Application Note 1073 Design Consideration with AP3502E/3E Prepared by Cheng Zhi Peng System Engineering Dept. 1. Introduction 2. General Description The AP3502E/3E are current-mode step-down DC-DC converters, capable of driving a 2A/3A load with high efficiency, especially high efficiency at light load, excellent line and load regulation. The AP3502E/3E integrate cycle-by-cycle current limit protection, programmable soft-start, hiccup mode for short circuit protection and over temperature protection, which can notably increase the system reliability. The AP3502E/3E are synchronous step-down converters with internal power MOSFETs. Turn on/off M1 and M2 alternately to chop the input voltage. The current sense signal is compared with the EA output signal to regulate the output voltage and adjust the MOSFETs’ duty cycle. The AP3502E/3E are also high reliability ICs with integrated OCP, OVP, OTP, UVLO circuit. For more information please refer to the functional block diagram (Figure 1). IN 2 SHUTDOWN COMPARATOR 1.5V VA INTERNAL REGULATOR 0.3V VB CURRENT SENSE AMPLIFIER SLOP COMP VA + OSCILLATOR SCP 90K/340K 1 BS M1 CLK LOCK EN COMPARATOR 7 S Q R Q 3 PWM COMPARATOR SW M2 2.5V VB 4 1.1V 5 A SS 8 GND EA 5 0. 925V FB 6 COMP Figure 1. Functional Block Diagram of AP3502E/3E Where CSS is the required capacitor between SS pin and GND, tSS is the desired soft-start time and VFB is the feedback voltage. 2.1 Programmable Soft-start The soft-start time of the AP3502E/3E is fully user programmable by selecting different CSS value. The CSS is charged by a 5μA current source, generating a ramp signal fed into non-inverting input of the error amplifier. And this ramp signal will regulate the voltage on COMP pin when starting the system, thus realizing soft-start. The capacitor value required for a given soft-start ramp time can be expressed as: 5μA C SS = t SS × Nov. 2012 2.2 Over Current Protection The AP3502E/3E have internal over current protection function to protect themselves from catastrophic failure. The AP3502E/3E can monitor the drain-to-source current of M1. The peak current-limit threshold is internally set at 3.5A/5.6A. When the inductor current is higher than the current limit threshold, OCP function will be triggered, VFB Rev. 1. 2 BCD Semiconductor Manufacturing Limited 1 Application Note 1073 forcing M1 to turn off, and this will last until the next switching cycle. be turned off. The AP3502E/3E will restart once released from OVP condition. 2.3 Short Circuit Protection When the circuit is shorted, the output is connected to GND and FB voltage is lower than 0.3V, reducing the switching frequency to 90kHz. Meanwhile, the current flowing through inductor reaches the current limit threshold of upper-side switch, then the OCP function is triggered. After that, inductor current will be decreased and SS pin begins to discharge; when SS discharge voltage reaches to about 0.2V, the IC enter soft-start mode and charge to SS pin. Thus, inductor current increases slowly and triggers the current limit threshold of upper-side switch again. With that, SS pin discharges and inductor current decreases once more. This process will be repeated continually until the system release from SCP(short circuit protection) function and restart normally, as showed in Figure 2. 2.5 Over Temperature Protection The OTP circuitry is provided to protect the IC if the maximum junction temperature is exceeded. When the junction temperature exceeds 160ºC, it will shut down the internal control circuit, M1 and M2. The AP3502E/3E will restart automatically under the control of soft-start circuit when the junction temperature decreases to 130ºC/140ºC. 2.6 High Efficiency at Light Load When the systems work in light load, Discontinuous conduction mode (DCM) is usually more advantage than continuous conduction mode (CCM). Since there’ll be higher efficiency in DCM mode than CCM, it can supply more power if the loss power are same. The AP3502E/3E is available for DCM, so it can achieve high efficiency at light load. 100 90 VOUT 80 Efficiency (%) VSS VSW 70 60 50 VIN=12V,VOUT=3.3V,L1=10μH 40 AP3502E AP3503E 30 IL 20 0.1 0.2 0.3 0.4 Load Current (A) Figure 2. AP3502E Short Circuit Protection and Recovery Figure 3. Efficiency vs. Load Current 2.4 Over Voltage Protection The AP3502E/3E have internal OVP circuits. When VOUT is higher than the OVP threshold, the power switching will C5 10nF VIN=12V R4 100k 2 IN 7 C1 10µF/25V (10µF/25V x2) EN C1, C3, R3 A (B) A for AP3502E B for AP3503E Nov. 2012 AP3502E/3E 8 SS GND 4 C4 0.1µF 1 BS L1 10µH SW 3 R1 26.1k FB 5 COMP 6 C3 4.7nF (3.3nF) C6 Optional VOUT=3.3V D1 Optional R2 10k C2 22µF/6.3V x2 R3 13k Figure 4. Typical Application of AP3502E/3E Rev. 1. 2 BCD Semiconductor Manufacturing Limited 2 Application Note 1073 3. Application Information I PEAK = I OUT + Typical application circuit is shown in Figure 4. For circuit parameters setting please refer to the following descriptions. The current rating of the selected inductor should be ensured to be 1.5 times of the peak inductor current. 3.3 Input Capacitor Setting A high-quality input capacitor with big value is needed to filter noise at input voltage source and limit the input ripple voltage while supplying most of the switch current during ON time. For input capacitor selection, a ceramic capacitor is highly recommended due to its low impedance and small size. However, tantalum or low electrolytic capacitor is also sufficed. ⎛ R2 ⎞ V FB = VOUT × ⎜ ⎟ ⎝ R1 + R 2 ⎠ Where VFB is the feedback voltage, and VFB=0.925V. Thus, VOUT can be expressed as: ⎛ R1 + R 2 ⎞ VOUT = 0.925 × ⎜ ⎟ ⎝ R2 ⎠ There are two important parameters of the input capacitor: the voltage rating and RMS current rating. The voltage rating should be at least 1.25 times greater than the maximum input voltage, and the RMS current of input capacitor can be expressed as: First, fix R2 based on the recommended value, 10kΩ. Then, R1 can be expressed as: I CIN _ RMS = I OUT ( MAX ) × ⎛V ⎞ R1 = R 2 × ⎜ OUT − 1⎟ ⎝ 0.925 ⎠ VOUT V IN ⎛ VOUT ⎜⎜1 − V IN ⎝ ⎞ ⎟⎟ ⎠ Where ICIN_RMS is the RMS current of input capacitor. 3.2 Inductor Setting The inductor is used to supply smooth current to output when driven by a switching voltage. Its value relies on the operating frequency, load current, ripple current, and duty cycle. As indicated by the RMS current equation above, ICIN_RMS reaches the highest level at the duty cycle of 50%. So the RMS current of input capacitor should be greater than half of the output current under this worst case. For reliable operation and best performance, ceramic capacitors are preferred for input capacitor because of their low ESR and high ripple current rating. And X5R or X7R type dielectric ceramic capacitors are preferred due to their better temperature and voltage characteristics. Additionally, when selecting ceramic capacitor, make sure its capacitance is big enough to provide sufficient charge to prevent excessive voltage ripple at input. The input ripple voltage can be approximately expressed as below: A higher-value inductor can decrease the ripple current and output ripple voltage, however usually with larger physical size. So some compromise needs to be made when selecting the inductor. The peak-to-peak inductor ripple current is 26% of the maximum output current when operating in continuous mode (In most applications, a good compromise is from 20% to 30% of the maximum load current of the converter), and the inductor L1 can be selected according to: f SW 2 × VIN × f SW × L1 Where IPEAK is the peak inductor current. 3.1 Output Voltage Setting The output voltage can be set using a voltage divider from the output to FB pin. VOUT is divided by the voltage divider as below: L1 = VOUT × (VIN − VOUT )× VOUT ΔVIN = V IN − VOUT × V IN × I OUT × 26% ⎛ V I OUT × ⎜1 − OUT f SW × C IN ⎜⎝ VIN ⎞ VOUT ⎟⎟ × ⎠ VIN Where ΔVIN is the input ripple voltage. Where VIN is the input voltage, IOUT is the output current, and fSW is the oscillator frequency. 3.4 Output Capacitor Setting The output capacitor can be selected based upon the desired output ripple and transient response. The output voltage ripple depends directly on the ripple current and is affected by two parameters of the output capacitor: total capacitance and the Equivalent Series Resistance (ESR). Another important parameter for the inductor is the current rating. After fixing the inductor value, the peak inductor current can be expressed as: Nov. 2012 Rev. 1. 2 BCD Semiconductor Manufacturing Limited 3 Application Note 1073 The output ripple voltage can be expressed as: ⎡ ⎛ ⎣ ⎝ 8 × COUT × f SW ΔVO = ΔI L × ⎢ RESR + ⎜⎜ 1 VIN/VOUT (V) 12/1.2 12/1.8 12/2.5 12/3.3 12/5 ⎞⎤ ⎟⎟⎥ ⎠⎦ Where ΔVO is the output ripple voltage, and RESR is ESR of output capacitor. L × ΔI TRAN C OUT × (VIN − VOUT ) 2 Where ΔITRAN is the output transient load current step, and VDROP is the output voltage drop (ignoring loop bandwidth). Both the voltage rating and RMS current rating of the capacitor needs to be carefully examined when designing a specific output ripple or transient drop. The output capacitor voltage rating should be greater than 1.5 times of the maximum output voltage. In the buck converter, output capacitor current is continuous. The RMS current is decided by the peak-to-peak inductor ripple current. It can be expressed as: ⎛ G × GCS × R3 V FB f C = ⎜⎜ EA × VOUT ⎝ 2π × C OUT ⎞ ⎟⎟ < 0.1× f SW ⎠ Where fC is the crossover frequency, GEA is the error amplifier transconductance, GCS is the current sense trans-conductance. And the desired crossover frequency can be set via compensation resister R3. ΔI L 12 For sufficient phase margin, the loop gain slope should be -20db/decade at the cross frequency. To suffice this requirement, the output filter pole (fP_OUT), which is product by output capacitor and the load resister, should be cancelled by the zero point of error amplifier (fZ_EA) due to the compensation capacitor (C3) and the output resistor of the error amplifier. They can be expressed as: Where ICOUT_RMS is the RMS current of output capacitor. 3.5 Loop Compensation The AP3502E/3E employs current-mode control to achieve easy compensation and fast dynamic response. Optimal loop compensation depends on the output capacitor, inductor, load, compensation network and also the device itself. For a stable system, the values for the compensation network are shown in Table 1 and Table 2. VIN/VOUT (V) 12/1.2 12/1.8 12/2.5 12/3.3 12/5 R1 (kΩ) 3 9.53 16.9 26.1 44.1 C3 (nF) 4.7 6.8 5.6 4.7 3.3 ⎛ 1 f P _ OUT = ⎜⎜ 2 π C × OUT × ROUT ⎝ ⎞ ⎟⎟ ⎠ 1 ⎞ ⎛ f Z _ EA = ⎜ ⎟ ⎝ 2π × C 3 × R3 ⎠ R3 (kΩ) 7.5 15 13 13 13 Where, fP_OUT is the output filter pole and fZ_EA is the zero point of error amplifier. In general, we can set fZ_EA below one-forth of the fC. So the value of C3 is determined by the following equation: Table 1. AP3502E Compensation Value R-C Combination Nov. 2012 R3 (kΩ) 7.5 10 10 13 13 If the VIN/VOUT value of desired solution is not found from the table above, the loop transfer function should be analyzed to optimize the loop compensation. The overall loop transfer function is the product of the power stage and the feedback network transfer function. The power stage transfer function is dictated by the modulator, the output LC filter and load. The feedback transfer function is dictated by the error amplifier gain, external compensation network and feedback resistor ratio. The purpose of loop compensation is to shape the loop transfer function in order to meet the desired loop gain. The crossover frequency should be set firstly. Because lower crossover frequency may result in slower line/load transient responses, while higher crossover frequency may result in system instability. A good compromise is to set the crossover frequency below 10% of the switching frequency. The crossover frequency (fC) can be expressed as below: The output capacitor selection will also affect the output drop voltage during load transient. The output drop voltage during load transient is dependent on many factors. However, an approximation of the transient drop ignoring loop bandwidth can be expressed as: I COUT _ RMS = C3 (nF) 6.8 6.8 5.6 3.3 2.2 Table 2. AP3503E Compensation Value R-C Combination For lower output ripple voltage across the entire operating temperature range, X5R or X7R ceramic dielectric capacitor, or other low ESR tantalum capacitor or aluminum electrolytic capacitor are recommended. VDROP = ΔI TRAN × RESR + R1 (kΩ) 3 9.53 16.9 26.1 44.1 Rev. 1. 2 BCD Semiconductor Manufacturing Limited 4 Application Note 1073 C3 > 4. PCB Layout Guidance 4 2π × R3 × f C PCB layout is an important part for DC-DC converter design. Poor PCB layout may reduce the converter performance and disrupt its surrounding circuitry due to EMI. A good PCB layout should follow below guidance: R3 and C3 should be set appropriately to make sure the system work at the desired transient voltage drop and setting time. If the output capacitor has a large capacitance and/or a high ESR value, the zero point resulting from the output capacitor as well as its ESR should be considered. In this case, the additional capacitor (C6) should be placed between the COMP pin and GND. And, C6 can add a pole to the circuit, thus increasing the mid-frequency width of the control circuit. 4.1 Power Path Length The power path of AP3502E/3E includes an input capacitor, output inductor and output capacitor. Place them on the same side of PCB and connect them with thick traces or copper planes on the same layer. The power components must be kept together closely. The longer the paths, the more they act as antennas, radiating unwanted EMI. ⎛ ⎞ 1 ⎟⎟ f Z _ ESR = ⎜⎜ × × 2 π C R OUT ESR ⎠ ⎝ 4.2 Coupling Noise The external control components should be placed as close to the IC as possible. Where fZ_ESR is the zero point of output filter. If needed, the value of C6 can be expressed as: C6 = 4.3 Feedback Net Special attention should be paid to the route of the feedback ring. The feedback trace should be routed far away from the inductor and noisy power traces. Try to minimize trace length to the FB pin and connect feedback network behind the output capacitors. C OUT ×RESR R3 3.6 Bootstrap Capacitor The bootstrap capacitor provided is used to drive the power switch’s gate above the supply voltage. The bootstrap capacitor is supplied by an internal 5V supply and placed between SW pin and BS pin to form a floating supply across the power switch driver. So the bootstrap capacitor should be a good quality and high-frequency ceramic capacitor. For best performance, the bootstrap capacitor should be X5R and X7R ceramic capacitor, and is recommended to be 10nF. 4.4 Via Hole Be careful to the via hole. Via hole will result in high resistance and inductance to the power path. If heavy switching current must be routed through via holes and/or internal planes, use multiple parallel via holes to reduce their resistance and inductance. Typical examples of AP3502E/3E PCB layer are shown in Figure 5, 6. Figure 5. Top Layer Nov. 2012 Figure 6. Bottom Layer Rev. 1. 2 BCD Semiconductor Manufacturing Limited 5