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  • Is it possible to drive the TR beyond the maximum ratings, even for a moment? 
    • No. Operation above the maximum ratings will most likely result in breakdown or, at the very least, deterioration (i.e. hFE drop). Please refer to the SOA (Safe Operating Area) for single pulse operation. For, continuous pulse situations, calculation of the power and die temperature are required.
      Please refer to ROHM’s website for more information (e.g. derating curves).
  • What is the maximum rating of the Base current? 
    • The maximum Base current rating is 1/3rd the Collector current (1/10th in the case of Darlington transistors). In the case of 2SD2656:
      IC max is 1A for DC and 2A for pulse. Therefore, the max. ratings for Base current is 333mA for DC and 666mA pulse. Digital transistors are designed to ensure that the input current will be within the rated value as long as Vin is within the normal range.
  • Is it possible to supply voltage to the Base-Emitter in reverse? 
    • For NPN transistors, VCEO is in breakdown when positive voltage is supplied to the Collector pin while the Emitter is grounded. Conversely, VCEO in PNP transistors is in breakdown when positive voltage is supplied to the Emitter with Collector grounded.
      Breakdown in the opposite direction (e.g. NPN: +V to Emitter, Collector grounded) is roughly equivalent to the breakdown between the Emitter and Base. Typically this is between 5 to 7V, making the Collector-Emitter reverse voltage less than 5V. (If sufficient voltage is supplied to the Collector-Emitter in reverse, deterioration such as low hFE will occur. Otherwise leakage current will flow.)

      The same applies to digital transistors. Voltages up to 5V can be supplied to the Collector-Emitter in the reverse direction as below. However, current flows through what has resistance between an GND terminal and a IN terminal.
  • What is the hFE distribution in actual production lots? 
    • The hFE range is depicted in the data sheet. The upper and lower limits for some products are specified while for others only the lower limit is given. The hFE distribution for components with both limits defined will be within the given range, while for transistors with only a lower limit provided the hFE dispersion will be several times the stated lower limit. More detailed information can be obtained by contacting a ROHM sales representative.
  • What is a digital transistor? 
    • A digital transistor is a bipolar transistor that integrates resistors.

      ¦Concerning internal resistor R1

      • The role of R1 is to stabilize transistor operation by converting the input voltage to current.

      Bipolar operation tends to become unstable if the input (Base) is connected directly to the output terminal of an IC for voltage control. Inserting a resistor at the Base stabilizes operation. Please note that the output current will vary exponentially based on changes to the input voltage, but will maintain a linear relationship with the input current.

      Next we compare the difference between Voltage and Current control.
       Voltage Control
      Input : Emitter-Base Voltage VEB
      Current Control
      Input : Base Current IB
      Measurement Circuit
      Theoretical Formula
      Input-Output Characteristics

      As we can see in the above graphs, Voltage Control results in an exponential change in the output current based on the input voltage, while Current Control exhibits a linear relationship between the input and output currents.
      For example, in the Current Control graph at right a doubling of the input current, from 40uA to 80uA, results in twice the output current, from 9mA to 18mA. However, in the Voltage Control graph we see that a small change in the input voltage, from 0.7V to 0.8V, results in an increase in the output current by 7 times, from 10mA to 70mA. This is not desirable, since even a little noise introduced in the input voltage can result in significant changes to the output current.
      Thus it is evident that current control via Base resistor that converts the voltage into current results in stable drive. ROHM offers digital transistors, which integrate a resistor at the Base, reducing the number of external components and mounting area.

      ¦Regarding internal resistor R2

      • The role of R2 is to absorb leakage current and shunt it to ground in order to prevent malfunction.
      Please note that a large input current may cause the transistor to turn ON,
      regardless of the presence of resistor R2.

      All of the current flows to ground via R2, preventing TR power ON
      (and possible malfunction in the case of leakage current).

      Part of the current flows through the Base, causing the transistor to turn ON
      (inadvertently in the case of leakage current).
      VR2=VBE<(when E-B Forward Voltage ? 0.7V)VR2=VBE>(when E-B Forward Voltage ? 0.7V)
  • How do you calculate the Base current of a digital transistor? 
    • Using ROHM’s digital transistor DTC114EKA as an example:

      As forward current flows through E-B, approximately 0.7V exists (at 25C) between E and B. Since the internal resistor R2 is connected in parallel, the voltage across is identical. Therefore, the current flowing through R2 is calculated as follows:
      IR2 = 0.7V / 10kΩ = 70µA

      When 5V is supplied to the Base at IN a voltage of 4.3V (5V-0.7V) exists across R1, resulting in a current of:
      IR1=4.3V / 10KΩ = 430µA

      Therefore, 430µA-70µA=360µA flows through the Base of the transistor.

      For stable operation the input voltage Vin must be adjusted in order to maintain an output current 10-20 times the Base current or lower. If sufficient output current cannot be obtained a digital transistor with lower R1 should be used.

      At 25ºC the forward voltage (VF) across E-B is approximately 0.7V. However, please note that VF will decrease by about 2.2mV for each degree 1ºC above 25ºC. For example, at an ambient temperature of +50ºC the forward voltage is approximately: 0.7V-(50ºC-25ºC)×2.2mV=0.645V. Therefore, it is imperative to consider the effects of ambient temperature on VF. Please note that there is a voltage tolerance of ±0.1V.
      In the case of internal resistors R1 and R2 a tolerance of ±30% exists. Please calculate accordingly.
  • Are any considerations necessary in the case of ambient temperature changes? 
    • Yes. Pc (Collector power dissipation) must be derated based on the ambient temperature Ta.

      Derating must be performed based on the SOA as well.
      Also, please note that the input voltage (VBE for bipolar and VI(on)/VI(off) for digital) and gain (hFE and G) are affected by the ambient temperature, making it important to verify the electrical characteristics vs. temperature curves (for MOSFETs as well).
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