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UC3903N Datasheet(PDF) 5 Page - Texas Instruments |
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UC3903N Datasheet(HTML) 5 Page - Texas Instruments |
5 / 8 page Setting a Fault Window The fault thresholds on the UC1903 are generated by cre- ating positive and negative offsets, equal in magnitude, that are referenced to the chip’s 2.5V reference. The re- sulting fault window is centered around 2.5V and has a magnitude equal to that of the applied offsets. Simplified schematics of the fault window and reference circuits are shown in Figure 1 (see previous page). The magnitude of the offsets is determined by the voltage applied at the window adjust pin, Pin 4. A bias cancellation circuit keeps the input current required at Pin 4 low, allowing the use of a simple resistive divider off the reference to set the ad- just pin voltage. The adjust voltage at Pin 4 is internally applied across R4, and an 8k resistor. The resulting current is mirrored four times to generate current sources IOA, IOB, IOC, and IOD, all equal in magnitude. When all four of the sense inputs are inside the fault window, a no-fault condition, Q4 and Q5 are turned on. In combination with D1 and D2 this pre- vents LOB and LOD from affecting the fault thresholds. In this case, the OV and UV thresholds are equal to VREF + IOA(R5 + R6) and VREF - IOC(R7 + R8) respectively. The fault window can be expressed as: (1) 2.5V ± VADJ 4 . In terms of a sensed nominal voltage level, VS, the win- dow as a percent variation is: (2) VS ± ( 10 ⋅ VADJ) %. When a sense input moves outside the fault window given in equation(1), the appropriate hysteresis control signal turns off Q4 or Q5. For the under-voltage case, Q5 is dis- abled and current source IOB flows through D2. The net current through R7 becomes zero as IOB cancels IOC, giv- ing an 8% reduction in the UV threshold offset. The over- voltage case is the same, with Q4 turning off, allowing IOD to cancel the current flow, IOA, through R6. The result is a UC1903 UC2903 UC3903 Figure 2. The fault window and threshold hysteresis scale as a function of the voltage applied at Pin 4, the window adjust pin. Figure 4. The general purpose op-amp on the UC1903 can be used to create a sense input with an independently tighter fault window. Figure 3. Using the reference output and a resistive divider, a sense input with an independently wider fault window can be generated. Figure 4 demonstrates one of many auxiliary functions that the uncommitted op-amp on the UC1903 can be used for. Alternatively, this op-amp can be used to buffer high impedance points, perform logic functions, or for sensing and amplification. For example, the G.P. op-amp, combined with the 2.5V reference, can be used to pro- duce and buffer an optically coupled feedback signal in isolated supplies with primary side control. The output stage of this op-amp is detailed in Figure 5. The NPN emitter follower provides high source current capability. ≥20mA while the substrate device, Q3, provides good transient sinking capability. Fault window for the Sense Input, in percent, is: ±10 (VADJ) • R3 + R1R2/(R1 + R2) R3 , for: VS (NOM) • R2 R1 + R2 = 2.5V Fault window for the sense input, in percent, is: ±10 (VADJ) • R2 R1 + R2 OPERATION AND APPLICATION INFORMATION hysteresis at the sense inputs which is always 8% of the window magnitude. This is shown graphically in Figure 2. Fault Windows Can Be Scaled Independently In many applications, it may be desirable to monitor vari- ous supply voltages, or voltage levels, with varying fault windows. Using the reference output and external resis- tive dividers this is easily accomplished with the UC1903. Figures 3 and 4 illustrate how the fault window at any sense input can be scaled independently of the remaining inputs. 5 |
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