SC2308A

SC2308A SEMTECH | Alldatasheet

Document overview

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Technical content

Features

Input Voltage Range: 2.6V to 20V Boost and SEPIC Topologies Up to 40V Output in Boost Topology Integrated 2.2A/45V Switch 600kHz Constant Switching Frequency Current-Mode Control Eases Compensation Cycle-by-Cycle Current-Limiting Internal Soft-Start Thermal Shutdown Protection Low Shutdown Current (< µA) 8-Pin SO Lead-Free Package Fully WEEE and RoHS Compliant

Applications

Telecommunication Equipment Point of Load DC-DC Converters Portable Devices

Description

The SC2308A is a 600kHz current-mode switching regulator with an integrated low-side 2.2A power transistor. The oper- ating supply voltage of the SC2308A ranges from that of a single Li-ion cell to various PC board power supplies. The internal switch is rated at 45V, making the device suitable for high voltage boost and SEPIC applications. The SC2308A shuts down to less than µA of supply current. The SC2308A uses peak current-mode PWM control for ease of loop compensation and excellent transient response. Cycle-by-cycle current limiting lowers power transistor dissipation. An internal soft-start timer prevents output overshoot and limits the input current during start-up. Thermal shutdown prevents the chip from overheating. Rev. 2.0 4.7µF VIN VOUT GND EN IN SW FB ONOFF SS22 22µF 11.3k 100k 12V, 0.8A L 10µH COMP 200k 470pF SC2308A 22µF C1: Murata GRM31CR61A475K C2,C3: Murata GRM31CR61C226K L: Coilcraft DO3316P103 D1: ON SS22 Figure 1. 5V to 12V Step-Up Converter

Ordering Information

SC2308ASTRT() (2) SO-8 SC2308AEVB Evaluation Board Notes: () Available in tape and reel only. A reel contains 2,500 devices. (2) Available in lead-free package only. Device is WEEE and RoHS compliant and halogen free. SC2308A yyww xxxxx IN FB GND SW EN COMP NC NC Top View θJA = 160°C/W 8-Lead SOIC yyww - Date Code xxxxx - Semtech Lot Number Top View SC2308A

Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. Absolute Maximum Ratings Recommended Operating Conditions Junction Temperature Range………………… -40°C to +05°C Thermal Information Maximum Junction Temperature …………………… +50 °C Storage Temperature Range ……………… -65°C to +50°C Peak IR Reflow Temperature (0s to 30s) …………… +260°C Unless otherwise noted: VIN = VEN = 3V, TJ = -40°C to 05°C. Typical values are at TJ = 25°C. Parameter Symbol Conditions Min Typ Max Units Input Supply Maximum Operating VIN VIN(MAX) 20 V VIN Start Voltage VIN Rising 2.45 2.6 V Shutdown Supply Current VEN = 0 0.0 µA Quiescent Supply Current IQ VFB = .5V (Not Switching) .3 .8 mA Control Loop Feedback Regulation Voltage VREF .20 .22 .24 V VREF Line Regulation VIN = 3V to 20V 0.002 0.005 %/V FB Pin Input Bias Current IFB FB in Regulation -5 -25 nA Error Amplifier Transconductance gm VCOMP = .V, DICOMP = ± 0.5µA 47 µW- Error Amplifier Open-Loop Gain AV 5 dB COMP to Switch Current Gain 4 A/V Soft-Start Soft-Start Time(3) tSS 3 ms

Electrical Characteristics

Notes: () Tested according to JEDEC standard JESD22-A4-B. (2) Calculated from package in still air, mounted to 3” x 4.5” , 4 layer FR4 PCB with thermal vias under the exposed pad per JESD5 standards. Notes: (3) Time taken for the error amplifier soft-start input to rise from 0 to .22V. SC2308A

Electrical Characteristics (continued) SC2308A Parameter Symbol Conditions Min Typ Max Units Oscillator Switching Frequency fSW 500 630 750 kHz Minimum Switch Off-Time tOFF(MIN) 60 ns Minimum Switch On-Time tON(MIN) 200 ns Minimum Duty Cycle DMIN 0 % Maximum Duty Cycle DMAX 87 96 % Power Switch Switch Current Limit(4) ILIM 2.2 2.9 3.7 A Switch Saturation Voltage VCESAT ISW=2.2A 320 480 mV Switch Leakage Current ILK VSW =2V 0. 0.5 µA Enable Pin High Voltage Threshold VIH 2 V Low Voltage Threshold VIL 0.3 V Enable Pin Current IEN VEN =0V 0.0 0. µAVEN =2V 3.3 5. VEN =6V 3 25 Over Temperature Protection Thermal Shutdown Temperature TSHDN Tj rising 60 OC Hysteresis THYST 2 OC Unless otherwise noted: VIN = VEN = 3V, TJ = -40°C to 05°C. Typical values are at TJ = 25°C. Notes: (4) Switch current limit does not vary with duty cycle.

-15 -10 -50 -25 0 25 50 75 100 125 Temperature (oC) Percenatge Variation (%) VIN = 3V Switch Current Limit vs Temperature 2.0 2.2 2.4 2.6 2.8 3.0 -50 -25 0 25 50 75 100 125 Temperature (oC) Current (A) VIN = 3V Error Amplifier Open-Loop Gain vs Temperature -50 -25 0 25 50 75 100 125 Temperature (oC) Gain (dB) VIN = 3V Error Amplifier Transconductance vs Temperature -50 -25 0 25 50 75 100 125 Temperature (oC) Transconductance (P:-1) VIN = 3V Feedback Voltage vs Temperature 1.20 1.21 1.22 1.23 -50 -25 0 25 50 75 100 125 Temperature ( o Voltage (V) VIN = 3V Efficiency vs Load Current VOUT = 5V 100 0 200 400 600 800 1000 1200 Load Current (mA) Efficiency (%) VIN = 3.3V E fficiency vs Load C urrent VOUT = 12V 100 0 200 400 600 800 Load Current (mA) E fficiency (%) V IN = 5V V IN = 3.3V Switc h Saturation Voltage vs Switc h C urre nt 100 200 300 400 500 S witch Current (A) S aturation Voltage (mV) V IN = 3V 105oC -45oC 25oC IN Pin C urre nt v s Switc h Curre nt S witch Current (A) IN P in Current (mA) V IN = 3V 25oC SC2308A

VEN (V) Current (PA) -40oC 25oC 105oC VIN Quiescent Current vs Temperature 0.0 0.5 1.0 1.5 2.0 -50 -25 0 25 50 75 100 125 Temperature (oC) Current (mA) VEN = 3V VIN = 3V Soft-Start Time (1) vs Temperature 2.0 2.5 3.0 3.5 4.0 -50 -25 0 25 50 75 100 125 Temperature (oC) Time (ms) VIN = 3V Typical Characteristics (Cont.) VIN Quiescent Current vs VIN 0.0 0.5 1.0 1.5 2.0 0 5 10 15 20 VIN (V) Current (mA) 25oC 105oC-40oC Minimum VIN vs Temperature 2.0 2.2 2.4 2.6 2.8 -50 -25 0 25 50 75 100 125 Temperature (oC) Input Voltage (V) Notes: () Time taken for the error amplifier soft-start input to rise from 0 to .22V.

to ground compensates the control loop. 2 FB The Inverting Input of the Error Amplifier. Tie to an external resistive divider to set the output voltage. 4 GND Ground. Tie to the ground plane. The converter output capacitor must be closely bypassed to the ground pin. switching voltage spike at this pin should be limited to less than 45V. 6 IN Power Supply Pin. Bypassed with capacitor close to the pin. Figure 2. SC2308A Block Diagram

General Description and Operation The SC2308A is a 600kHz peak current-mode switching reg- ulator with an integrated 2.2A (minimum) low-side power transistor. The voltage reference runs off the input supply and is enabled by applying at least 2V at the EN pin, as shown in the block diagram in Figure 2. The reference also senses VIN and produces a lockout signal “REF NOT READY” . This signal does not go low until there is enough VIN head- room for the reference to achieve regulation (typically VIN = 2.45V). The “REF NOT READY” signal and the temperature sensor control the internal regulator, which powers all of the internal control circuits. The error amplifier EA has two non-inverting inputs. The non-inverting input with the lower voltage predominates. One of the non-inverting inputs is biased to a precision .22V reference and the other non-inverting input is tied to a soft-start timer. Before the internal regulator turns on, the output SS of the soft-start timer is discharged to ground. As the internal regulator turns on, it also releases the timer. The soft-start timer generates a slow rising SS ramp, which is fed into one of the non-inverting inputs of the EA. During power-up, the SS voltage becomes the EA effective non-inverting input voltage. In a boost converter, the part starts switching as V SS exceeds the FB voltage. If the soft-start ramp is sufficiently slow, then the FB voltage (hence the output voltage) will track VSS and there will be no output overshoot during start-up. It takes about 3ms to charge VSS from ground to the nominal feedback voltage. The end of charge VSS is significantly higher than .22V so that it has no effect on the error amplifier. Soft-start also reduces the input start-up current. The clock CLK resets the latch and blanks the power transis- tor Q conduction. Q is switched on at the trailing edge of the clock. The switch current is sensed with an integrated 6.3mW sense resistor. The sensed current summed with the slope-compensating ramp is fed into the modulating ramp input of the PWM comparator. The latch is set and Q conduction is terminated when the modulating ramp intersects the error amplifier output. If the switch current exceeds 2.9A (the typical current limit), then the current- limit comparator ILIM will set the latch and turn off Q. Due to separate pulse-width modulating and current limiting paths, cycle-by-cycle current limiting is not affected by slope compensation. The current-mode switching regulator is a dual-loop feed- back control system, designed to simplify loop compensa- tion. In the inner current loop, the EA output controls the peak inductor current. In the outer loop, the error amplifier regulates the output voltage. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, easing loop compensation. A simple, two-pole, single-zero compensator network con - nected from COMP to ground is adequate to stabilize the converter.

Applications Information (Continued) SC2308A losses in both the switch and the diode, an expression for the maximum available output current of a boost converter can be derived using Equation (5): º ª IN CESATDD OUT INLIM )MAX(OUT V VVDV D1V VII (5) Since switching losses are excluded in the derivation, the actual output current is over-estimated in Equation (5). Nevertheless, this calculation still provides a useful initial approximation. Inductor Selection The inductor must be able to handle the peak current ILIM. First, the inductor should not saturate at I LIM. Second, the inductor needs to have low core loss at the switching fre- quency. Inductors with ferrite cores are preferrable. More- over, the inductor should have low DCR for low copper loss. The inductance can be selected such that the inductor ripple current is between 20% to 40% of its average current for improved efficiency. The inductance can be calculated using Equation (6): SWL CESATIN fI VVDL ˜' (6) The Coilcraft DO336P series and the Sumida CDRH8D38NP series inductors perform well in boost converters. The inductors selected must be suitable for a 750kHz switch - ing frequency. Input Capacitor Selection The input current in a boost converter is the inductor cur- rent, which is continuous with low RMS current ripples. A 2.2mF~4.7mF ceramic capacitor is adequate for most applications. Use X5R or better ceramic capacitors, since they have stable temperature and voltage coefficients. The voltage rating for the input capacitor should exceed the maximum input voltage by 0% to 25%. Murata and TDK are two ceramic capacitor suppliers. Output Capacitor Selection Ceramic and low equivalent series resistance (ESR) tantalum or polymer capacitors can be used for output filtering. In a buck converter, the inductor ripple current flows in the output capacitor, whereas in a boost converter, the output capacitor current is the difference between the rectifying diode current and the output current (Figure 4). This cur - rent is discontinuous with high current amplitudes. For this reason, the output ripple voltage of a boost converter is always higher than that of a buck converter with the same inductor current and the same output capacitor. If tantalum or polymer capacitors are used at the converter output, then the converter output ripple voltage will be primarily determined by the capacitor ESR, due to the rel- atively high ESR of these capacitors. The output voltage ripple is the product of the peak inductor current and the output capacitor ESR. For example, if two Sanyo 6TPG00M (00mF, ESR=70mW) polymer capacitors are used for output filtering, then the output peak-to-peak ripple voltage will be 70mV, assuming a 2A peak inductor current. Tantalum capacitor voltage derating is generally 50%. Multi-layer ceramic capacitors, due to their extremely low ESR (<5mΩ), are particularly well suited for output filtering. It is worth noting that the output ripple voltage resulting from charging and discharging of a 0μF or a 22mF ceramic capacitor is higher than the ripple voltage resulting from the capacitor ESR. The output ripple voltage due to charging and discharging effects is calculated using the following equation: OUTSW OUT OUT Cf DIV ˜ ' (7) X5R and X7R ceramic capacitors are the preferred types. Rectifying Diode For high efficiency, Schottky barrier diodes should be used as rectifying diodes for the SC2308A. These diodes should have an average forward current rating at least equal to the output current. The reverse blocking voltage of the Schottky diode should be derated by 0%-20% for reliability. The Schottky diode used in a 2V output step- up converter should have a reverse voltage rating of at least 5V (20% derating).

IN_EN_OUT_IL_3.3V to 5V@1.1A_EN Start VOUT 2V/div IL1 1A/div VIN 2V/div VEN 2V/div 1ms/div (c) (a) IN_EN_OUT_IL_5V to 12V@10mA_EN Start VOUT 5V/div IL1 0.5A/div VIN 5V/div VEN 2V/div 2ms/div (b) 2ms/div VOUT 5V/div IL1 1A/div VIN 5V/div IN_EN_OUT_IL_5V to 12V@800mA_EN Start VEN 2V/div SC2308A Applications Information (Continued) SS22 and SS24 from ON Semiconductor and 0BQ020 and 0BQ040 from International Rectifier are widely used Schottky diodes. Soft-Start The SC2308A comprises an internal soft-start timer. The output (SS) of the soft-start timer (see Figure 2), which forms the second non-inverting input of the feedback amplifier, is reset to zero before VIN rises above its turn-on threshold. The SS voltage is subsequently charged from zero to the nominal feedback voltage (.22V) in about 3ms. If a step-up converter is enabled by stepping the EN input while connected to a live power supply, then its output voltage will rise linearly from approximately V IN to its set voltage. The current drawn from the input power supply will be less than the switch current limit and there will be no output overshoot during start-up. Figure 5 shows the start-up waveforms of the 5V to 2V step-up converter in Figure and the 3.3V to 5V step-up converter in Figure 0. Notice that the regulator does not switch until the internal SS voltage exceeds the FB voltage. If the input power supply to a step-up converter is turned on with the EN and the IN pins shorted, then the start-up waveforms will depend on the input voltage ramp rate and the output load. The internal 3ms soft-start interval may be insufficient to keep the input start-up current below the switch current limit, especially with heavy loads and slow VIN ramp. Figure 6 shows the start-up waveforms of the step- up converters in Figure and Figure 0 when powering on using the Agilent 6652A DC power supply. Before VIN rises above the input start voltage, there is no switching and the converter output simply follows VIN. When starting into an 800mA constant-current load, the 5V to 2V converter reaches the cycle-by-cycle current limit and the output voltage ramp becomes non-linear {Figure 6(c)}. There is, however, very little output voltage overshoot. Boost Converter Start-Up Waveforms. EN is Stepped with Input Applied. (a) 5V to 12 V Step-Up Regulator (Figure 1), IOUT = 10mA (b) 5V to 12 V Step-Up Regulator, IOUT = 800mA (c) 3.3V to 5V Step-Up Regulator (Figure 10), IOUT = 1.1A Figure 5.

Applications Information (Continued) The power stage also has a right half plane (RHP) zero at: RD1f L S (9) The ESR zero frequency is: OUTC CR2 1f S (0) where RC is the ESR of the output capacitor. R3 and C4 form a zero at: CR2 1f S With the assumption that C 4>>C6 , R3 and C6 also form a pole p3 at frequency: CR2 1f S (2) There is also a low-frequency integrator pole p formed by C4 and the equivalent output resistance of the trans - conductance amplifier. The corresponding bode plots are shown in Figure 8. Bode Magnitude Plots of the Power Stage, the Compensator, and the Overall Loop Gain Figure 8. Power-Stage Compensator Loop Gain fz1 fp3 fp2 fz2 fz3 fp3,4 f Gain (dB) Crossover Frequency, fC The poles p, p2 and the RHP zero z2 all increase phase shift in the loop response. For stable operation, the overall loop gain should cross 0dB with -20dB/decade slope. Due to the presence of the RHP zero, it is suggested that the 0dB crossover frequency should not be more than 3 f 2z A simple two-pole, single-zero compensator network is adequate. The loop is compensated with R3, C4 and C6 from the COMP pin to ground. The compensating zero z pro- vides phase boost beyond p2. In general, the converter will be more stable if the filter pole p2 and the RHP zero z2 are widely separated. The RHP zero moves to low frequency when either the duty cycle D or the output current I OUT increases. It is beneficial to use small inductors and larger output capacitors, especially when stepping up from low VIN to high VOUT. An optional second pole can be placed at the power stage ESR zero to attentuate any high-frequency noise. Thermal Shutdown Thermal shutdown turns off the power switch and the control circuit as the junction temperature exceeds 60°C. Switching resumes when the junction temperature falls by 2°C.

Figure 9. Suggested PCB Layout for the SC2308A loop formed by these components should be minimized. The main power switch is integrated inside the SC2308A.

Outline Drawing — SOIC-8 SEE DETAIL DETAIL AA .050 BSC .236 BSC .010 .150 .189 .154 .193 .012 - 0.25

1.27 BSC

6.00 BSC

3.90 4.90 .157 .197 3.80 4.80 .020 0.31 4.00 5.00 0.51 bxN 2X N/2 TIPS SEATING aaa C E/22X 1 2 N A D bbb C A-B D ccc C e/2 (.041) .004 .008 .028 .016 .007 .049 .004 .053 8° 0° 0.20 0.10 - 8° 0.40 0.17 1.25 0.10 .041 .010 .069 .065 .010 1.35 (1.04) 0.72 1.04 0.25 - 1.75 1.65 0.25 c L (L1) 01 0.25 GAGE PLANE h h 3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. -B- CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). DATUMS AND TO BE DETERMINED AT DATUM PLANE NOTES: 2. -A- -H- SIDE VIEW A B C De H PLANE REFERENCE JEDEC STD MS-012, VARIATION AA.4. N bbb aaa ccc A b D E L h e c DIM MIN MILLIMETERS NOM DIMENSIONS INCHES MIN MAX MAXNOM E Land Pattern – SOIC-8 (.205) (5.20) ZG Y P (C) 3.00.118 1.27.050 0.60.024 2.20.087 7.40.291 X INCHES DIMENSIONS Z P Y X DIM C G MILLIMETERS THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. NOTES: REFERENCE IPC-SM-782A, RLP NO. 300A.2. SC2308A

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