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NCP3163MNR2G Datasheet(PDF) 9 Page - ON Semiconductor |
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NCP3163MNR2G Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 20 page NCP3163, NCV3163 http://onsemi.com 9 Feedback and Low Voltage Indicator Comparators Output voltage control is established by the Feedback comparator. The inverting input is internally biased at 1.25 V and is not pinned out. The converter output voltage is typically divided down with two external resistors and monitored by the high impedance noninverting input at Pin 2. The maximum input bias current is ±0.4 mA, which can cause an output voltage error that is equal to the product of the input bias current and the upper divider resistance value. For applications that require 5.0 V, the converter output can be directly connected to the noninverting input at Pin 3. The high impedance input, Pin 2, must be grounded to prevent noise pickup. The internal resistor divider is set for a nominal voltage of 5.05 V. The additional 50 mV compensates for a 1.0% voltage drop in the cable and connector from the converter output to the load. The Feedback comparator’s output state is controlled by the highest voltage applied to either of the two noninverting inputs. The Low Voltage Indicator (LVI) comparator is designed for use as a reset controller in microprocessor−based systems. The inverting input is internally biased at 1.125 V, which sets the noninverting input thresholds to 90% of nominal. The LVI comparator has 15 mV of hysteresis to prevent erratic reset operation. The Open Collector output is capable of sinking in excess of 6.0 mA (see Figure 13). An external resistor (RLVI) and capacitor (CDLY) can be used to program a reset delay time (tDLY) by the formula shown below, where Vth(MPU) is the microprocessor reset input threshold. Refer to Figure 20. tDLY = RLVI ⋅ CDLY ⋅ In 1 1 − Vth(MPU) Vout Figure 20. Partial Application Schematic Showing Implementation of LVI Delay with RLVI and CDLY L LVI 1 + + - 3 2 (Bottom View) + + - 16 14 15 1.125 V 1.25 V Feedback Comparator CO Vout Low Voltage Indicator Output CDLY RLVI Current Limit Comparator, Latch and Thermal Shutdown With a voltage mode ripple converter operating under normal conditions, output switch conduction is initiated by the oscillator and terminated by the Voltage Feedback comparator. Abnormal operating conditions occur when the converter output is overloaded or when feedback voltage sensing is lost. Under these conditions, the Current Limit comparator will protect the Output Switch. The switch current is converted to a voltage by inserting a fractional ohm resistor, RSC, in series with VCC and output switch transistor Q2. The voltage drop across RSC is monitored by the Current Sense comparator. If the voltage drop exceeds 250 mV with respect to VCC, the comparator will set the latch and terminate output switch conduction on a cycle−by−cycle basis. This Comparator/Latch configuration ensures that the Output Switch has only a single on−time during a given oscillator cycle. The calculation for a value of RSC is: RSC + 0.25 V Ipk (Switch) Figures 14 and 15 show that the Current Sense comparator threshold is tightly controlled over temperature and has a typical input bias current of 1.0 mA. The propagation delay from the comparator input to the Output Switch is typically 200 ns. The parasitic inductance associated with RSC and the circuit layout should be minimized. This will prevent unwanted voltage spikes that may falsely trip the Current Limit comparator. Internal thermal shutdown circuitry is provided to protect the IC in the event that the maximum junction temperature is exceeded. When activated, typically at 170 °C, the Latch is forced into the “Set” state, disabling the Output Switch. This feature is provided to prevent catastrophic failures from accidental device overheating. It is not intended to be used as a replacement for proper heatsinking. Driver and Output Switch To aid in system design flexibility and conversion efficiency, the driver current source and collector, and output switch collector and emitter are pinned out separately. This allows the designer the option of driving the output switch into saturation with a selected force gain or driving it near saturation when connected as a Darlington. The output switch has a typical current gain of 70 at 2.5 A and is designed to switch a maximum of 40 V collector to emitter, with up to 3.4 A peak collector current. The minimum value for RSC is: RSC(min) + 0.25 V 3.4 A + 0.0735 W |
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