AN-1001 Over Current Protection (OCP) Analysis Using AT7576

Transcription

1 A. Resistor to Detect the Over-Current Figure-01 shows the current detecting circuit of AT7576. As the means to detect the output current, a resistor series is added between the output capacitor and load. The resistance should be low to reduce the power dissipation. The voltage drop across the current-sensing resistor is fed back to control the output transistor. Figure-01 The equations below show the input signals of OCP comparator. V_CCN = Vo (Equation-1) V_CCP = (Vo + Io Rs) R4 / ( R3 + R4 ) (Equation-2) In normal operation, the voltage of CCP is lower than CCN. The output of OCP comparator does not interfere in PWM control. The voltage across the current-sensing resistor is following the output current. While the output current increases, the voltage across Rs and the voltage of CCP increase also. When the voltage of CCP is higher than CCN, the OCP comparator pulls the FB voltage up to turn off the output transistor. In other word, the OCP occurred while V_CCP equal to V_CCN. V_CCP = V_CCN (Vo + I OCP Rs) R4 / ( R3 + R4 ) = Vo I OCP = (Vo R3) / (Rs R4) (Equation-3) The over-current protection threshold current, I OCP, can be calculated as Equation-3.

2 CCN, CCP Voltage (V) CCN and CCP vs. Output Current R1 = 7.68kΩ, R2 = 2.49kΩ Vo = 5.024V R3 = 475Ω, R4 = 16.5kΩ I OCP = 1.446A CCN CCP CCP CCN Output Current (A) Figure-02 Figure-02 is drawn stand on Equation-3. It shows clearly the relation of V_CCN and V_CCP to output current. B. The errors in real PCB In the real PCB, there are many error factors make the OCP threshold current deviates from the designed value. In equation-3, we know the OCP threshold, I OCP, is determined by Vo, Rs, R3 and R4. Then find out the error factors which output tolerance, output load regulation, output ripple, tolerance of resistors, TCR (Temp. coefficient of resistance) of current sensing resistor, voltage drop on Rs trace and input offset voltage (V IO ) of OCP comparator, etc. B-1. Output Voltage Tolerance The output error included the tolerance of feed-back resistors and reference voltage of IC. Use high accuracy resistor of 1% for feed-back resistors R1 and R2 to minimize the output tolerance. The tolerance of reference voltage of IC shows in datasheet as below.

3 B-2. IC Load Regulation Over current protection is designed at heavy load condition. We need to consider the output load regulation. When the OCP condition occurs, the output voltage might deviate from the output at mid-load condition. An output voltage offset of 0.5% for load regulation is reasonable. Output Load Regulation Output Voltage (V) Figure-03 VIN = 12V, Inductor = 100uH, Cout = 470uF Load Current (A) B-3. Output Voltage Ripple In Equation-3, we use average output voltage in substitution for Vo. But the output voltage is actually a triangle wave and the OCP function is triggered at the valley. To calculate the output ripple of a step down converter, the switching frequency, inductor and output capacitor are the most decisive factor. At the most basic level the output voltage will raise and fall as a result of the output capacitor charging and discharging. In this case, the output ripple makes OCP threshold offset around 0.23% lower. Output Voltage (AC) Output Voltage (DC) Figure-04 VIN = 12V, Vo = 5V, Inductor = 100uH, Cout = 470uF. Figure-04

4 B-4. Tolerance of Resistors Except for Vo, the other elements in Equation-3 are R3, R4 and Rs. To choose high precisely resistor for accurately OCP design. The maximum error of OCP threshold will be controlled fewer than 3% if all these resistors are in tolerance of 1%. B-5. TCR of current-sensing resistor Choose a low TCR (Temperature Coefficient of Resistance) resistor as the current-sensing resistor. It reduces the variation of OCP threshold especially in high current and high temperature applications. If choose current-sensing resistor as a low TCR resistor of 200ppm/. The variation of OCP will be 0.8% as the temperature changes from 25 to 65. B-6. Voltage drop on the PCB pattern It is convenient to add a resistor to the network to measure the electric current. Measuring the voltage across the resistor indicates the current that flows through it. The resistance should be low enough not to disturb the circuit condition and dissipate the spurious power. But when the resistance is very low, the resistance of the printed circuit pattern may cause error to the measurement. A precision resistor is not applicable in that case. A practical circuit pattering solution is known as Kelvin s four-terminal method, where the terminals to supply current and the terminals to detect voltage are prepared as illustrated in Figure-5 Figure-05 In any high current, >1Amp, application, Kelvin contacts to the sense resistor are important to maintain accuracy.

5 B-7. Input offset voltage of OCP comparator Consider the V IO (input-offset voltage) of OCP comparator. A worse case, V IO up to 11mV is possible. Assume Rs is 100mΩ and the OCP is designed at 1.7A. The voltage drop across Rs at COP condition is 170mV. The VIO might cause an error of OCP threshold around 6%. C. Conclusions For the applications that need high accuracy OCP function, following the principles below is important. Appropriately adjust the OCP designed value to compensate for output load regulation output ripple. Choose low TCR resistor for current-sensing resistor Follow Kelvin s method to layout the PCB pattern for the current sensing resistor. However, there is a seasonable tolerance of OCP threshold in the actual products. The errors cause by V IO of comparator (6%), tolerance of resistors (3%), TCR of current-sensing resistor (0.8%) and tolerance of reference voltage inside IC (2%). Amount all these errors we can t avoid, the seasonable tolerance of OCP should be define as 10%.