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BD18377EFV-M 数据表(PDF) 9 Page - Rohm |
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BD18377EFV-M 数据表(HTML) 9 Page - Rohm |
9 / 32 page BD18377EFV-M Datasheet . 9/28 TSZ02201-0W1W0C500020-1-2 © 2016 ROHM Co., Ltd. All rights reserved. 19.APR.2016 Rev.005 TSZ22111 ・ ・ ・ ・ 15 ・ ・ ・ ・ 001 www.rohm.com ● ● ● ● Functional Description Constant current driver block The BD18377EFV-M uses a constant current output driver with a provision for dot calibration per channel. The constant current ID is derived from referring an internal reference voltage over the external resistor REXT, see Equation 1. Reference current. The external resistor is chosen to set a reference current IREF, in the range of 30~100uA. In operating conditions the resistor value is in the range of 12k-40kΩ. The maximum output current IDmax is 500 x IREF, see Equation 2. The external resistor value, the 6 bit current calibration (CAL) and output current relate as follows: see Equation 3. EXT REF REF R V I = Equation 1. Reference current 500 max ⋅ = REF D I I Equation 2. Maximum output current 64 ) 1 ( max + ⋅ = CAL I I D D Equation 3. Output current as function of CAL value SC D LED I I I + = , where Ω ≤ k V I D SC 430 Equation 4. Total LED current. Finally the total LED current, including the offset current due to the SHORT detection is shown in Equation 4. Note that the short detection current flows at all times when the channel is enabled. For example in case of a maximum output current of 20mA REXT shall be 30kΩ. See Figure 8 the global reference current, IREF, is mirrored into the channel current to generate a local reference voltage. The error amplifier A1 forces the voltage across the LED current sense resistor to match the local reference voltage. The output device is scaled to give 6 bit output range. Thus a factor IDmax/ID can be achieved in the range from 0 to 63 in 64 steps. The output devices driving analog current amplitude are timed by the global PWM. The active drivers have a low leakage current to keep the LED in firm OFF condition when the channel is not active. To ensure low leakage current ( IDLeak ), the gate voltage of the output transistors will be pulled low during MASK = LOW or PWM_B = HIGH or when the device is not required by the CAL setting. When there is an open load, half open load or a weak battery supply, the channel will be unable to sink enough current. In this case the output of error amplifier A1 will latch to maximum voltage pulling the output voltage VD below VOCth. This is detected by comparator A2 as an OPEN event. There is a glitch reject filter that will reject any OPEN that is < tglitch_reject. The voltage VD at pin DN (Figure 8) is divided by a high ohmic voltage divider and compared to a reference voltage in order to detect the short circuit. When comparator A3 detects a voltage at pin Dn > VSCth this indicates a LED SHORT circuit condition. SHORT detection requires a minimum PWM ON time > tON. This is because the glitch reject filter will reject any SHORT that is < tglitch_reject. The same circuit is used for both OPEN and SHORT rejection and therefore the period is the same. Both the OPEN and SHORT detection circuits are disconnected during PWM_B = “HIGH” or MASK = LOW, but the SHORT detection bypass current will flow when MASK _ HIGH. The status of OPEN, SHORT for each channel can be read out from the OPEN / SHORT registers using the command [READ OPEN] and [READ SHORT]. |
类似零件编号 - BD18377EFV-M |
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类似说明 - BD18377EFV-M |
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