FAN8303 ONSEMI | Alldatasheet
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© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN8303 • Rev. 1.2 FAN8303 — 2 A 23 V Non-Synchronous Step-Down DC/DC Regulator FAN8303
2 A 23 V Non-Synchronous Step-Down DC/DC
Features
2 A Output Current 0.22 Internal Power MOSFET Switch Wide 5 V to 23 V Operating Input Range Output Adjustable from 0.6 to 20 V Stable with Low ESR Output Ceramic Capacitors Up to 90% Efficiency Less than 40 µA Shutdown Current Fixed 370 kHz Frequency Thermal Shutdown with Hysteresis Cycle-by-Cycle Over-Current Protection Available in 8-Pin SOIC Package
Applications
Set-Top Box DSL and Cable Modems Distributed Power Systems Consumer Appliances (DVD) Auxiliary supplies
Description
The FAN8303 is a monolithic, non-synchronous, step- down (buck) regulator with internal power MOSFETs. It achieves 2 A continuous out put current over a wide input supply range with excellent load and line regulation. Current -mode operation provides fast transient response and eases loop stabilization. Fault condition protection includes cycle -by-cycle current limiting and thermal shutdo wn. The regulator draws less than 40 µA shutdown current. FAN8303 requires a minimum number of readily available standard external components. External compensation, enable, and programmable soft-start features allow design optimization and flexibility. Cycle -by-cycle current limit, frequency foldback, and thermal shutdown provide protection against shorted outputs. FAN8303 SW FB BSVIN EN SS GND COMP RC 22k CC 1nF CA OPEN INPUT 5~23V ENABLE SHUTDOWN OUTPUT 2.5V/2A CBS 10nF CSS 10nF 18k 5.6k COUT 22µF CIN 10µF 15µH Figure 1. Typical Application
Ordering Information
Part Number Operating Temperature Range Package Packing Method FAN8303MX -40°C to +85°C 8-SOIC Reel For Fairchild’s definition of “green” Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html.
Figure 2. Functional Block Diagram
Figure 3. Pin Configuration (Top View) High-Side Drive BOOT Voltage. Connect through capacitor (CBS) to SW. capacitor on this pin to VCC when SW is LOW. 10 µF or greater ceramic capacitor. resistive divider across the output. node by connecting a series R-C to ground. startup, leave EN unconnected. be used to set soft-start time.
© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN8303 • Rev. 1.2 4 FAN8303 — 2 A 23 V Non-Synchronous Step-Down DC/DC Regulator Absolute Maximum Ratings Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be operable above the recommended operating conditions and stressing the parts to these levels is not recommended. In addition, extended exposure to stre sses above the recommended operating conditions may affect device reliability. The absolute maximum ratings are stress ratings only. All voltage values, except differential voltages, are given with respect to the network ground terminal. Stresses beyond th ose listed under Absolute Maximum Ratings may cause permanent damage to the device Symbol Parameter Min. Max. Unit VIN Supply Voltage, VIN to GND 25 V VSW Switch Voltage, SW to GND -0.3 VIN+0.3 V VBS Boost Voltage VSW + 6 V VFB Feedback Voltage -0.3 6.0 V VEN Enable Voltage -0.3 6.0 V VCOMP Compensation Voltage -0.3 6.0 V VSS Soft-Start Voltage -0.3 6.0 V ΘJA Thermal Resistance, Junction-Air 105 °C/W ΘJC Thermal Resistance, Junction-Case 40 °C/W TJ Operating Junction Temperature -40 +125 °C TL Lead Temperature (Soldering, 5 Seconds) +260 °C TSTG Storage Temperature Range -65 +150 °C ESD Electrostatic Discharge Protection Level Human Body Model, JEDEC JESD22-A114 3.0 kV Charged Device Model, JEDEC JESD22-C101 2.5 Recommended Operating Conditions The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended operating conditions are spec ified to ensure optimal performance to the datasheet specifications. Fairchild does not recommend exceeding them or designing to absolute maximum ratings. Symbol Parameter Min Max. Unit VIN Supply Voltage 5 23 V TA Operating Ambient Temperature -40 +85 C
© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN8303 • Rev. 1.2 5 FAN8303 — 2 A 23 V Non-Synchronous Step-Down DC/DC Regulator
Electrical Characteristics
VIN=12 V, TA= -40 to +85C, unless otherwise noted. Symbol Parameter Condition Min. Typ. Max. Unit VFB Feedback Voltage 25°C, 5V<VIN<23 V 0.58 0.60 0.62 V RON_H Upper Switch On Resistance 0.22 RON_L Lower Switch On Resistance 4 ILKG Upper Switch Leakage Current VEN=0 V,VSW=0 V 0 10 µA IPK Peak Inductor Current 3.5 A fOSC Oscillator Frequency VFB>0.3 V 315 370 435 kHz VUVLO Under-Voltage Lockout Rising VIN 4.2 4.6 5.0 V fSHORT Short Circuit Frequency VFB<0.3 V 25 45 55 kHz DMAX Maximum Duty Cycle 90 % TON_MIN Minimum On Time 210 ns VEN Enable Threshold 1.2 1.6 2.0 V VEN_H Enable Threshold Hysteresis 150 mV IOFF Supply Current (Shutdown) VEN=0 V 10 40 µA IQ Supply Current (Quiescent) VEN>1.6 V; VFB=0.8 V 1.0 2.0 mA GCS Current Sense Gain 2 A/V GEA Error Amplifier Transconductance 380 µA/V AVEA Error Amplifier Voltage Gain 400 V/V ISS Soft-Start Current 6 µA TSD Thermal Shutdown Temperature 155 °C
© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN8303 • Rev. 1.2 8 FAN8303 — 2 A 23 V Non-Synchronous Step-Down DC/DC Regulator Functional Description The FAN8303 is a monolithic, non-synchronous, current-mode, step-down regulator with internal power MOSFETs. It achieves 2 A continuous output current over a wide input supply range from 5 V to 23 V with excellent load and line regulation. The output voltage can be regulated as low as 0.6 V. The FAN8303 uses current-mode operation that provides fast transient response and eases loop stabilization. The FAN8303 requires a minimum number of readily avai lable standard external components. Current Mode PWM Control Loop FAN8303 uses current -mode PWM control scheme. The peak inductor current is modulated in each switching cycle by an internal op-amp output signal to achieve the output voltage regulat ion. An internal slope compensation circuit is included to avoid sub -harmonic oscillation at duty cycle greater than 50%. Current - mode control provides cycle -by-cycle current limit protection and superior regulation control loop response compared to the traditional voltage-mode control. In normal operation, the high-side MOSFET is turned on at the beginning of each switching cycle , which causes the current in the inductor to build up. The current - control loop senses the inductor current by sensing the voltage across the high -side senseFET during on time. The output of the current -sense amplifier is summed with the slope compensation signal and the combined signal is compared with the error amplifier output to generate the PWM signal. As the inductor cur rent ramps up to the controlled value, the high -side MOSFET is turned off and the inductor current reaches zero through a freewheeling diode . In light -load condition, the high -side switch may be kept off for several cycles to improve efficiency. Short-Circuit Protection The FAN8303 protects output short circuit by switching frequency fold-back. The oscillator frequency is reduced to about 4 5 kHz when the output is short ed to ground. This frequency fold -back allows the inductor current more time to decay to prevent potential r un-away condition. The oscillator frequency switch es to 370 kHz as VOUT rises gradually from 0V back to regulated level. Slope Compensation and Inductor Peak Current The slope compensation provides stability in constant frequency architecture by preventing sub -harmonic oscillations at high duty cycles. It is accomplished internally by adding a compen sating ramp to the inductor current signal at duty cycles in excess of 50%. Maximum Load Current at Low VIN The FAN8303 is able to operate with input supply voltage as low as 5 V, although the maximum allowable output current is reduced a s a funct ion of duty cycle (see Figure 15). Additionally, at this low input voltage ; if the duty cycle is greater than 50%, slope compensation reduces allowable output current. Inductor Selection A higher inductor value lower s ripple current. The inductor value can be calculated as: IN OUT LS OUT V V If VL 1 (1) where: fs is the switching frequency; VOUT is the output voltage; VIN is the input supply voltage; and IL Is the inductor ripple current. Considering worst case, the equation is changed to: MAXIN OUT MAXLS OUT V V If VL (2) Input Capacitor Selection To prevent high -frequency switching current passing to the input, the input capacitor impedance at the switching frequency must be less than input source impedance. High -value, small, inexpensive, lower -ESR ceramic capacitors are recommended. 10 µF ceramic capacitors should be adequate for 2 A applications. Output Capacitor Selection A larger output capacitor value keeps the output ripple voltage smaller. The formula of output ripple ΔVOUT is: SOUT LOUT fC ESRIV (3) where C OUT is the output capacitor and ESR is the equivalent series resistance of the output capacitor. Output Voltage Programming The output voltage is set by a resist or divider, according to the following equation: 3 216.0 R RVOUT (4) Freewheeling Diode An output freewheeling diode carries load current when the high -side switch is turn ed off. Therefore, use a Schottky diode to reduce loss due to diode forward voltage and recovery time. The diode should have at least 2 A current rating and a reverse blocking voltage greater than the maximum input voltage. The diode should be close to the SW node to keep traces short and reduce ringing.
GND helps control the rate of rise on the output voltage. to the SS pin, causing the voltage to rise. and fast transient , while maintaining loop stability. one-pole and one-zero system. where RL is the load resistor value (VOUT/IOUT). the COMP pin to set the pole and zero. CC is the compensation capacitor. where RC is compensation resistor. in instability if not properly compensated. by the voltage at COMP (2 A/V). crossover frequency provides sufficient phase margin. (CA) to set the pole f P3 at the location of the ESR zero. Figure 16. Block Diagram of Compensation
Assume the V IN voltage is 12 V with a 10% tolerance. A 15 µH inductor is chosen for this application. If RC=22.72 kΩ, choose 22 kΩ for the design. CC= 0.965 nF, choose 1 nF for the design. Table 1. Recommended Compensation Values
1.8 V 10 µH
3.3 V 15 µH 4 kΩ 27 kΩ 820 pF
close as possible to the IC terminals. wide land areas with appropriate thermal vias. Figure 17. Recommended PCB Layout The table below pertains to Marketing outline drawing on the following page.
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