FR9881AS6

FR9881A /a85T 18V, 2A, 1MHz Synchronous Step-Down DC/DC Converter Description Features The FR9881A is a synchronous step-down DC/DC converter that provides wide 4.5V to 18V input voltage range and 2A load current capability. At light load condition, the FR9881A can operate at power saving mode to support high efficiency and reduce power loss.  The FR9881A fault protection includes cycle-by-cycle current limit, UVLO and thermal shutdown. Soft-start period depends on external EN/SS resistor adjustment. Soft-start function prevents inrush current at turn-on. This device uses current mode to control scheme which provides fast transient response. Internal compensation function reduces external compensation components and simplifies the design process. In shutdown mode, the supply current is about 1μA. The FR9881A is offered in SOT-23-6 package, which provides a very compact system solution.            Low RDS(ON) Integrated Power MOSFET (150mΩ/85mΩ) Internal Compensation Function Wide Input Voltage Range: 4.5V to 18V 0.6V Reference Voltage 2A Output Current 1MHz Switching Frequency Soft-Start Period Depends on EN/SS Resistor Adjustment Cycle-by-Cycle Current Limit Hiccup Short Circuit Protection Over-Temperature Protection with Auto Recovery Input Under Voltage Lockout SOT-23-6 Package Applications     STB (Set-Top-Box) LCD Display, TV Distributed Power System Networking, XDSL Modem Pin Assignments Ordering Information S6 Package (SOT-23-6) FR9881A□ Package Type S6: SOT-23-6 LX VIN EN/SS 6 5 4 (Marking) 1 2 3 BST GND FB Figure 1. Pin Assignment of FR9881A FR9881A-Preliminary 0.2-MAR-2016 SOT-23-6 Marking Part Number Product Code FR9881AS6 FR6 1 FR9881A /a85T Typical Application Circuit C3 0.1μF R3 100kΩ 4 1 EN/SS 5 VIN BST LX 6 VIN L1 1.8μH VOUT 1.2V 4.5V to 18V FR9881A C1 FB C5 10μF/25V CERAMIC x 1 10μF/25V CERAMIC x 1 3 GND 2 R1 4.99kΩ 1% C4 C2 22μF/6.3V CERAMIC x 2 (optional) R2 4.99kΩ 1% Figure 2. CIN /COUT use Ceramic Capacitors Application Circuit C3 0.1μF R3 100kΩ 4 1 EN/SS 5 VIN BST LX 6 VIN L1 1.8μH VOUT 1.2V 4.5V to 18V FR9881A C1 FB C5 100μF/25V EC x 1 0.1μF/25V CERAMIC x 1 3 GND 2 R1 4.99kΩ 1% C4 C2 100μF/6.3V EC x 1 (optional) R2 4.99kΩ 1% Figure 3. CIN /COUT use Electrolytic Capacitors Application Circuit VIN=12V, the recommended BOM list is as below. VOUT C1 R1 R2 C5 C4 L1 C2 1.2V 10μF MLCC 4.99kΩ 4.99kΩ 10μF MLCC 10pF~10nF 1.8uH 22μF MLCC x2 1.8V 10μF MLCC 30.9kΩ 15.4kΩ 10μF MLCC 10pF~10nF 3.3uH 22μF MLCC x2 2.5V 10μF MLCC 30.9kΩ 9.76kΩ 10μF MLCC 10pF~10nF 3.3uH 22μF MLCC x2 3.3V 10μF MLCC 30kΩ 6.65kΩ 10μF MLCC 10pF~10nF 4.7uH 22μF MLCC x2 5V 10μF MLCC 30.9kΩ 4.22kΩ 10μF MLCC 10pF~10nF 4.7uH 22μF MLCC x2 1.2V 100μF EC 4.99kΩ 4.99kΩ 0.1μF -- 1.8uH 100μF EC 1.8V 100μF EC 30.9kΩ 15.4kΩ 0.1μF -- 3.3uH 100μF EC 2.5V 100μF EC 30.9kΩ 9.76kΩ 0.1μF -- 3.3uH 100μF EC 3.3V 100μF EC 30kΩ 6.65kΩ 0.1μF -- 4.7uH 100μF EC 5V 100μF EC 30.9kΩ 4.22kΩ 0.1μF -- 4.7uH 100μF EC Table 1. Recommended Component Values FR9881A-Preliminary 0.2-MAR-2016 2 FR9881A /a85T Functional Pin Description Pin Name Pin No. Pin Function BST 1 High side gate drive boost pin. A capacitance between 10nF to 100nF must be connected from this pin to LX. It can boost the gate drive to fully turn on the internal high side NMOS. GND 2 Ground pin. FB 3 EN/SS 4 VIN 5 LX 6 Voltage feedback input pin. Connect FB and VOUT with a resistive voltage divider. This IC senses feedback voltage via FB and regulates it at 0.6V. Enable/Soft-start pin. This pin include two function controls: 1. Enable function: control input that turns the converter on or off 2. Soft-start function: soft-start period depends on external EN/SS resistor adjustment. Connect a resistor to VIN for self-startup. (Soft-start setting, please refer to the following page 10 table 2). Power supply input pin. Placed input capacitors as close as possible from VIN to GND to avoid noise influence. Power switching node. Connect an external inductor to this switching node. Block Diagram VIN ISEN UVLO EN/SS Internal Regulator VCC OTP VCC Oscillator BST Soft-Start High-Side MOSFET S FB Current Comp R OTP Control Logic Driver Logic LX UVLO Low-Side MOSFET Vref Current Limit GND Figure 4. Block Diagram of FR9881A FR9881A-Preliminary 0.2-MAR-2016 3 FR9881A Absolute Maximum Ratings (Note 1) /a85T ● Supply Voltage VIN ------------------------------------------------------------------------------------------- -0.3V to +20V ● Enable Voltage VEN/SS --------------------------------------------------------------------------------------- -0.3V to +20V ● LX Voltage VLX ------------------------------------------------------------------------------------------------ -1V to VIN+0.3V ● Dynamic LX Voltage in 15ns Duration------------------------------------------------------------------- -5V to VIN+5V ● BST Pin Voltage VBST --------------------------------------------------------------------------------------- VLX-0.3V to VLX+6V ● All Other Pins Voltage -------------------------------------------------------------------------------------- -0.3V to +6V ● Maximum Junction Temperature (TJ) ------------------------------------------------------------------- +150°C ● Storage Temperature (TS) --------------------------------------------------------------------------------- -65°C to +150°C ● Lead Temperature (Soldering, 10sec.) ----------------------------------------------------------------- +260°C ● Package Thermal Resistance (θJA) SOT-23-6 --------------------------------------------------------------------------------------------- 250°C/W ● Package Thermal Resistance (θJC) SOT-23-6 --------------------------------------------------------------------------------------------- 110°C/W Note 1：Stresses beyond this listed under “Absolute Maximum Ratings" may cause permanent damage to the device. Recommended Operating Conditions ● Supply Voltage VIN ------------------------------------------------------------------------------------------- +4.5V to +18V ● Operation Temperature Range --------------------------------------------------------------------------- -40°C to +85°C FR9881A-Preliminary 0.2-MAR-2016 4 FR9881A /a85T Electrical Characteristics (VIN=12V, TA=25°C, unless otherwise specified.) Parameter Symbol Conditions Min Typ 4.5 Max Unit 18 V VIN Input Supply Voltage VIN VIN Quiescent Current IDDQ VEN/SS=2V, VFB=1.0V 1 1.2 mA VIN Shutdown Supply Current ISD VEN/SS =0V 1 3 μA Feedback Voltage VFB 4.5V≦VIN≦18V 0.6 0.612 V 0.588 High-Side MOSFET RDS(ON) (Note 2) RDS(ON) 150 mΩ Low-Side MOSFET RDS(ON) (Note 2) RDS(ON) 85 mΩ High-Side MOSFET Leakage Current ILX(leak) VEN/SS =0V, VLX=0V ILIMIT(HS) Minimum Duty High-Side MOSFET Current Limit (Note 2) Oscillation Frequency Short Circuit Oscillation Frequency FOSC FOSC(short) Maximum Duty Cycle DMAX Minimum On Time (Note 2) TMIN Input Supply Voltage UVLO Threshold VUVLO(Vth) Input Supply Voltage UVLO Threshold Hysteresis VUVLO(HYS) Adjustable Soft-Start Period TSS E Input Low Voltage VEN/SS(L) E Input High Voltage VEN/SS(H) Thermal Shutdown Threshold (Note 2) TSD 10 3.5 0.8 1 μA A 1.2 MHz VFB=0V 150 kHz VFB=0.4V 80 % 100 ns 4.3 V 400 mV 1 ms VIN Rising VEN/SS=5V,R3=100kΩ 0.4 2 V V 160 °C Note 2：Not production tested. FR9881A-Preliminary 0.2-MAR-2016 5 FR9881A /a85T Typical Performance Curves VIN=12V, VOUT=3.3V, C1=10μF×2, C2=22μF×2, L1=4.7μH, TA=+25°C, unless otherwise noted. IOUT=0A IOUT=2A VOUT 10mV/div. VOUT 10mV/div. IL 1A/div. IL 1A/div. VLX 5V/div. VLX 5V/div. 2ms/div. 1μs/div. Figure 5. Steady State Waveform IOUT=0A Figure 6. Steady State Waveform IOUT=2A VEN/SS VOUT VEN/SS 5V/div. 1V/div. VOUT 5V/div. 1V/div. IL 1A/div. IL 1A/div. VLX 5V/div. VLX 5V/div. 400us/div. 400μs/div. Figure 7. Power On through E Waveform IOUT=0A Figure 8. Power On through E Waveform IOUT=2A VEN/SS VOUT 5V/div. 1V/div. VEN/SS VOUT 5V/div. 1V/div. IL 1A/div. IL 1A/div. VLX 5V/div. VLX 5V/div. 20ms/div. Figure 9. Power Off through E FR9881A-Preliminary 0.2-MAR-2016 200us/div. Waveform Figure 10. Power Off through E Waveform 6 FR9881A /a85T Typical Performance Curves (Continued) VIN=12V, VOUT=3.3V, C1=10μF×2, C2=22μF×2, L1=4.7μH, TA=+25°C, unless otherwise noted. IOUT=0A IOUT=2A VIN 5V/div. VIN 5V/div. VOUT VOUT 1V/div. 1V/div. IL 1A/div. IL 1A/div. VLX 5V/div. VLX 5V/div. 10ms/div. 10ms/div. Figure 11. Power On through VIN Waveform Figure 12. Power On through VIN Waveform IOUT=0A IOUT=2A VIN 5V/div. VIN 5V/div. VOUT VOUT 1V/div. IL 1A/div. 1V/div. IL 1A/div. VLX 5V/div. VLX 5V/div. 200ms/div. 200ms/div. Figure 13. Power Off through VIN Waveform Figure 14. Power Off through VIN Waveform IOUT=1A to 2A VOUT 200mV/div. VOUT 1V/div. IL 1A/div. IL 2A/div. 400μs/div. 4ms/div. Figure 15. Load Transient Waveform Figure 16. Short Circuit Test FR9881A-Preliminary 0.2-MAR-2016 7 FR9881A /a85T Typical Performance Curves (Continued) VOUT=3.3V 100 100 90 90 80 80 70 70 Efficiency (%) Efficiency (%) VOUT=1.2V 60 50 40 30 20 60 50 40 30 20 5V to 1.2V 12V to 1.2V 10 0 0.01 0.1 1 Load Current(A) 5V to 3.3V 12V to 3.3V 18V to 3.3V 10 0 0.01 10 Figure 17. Efficiency vs. Load Current 0.1 1 Load Current(A) 10 Figure 18. Efficiency vs. Load Current IOUT=600mA VOUT=5V 100 615 90 610 Feedback Voltage (mV) Efficiency(%) 80 70 60 50 40 30 20 12V to 5V 18V to 5V 10 0 0.01 605 600 595 590 585 0.1 1 Load current(A) 10 Figure 19. Efficiency vs. Load Current -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) Figure 20. Feedback Voltage vs. Temperature IOUT=600mA Switching Frequency (M Hz) 1.2 1.1 1 0.9 0.8 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) Figure 21. Switching Frequency vs. Temperature FR9881A-Preliminary 0.2-MAR-2016 8 FR9881A /a85T Function Description The FR9881A is a high efficiency, internal compensation, and constant frequency current mode step-down synchronous DC/DC converter. It has integrated high-side (150mΩ, typ) and low-side (85mΩ, typ) power switches, and provides 2A load current. It regulates input voltage from 4.5V to 18V, and down to an output voltage as low as 0.6V. Control Loop Under normal operation, the output voltage is sensed by FB pin through a resistive voltage divider and amplified through the error amplifier. The voltage of error amplifier output is compared to the switch current to control the RS latch. At the beginning of each clock cycle, the high-side NMOS turns on when the oscillator sets the RS latch, and turns off when current comparator resets the RS latch. Then the low-side NMOS turns on until the clock period ends. Internal Compensation Function The stability of the feedback circuit is controlled through internal compensation circuits. This internal compensation function is optimized for most applications and this function can reduce external R, C components. Enable/Soft-Start pin includes enable and soft-start function. Enable function provides digital control to turn on/off the converter. When voltage exceeds threshold voltage, soft-start function will start. If voltage is below the shutdown threshold voltage, the converter will turn into the shutdown mode. Soft-start function employs external EN/SS resistor to control soft-start period to reduce input inrush current during start up. Please refer to P.10 for soft-start setting. Over Current Protection The FR9881A over current protection function is implemented using cycle-by-cycle current limit architecture. The inductor current is monitored by measuring the high-side MOSFET series sense resistor voltage. When the load current increases, the inductor current also increases. When the peak inductor current reaches the current limit threshold, the output voltage starts to drop. When the over current condition is removed, the output voltage returns to the regulated value. Short Circuit Protection The FR9881A provides short circuit protection function to prevent the device damage from short condition. When the short condition occurs and the feedback voltage drops lower than 0.3V, the oscillator frequency will be reduced to 150kHz and hiccup mode will be triggered to prevent the inductor current increasing beyond the current limit. Once the short condition is removed, the frequency will return to normal. Over Temperature Protection The FR9881A incorporates an over temperature protection circuit to protect itself from overheating. When the junction temperature exceeds the thermal shutdown threshold temperature, the regulator will be shutdown. And the hysteretic of the over temperature protection is 40°C (typ). Input Under Voltage Lockout When the FR9881A is power on, the internal circuits are held inactive until VIN voltage exceeds the input UVLO threshold voltage. And the regulator will be disabled when VIN is below the input UVLO threshold voltage. The hysteretic of the UVLO comparator is 400mV (typ). FR9881A-Preliminary 0.2-MAR-2016 9 FR9881A /a85T Soft-Start Time Setting Power on(VIN control) R3 t1 5 VIN VIN=VEN/SS VIN=12V 4 EN/SS FR9881A VOUT t2 Power on(EN control) R3 t1 VEN/SS 4 EN/SS VIN 5 VIN=12V VIN VEN/SS FR9881A VOUT t2 Parameters Description Parameters Description Value VIN Input operation voltage 12V VEN/SS Enable & soft-start operation voltage 5V t1 VIN rising time t2 12ms soft-start period depends on R3 VOUT soft-start time resistor adjustment Figure 22. Soft-Start Setting Diagram Soft-start time depends on EN/SS resistor adjustment VIN control R3 (Ω) Soft-Start Time (ms) VEN/SS control 0 0.5 0.5 100k 1 1 500k 5 5 Table 2. Soft-Start Setting FR9881A-Preliminary 0.2-MAR-2016 10 FR9881A /a85T Application Information Output Voltage Setting The output voltage VOUT is set using a resistive divider from the output to FB. The FB pin regulated voltage is 0.6V. Thus the output voltage is: V T =0. V× 1+ R1 R2 A low ESR capacitor is required to keep the noise minimum. Ceramic capacitors are better, but tantalum or low ESR electrolytic capacitors may also suffice. When using tantalum or electrolytic capacitors, a 0.1μF ceramic capacitor should be placed as close to the IC as possible. Output Capacitor Selection Table 3 lists recommended values of R1 and R2 for most used output voltage. Table 3 Recommended Resistance Values VOUT R1 R2 5V 30.9kΩ 4.22kΩ 3.3V 30kΩ 6.65kΩ 2.5V 30.9kΩ 9.76kΩ 1.8V 30.9kΩ 15.4kΩ 1.2V 4.99kΩ 4.99kΩ Place resistors R1 and R2 close to FB pin to prevent stray pickup. The output capacitor is used to keep the DC output voltage and supply the load transient current. When operating in constant current mode, the output ripple is determined by four components: VR PPLE t =VR PPLE C +VR PPLE(E t +VR PPLE L) t +V E R E t t The following figures show the form of the ripple contributions. VRIPPLE(ESR)(t) Input Capacitor Selection The use of the input capacitor is filtering the input voltage ripple and the MOSFETS switching spike voltage. Because the input current to the step-down converter is discontinuous, the input capacitor is required to supply the current to the converter to keep the DC input voltage. The capacitor voltage rating should be 1.25 to 1.5 times greater than the maximum input voltage. The input capacitor ripple current RMS value is calculated as: (RM ) = = V V T× 1.25 IIN(RMS) (A) 0.5 (t) T This function reaches the maximum value at D=0.5 and the equivalent RMS current is equal to IOUT/2. The following diagram is the graphical representation of above equation. 0.75 + VRIPPLE(C) (t) (t) ×1 Where D is the duty cycle of the power MOSFET. 1 + VRIPPLE(ESL) (t) 2A + VNOISE (t) (t) = VRIPPLE(t) 1.5A 1A (t) 0.25 0 10 20 30 40 50 60 70 80 90 D (%) FR9881A-Preliminary 0.2-MAR-2016 11 FR9881A /a85T Application Information (Continued) VR PPLE(E R) = VR PPLE(E L) = VR PPLE(C) = V F T C 1 L V V T E R E L V L+E L F V T C2 L C That will lower ripple current and result in lower output ripple voltage. The Δ L is inductor peak-to-peak ripple current: V L= T V 1 V F T C 1 L The following diagram is an example to graphical represent Δ L equation. 0.9 It is important to use the proper method to eliminate high frequency noise when measuring the output ripple. The figure shows how to locate the probe across the capacitor when measuring output ripple. Removing the scope probe plastic jacket in order to expose the ground at the tip of the probe. It gives a very short connection from the probe ground to the capacitor and eliminating noise. Probe Ground L=3.3μH 0.8 ΔIL (A) 0.7 Low ESR capacitors are preferred to use. Ceramic, tantalum or low ESR electrolytic capacitors can be used depending on the output ripple requirement. When using the ceramic capacitors, the ESL component is usually negligible. L=4.7μH 0.6 0.5 L=6.8μH 0.4 0.3 0.2 5 6 7 8 9 10 11 12 13 14 15 16 17 18 VIN (V) VOUT=3.3V, FOSC=1MHz A good compromise value between size and efficiency is to set the peak-to-peak inductor ripple current Δ L equal to 30% of the maximum load current. But setting the peak-to-peak inductor ripple current Δ L between 20%~50% of the maximum load current is also acceptable. Then the inductance can be calculated with the following equation: L =0.3× L= GND V V V F The output inductor is used for storing energy and filtering output ripple current. But the trade-off condition often happens between maximum energy storage and the physical size of the inductor. The first consideration for selecting the output inductor is to make sure that the inductance is large enough to keep the converter in the continuous current mode. Load Current PEA Inductor Selection T(MA ) V T C T L To guarantee sufficient output current, peak inductor current must be lower than the FR9881A high-side MOSFET current limit. The peak inductor current is as below: Ceramic Capacitor FR9881A-Preliminary 0.2-MAR-2016 T T Where FOSC is the switching frequency, L is the inductance value, VIN is the input voltage, ESR is the equivalent series resistance value of the output capacitor, ESL is the equivalent series inductance value of the output capacitor and the COUT is the output capacitor. VOUT V V = T(MA ) + L 2 IPEAK IOUT(MAX) ∆IL Time 12 FR9881A /a85T Application Information (Continued) Feedforward Capacitor Selection PCB Layout Recommendation Internal compensation function allows users saving time in design and saving cost by reducing the number of external components. The use of a feedforward capacitor C4 in the feedback network is recommended to improve the transient response or higher phase margin. The device’s performance and stability are dramatically affected by PCB layout. It is recommended to follow these general guidelines shown as below: VOUT R1 FR9881A C4 FB R2 1. Place the input capacitors and output capacitors as close to the device as possible. The traces which connect to these capacitors should be as short and wide as possible to minimize parasitic inductance and resistance. 2. Place feedback resistors close to the FB pin. 3. Keep the sensitive signal (FB) away from the switching signal (LX). 4. Multi-layer PCB design is recommended. C4= 1 2 ×FCR × 1 1 1 × + R1 R1 R2 VOUT L1 VIN LX 6 5 4 R3 For optimizing the feedforward capacitor, knowing the cross frequency is the first thing. The cross frequency (or the converter bandwidth) can be determined by using a network analyzer. When getting the cross frequency with no feedforward capacitor identified, the value of feedforward capacitor C4 can be calculated with the following equation: C1 C2 GND C3 1 2 3 R2 R1 C4 Figure 23. Recommended PCB Layout Diagram Where FCROSS is the cross frequency. To reduce transient ripple, the feedforward capacitor value can be increased to push the cross frequency to higher region. Although this can improve transient response, it also decrease phase margin and cause more ringing. In the other hand, if more phase margin is desired, the feedforward capacitor value can be decreased to push the cross frequency to lower region. In general, the feedforward capacitor range is between 10pF to 10nF. External Diode Selection For 5V input applications, it is recommended to add an external boost diode. This helps improving the efficiency. The boost diode can be a low cost one such as 1N4148. D1 1N4148 VIN 5V VIN BST FR9881A C3 LX FR9881A-Preliminary 0.2-MAR-2016 13 FR9881A /a85T Outline Information SOT-23-6 Package (Unit: mm) SYMBOLS UNIT DIMENSION IN MILLIMETER MIN MAX A 0.90 1.45 A1 0.00 0.15 A2 0.90 1.30 B 0.30 0.50 D 2.80 3.00 E E1 2.60 1.50 3.00 1.70 e 0.90 1.00 e1 1.80 2.00 L 0.30 0.60 Note 1：Followed From JEDEC MO-178-C. Carrier dimensions Life Support Policy Fitipower’s products are not authorized for use as critical components in life support devices or other medical systems . FR9881A-Preliminary 0.2-MAR-2016 14