancelation to the OFF state during an excessive high load in case it persists for a specified time (the protection delay time). The circuit can be released by changing the IC into the standby mode and back to the active mode by the CE pin, or by turning the power off and back on. There are some products which an adjustable protection delay time by a capacitor connected to the DLY pin (DELAY pin). • A Reset type protection circuit controls the power MOSFET to the OFF state during an excessive high load in case it persists for a specified time (the protection delay time). The circuit is automatically released after removal VOUT Reset Type VOUT Fold back Type Auto-release Auto-release Protection Delay Time Latch Protection Soft Start Normal Operation t restart by CE signal Protection Delay Time Reset Protection Soft Start Normal Operation t DC/DC of the excessive load and by triggering the soft start operation. The protection delay time is fixed within the IC. • A Fold back type protection circuit decreases the oscillator frequency when the output voltage decreases below the preset value. Therefore the minimum ON duty decreases and the inductor ripple current increases. As a result the Preset value VOUT reduce frequency t Auto-release Fold back Protection Normal Operation Soft Start output current decreases. The protection is automatically released after the excessive load is removed. Products Supporting Latch Type Protection Circuit R1211x R1212D R1213K R1215D R1225N R1232D R1240x R1242S R1243x R1245x R5220x R1283K R1286K R1290K R1293K RP401x RP500x RP501K RP502x RP503x RP504x RP505K RP506K RP550K RP904Z Products Supporting Reset Type Protection Circuit R1224N RP901K Products Supporting Fold Back Type Protection Circuit R1240x R1242S R1243x R1244N R1245x 127
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For Printing (7 pages/525KB) - Ricoh

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