SC4502/SC4502H 1.4Amp, 2MHz Step-Up Switching Regulator with Soft-Start POWER MANAGEMENT Description Features Low saturation voltage switch: 210mV (250mV for the SC4502H) Constant switching frequency current-mode control Programmable switching frequency up to 2MHz Soft-Start function Input voltage ranges from 1.4V to 16V Output voltage up to 32V (40V for the SC4502H) Low shutdown current Adjustable undervoltage lockout threshold Small low-profile thermally enhanced lead free package. This product is fully WEEE and RoHS compliant. The SC4502/SC4502H is a high-frequency current-mode step-up switching regulator with an integrated 1.4A power transistor. Its high switching frequency (programmable up to 2MHz) allows the use of tiny surface-mount external passive components. Programmable soft-start eliminates high inrush current during start-up. The internal switch is rated at 32V (40V for the SC4502H) making the converter suitable for high voltage applications such as Boost, SEPIC and Flyback. The operating frequency of the SC4502/SC4502H can be set with an external resistor. The ability to set the operating frequency gives the SC4502/SC4502H design flexibilities. A dedicated COMP pin allows optimization of the loop response. The SC4502/SC4502H is available in thermally enhanced 10-pin MLPD package. Applications Typical Application Circuit D1 L1 5V VOUT 12V 10BQ015 8 OFF ON C1 2.2µF 3 6,7 IN SW SHDN FB SS COMP GND C3 47nF 4,5 95 2 SC4502 10 Efficiency R1 866K 1 ROSC 90 C2 10µF R3 9 C6 R4 10.5µH, 700KHz 5.3µH, 1.4MHz 85 R2 100K Efficiency (%) VIN Flat screen LCD bias supplies TFT bias supplies XDSL power supplies Medical equipment Digital video cameras Portables devices White LED power supplies C4 All Capacitors are Ceramic. 80 75 3.3µH, 2MHz 70 65 60 VIN = 5V VOUT = 12V 55 50 f (MHz) R3 (KΩ ) R4 (KΩ ) C4 (pF) C6 (pF) L1 (µH) 0.7 33.2 23.7 1500 - 10.5 (Falco D08019) 1.4 59.0 9.53 560 - 5.3 (Sumida CDRH5D28) 2 73.2 5.36 330 22 3.3 (Coilcraf t DO1813P) 0.0 0.1 0.2 0.3 0.4 0.5 Load Current (A) Figure 1(b). Efficiencies of 5V to 12V Boost Converters at 700KHz, 1.4MHz and 2MHz. Figure 1(a). 5V to 12V Boost Converter. Revision: July 25, 2005 1 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Absolute Maximum Rating Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied. Parameter Symbol Typ Units Supply Voltage VIN -0.3 to 16 V SW Voltage V SW -0.3 to 32 V SW Voltage (SC4502H) V SW -0.3 to 40 V FB Voltages V FB -0.3 to 2.5 V VSHDN -0.3 to VIN + 1 V Operating Temperature Range TA -40 to +85 °C Thermal Resistance Junction to Ambient (MLPD-10) θJ A 40 °C/W Maximum Junction Temperature TJ 160 °C Storage Temperature Range TSTG -65 to +150 °C Lead Temperature (Soldering)10 sec TLEAD 260 °C ESD Rating (Human Body Model) ESD 2000 V SHDN Voltage Electrical Electrical Characteristics Characteristics Unless other specified: VIN = 2V, SHDN = 1.5V, ROSC = 7.68kΩ, -40°C < TA = TJ < 85°C Parameter Test Conditions Min Minimum Operating Voltage Typ Max Unit 1.3 1.4 V 16 V 1.260 V 1.267 V Maximum Operating Voltage Feedback Voltage Feedback Voltage Line Regulation TA = 25°C 1.224 -40°C < TA < 85°C 1.217 1.5V < VIN < 16V 1.242 0.01 % FB Pin Bias Current 40 80 nA Error Amplifier Transconductance 60 µΩ−1 Error Amplifier Open-Loop Gain 49 dB COMP Source Current VFB = 1.1V 5 µA COMP Sink Current VFB = 1.4V 5 µA VSHDN = 1.5V, VCOMP = 0 ( Not Switching ) 1.1 1.6 mA VSHDN = 0 10 18 µA 1.7 MHz VIN Quiescent Supply Current VIN Supply Current in Shutdown Switching Frequency 1.3 1.5 Maximum Duty Cycle 85 90 Minimum Duty Cycle % 0 Switch Current Limit 1.4 2 % A Switch Saturation Voltage ISW = 1.3A 210 340 mV Switch Saturation Voltage (SC4502H) ISW = 1.3A 250 390 mV 2005 Semtech Corp. 2 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Electrical Characteristics (Cont.) Unless other specified: VIN = 2V, SHDN = 1.5V, ROSC = 7.68kΩ, -40°C < TA = TJ < 85°C Parameter Test C onditions Swi tch Leakage C urrent Min V S W = 5V Shutdown Threshold Voltage 1.02 Typ Max U nit 0.01 1 mA 1.1 1.18 V µA VSHDN = 1.2V -4.6 VSHDN = 0 0 VSS = 0.3V 1.5 µA Thermal Shutdown Temperature 160 °C Thermal Shutdown Hysteresi s 10 °C Shutdown Pi n C urrent Soft-Start C hargi ng C urrent Pin Configurations µA Ordering Information TOP VIEW Device(1)(2) P ackag e Temp. Range( TA) SC4502MLTRT MLPD-10 -40 to 85°C SC4502HMLTRT MLPD-10 -40 to 85°C S C 4502E V B Evaluation Board SC4502HEVB Evaluation Board Notes: (1) Only available in tape and reel packaging. A reel contains 3000 devices for MLP package. (2) Lead free product. This product is fully WEEE and RoHS compliant. (10 Pin - MLPD, 3 x 3mm) 2005 Semtech Corp. 0.1 3 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Pin Descriptions Pin Pin Name Pin Function 1 COMP The output of the internal transconductance error amplifier. This pin is used for loop compensation. 2 FB The inverting input of the error amplifier. Tie to an external resistive divider to set the output voltage. 3 SHDN Shutdown Pin. The accurate 1.1V shutdown threshold and the 4.6uA shutdown pin current hysteresis allow the user to set the undervoltage lockout threshold and hysteresis for the switching regulator. Pulling this pin below 0.1V causes the converter to shut down to low quiescent current. Tie this pin to IN if the UVLO and the shutdown features are not used. This pin should not be left floating. 4,5 GND Ground. Tie both pins to the ground plane. Pins 4 and 5 are not internally connected. 6,7 SW Collector of the internal power transistor. Connect to the boost inductor and the rectifying diode. 8 IN 9 ROSC 10 SS Power Supply Pin. Bypassed with capacitors close to the pin. A resistor from this pin to the ground sets the switching frequency. Soft-Start Pin. A capacitor from this pin to the ground lengthens the start-up time and reduces startup current. Exposed Pad The exposed pad must be soldered to the ground plane on the PCB for good thermal conduction. Block Diagram IN 8 SW SW 6 7 4.6µA SHDN 3 + - INTERNAL SUPPLY CMP 1.1V VOLTAGE REFERENCE FB 2 1.242V COMP 1 + - REG ENABL E THERMAL SHUTDOWN CLK EA - R + S PWM REG Q 1.5µA SS 10 + ILIM - I-LIMIT REG_GOOD RSENSE ENABL E Σ ROSC 9 CLK OSCILLATOR + + SLOPE COMP + ISEN 4 5 GND GND Figure 2. SC4502/SC4502H Block Diagram. 2005 Semtech Corp. 4 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Typical Characteristics Feedback Voltage vs Temperature Switching Frequency vs Temperature ROSC vs Switching Frequency 1.3 1.7 100 1.2 25ºC 10 1.6 VIN = 12V 1.5 VIN = 2V 1.4 1.15 1.3 1 -50 -25 0 25 50 75 100 125 0.0 0.5 1.0 Switch Saturation Voltage vs Switch Current 500 1.5 2.0 2.5 -50 3.0 25 50 75 125 Switch Saturation Voltage vs Switch Current Minimum VIN vs Temperature 1.5 400 85ºC 200 100 1.4 Input Voltage (V) V CESAT (mV) 300 25ºC 300 85ºC 200 25ºC 100 0 0 1 1.5 2 1.3 1.2 1.1 1 0 0.5 Switch Current (A) 1 1.5 2 -50 -25 Switch Current (A) 1.3 0 25 50 75 100 125 Temperature (ºC) VIN Current in Shutdown vs Input Voltage VIN Quiescent Current vs Temperature Shutdown Threshold vs Temperature 50 1.20 Not Switching VIN = 2V 1.2 VIN = 16V 1.1 1 VIN = 2V 0.9 Shutdown Threshold (V) 40 VIN Current (µA) VIN Current (mA) 100 SC4502H 400 0.5 0 Temperature (ºC) 500 SC4502 0 -25 Frequency (MHz) Temperature (ºC) VCESAT (mV) Frequency (MHz) VIN = 2V 1.25 ROSC (KΩ ) Feedback Voltage (V) ROSC = 7.68KΩ -40ºC 30 125ºC 20 10 1.15 1.10 1.05 VSHDN = 0 0.8 0 -50 -25 0 25 50 75 Temperature (ºC) 2005 Semtech Corp. 100 125 1.00 0 5 10 Input Voltage (V) 5 15 20 -50 -25 0 25 50 75 100 125 Temperature (ºC) www.semtech.com SC4502/SC4502H POWER MANAGEMENT Typical Characteristics VIN Current vs SHDN Pin Voltage 1.2 -3 0.1 VIN = 2V VIN = 2V VSHDN = 1.25V 125ºC 0.8 0.6 0.4 125ºC -40ºC 0.2 25ºC 0.06 0.04 Current (µA) 0.08 VIN Current (mA) VIN Current (mA) 1 -4 VIN = 2V -5 VIN = 12V 0.02 -40ºC 0 0 0 0.5 1 1.5 -6 0 0.2 SHDN Voltage (V) 0.4 0.6 0.8 1 1.2 -50 1.4 1.2 1 70 60 50 75 Temperature (ºC) 2005 Semtech Corp. 100 100 125 125 2.2 2 1.8 40 1.6 30 50 75 2.4 Current Limit (A) -1 Transconductance (µΩ ) 1.6 25 50 VIN = 2V 1.8 0 25 Switch Current Limit vs Temperature 80 V SS = 0.3V -25 0 Temperature (ºC) Transconductance vs Temperature 2 -50 -25 SHDN Voltage (V) Soft-Start Charging Current vs Temperature Current (µA) Shutdown Pin Current vs Temperature VIN Current vs SHDN Pin Voltage -50 -25 0 25 50 75 Temperature (ºC) 6 100 125 -50 -25 0 25 50 75 100 Temperature (ºC) www.semtech.com SC4502/SC4502H POWER MANAGEMENT Operation Applications Information The SC4502/SC4502H is a programmable constantfrequency peak current-mode step-up switching regulator with an integrated power transistor. As shown in the block diagram in Figure 2, the power transistor is turned on at the trailing edge of the clock. Switch current is sensed with an integrated sense resistor. The sensed current signal is summed with the slope-compensating ramp before compared to the output of the error amplifier EA. The PWM comparator trip point determines the switch turn-on pulse width. The current-limit comparator ILIM turns off the power switch when the switch current exceeds the 2A current-limit threshold. ILIM therefore provides cycle-by-cycle current limit. Current-limit is not affected by slope compensation because the current limit comparator ILIM is not in the PWM signal path. Setting the Output Voltage An external resistive divider R1 and R2 with its center tap tied to the FB pin (Figure 3) sets the output voltage. V R1 = R 2 OUT − 1 1.242V VOUT SC4502/SC4502H R1 40nA 2 FB R2 Figure 3. The Output Voltage is set with a Resistive Divider Current-mode switching regulators utiilize a dual-loop feedback control system. In the SC4502/SC4502H the amplifier output COMP controls the peak inductor current. This is the inner current loop. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, easing loop compensation. Fast transient response can be obtained with a simple Type-2 compensation network. In the outer loop, the error amplifier regulates the output voltage. The input bias current of the error amplifier will introduce an error of: ∆VOUT 40nA ⋅ (R1//R 2 )⋅ 100 = % VOUT 1.242V (2) The percentage error of a VOUT = 5V converter with R1 = 100KΩ and R2 = 301KΩ is The switching frequency of the SC4502/SC4502H can be programmed up to 2MHz with an external resistor from the ROSC pin to the ground. For converters requiring extremely low or high duty cycles, the operating frequency can be lowered to maintain the necessary minimum on time or the minimum off time. ∆VOUT 40nA ⋅ (100 K Ω // 301K Ω ) ⋅ 100 = = 0.24% VOUT 1.242V Operating Frequency and Efficiency Switching frequency of SC4502/SC4502H is set with an external resistor from the ROSC pin to the ground. A graph showing the relationship between R OSC and switching frequency is given in the “Typical Characteristics”. The SC4502/SC4502H requires a minimum input of 1.4V to operate. A voltage higher than 1.1V at the shutdown pin enables the internal linear regulator REG in the SC4502/SC4502H. After VREG becomes valid, the softstart capacitor is charged with a 1.5µA current source. A PNP transistor clamps the output of the error amplifier as the soft-start capacitor voltage rises. Since the COMP voltage controls the peak inductor current, the inductor current is ramped gradually during soft-start, preventing high input start-up current. Under fault conditions (VIN<1.4V or over temperature) or when the shutdown pin is pulled below 1.1V, the soft-start capacitor is discharged to ground. Pulling the shutdown pin below 0.1V reduces the total supply current to 10µA. 2005 Semtech Corp. (1) High frequency operation reduces the size of passive components but switching losses are higher. The efficiencies of 5V to 12V converters operating at 700KHz, 1.4MHz and 2MHz are plotted in Figure 1(b) for SC4502. Duty Cycle The duty cycle D of a boost converter in continuous conduction mode is: 7 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information It is worth noting that IOUTMAX is directly proportional to the VIN VOUT + VD D= VCESAT 1− VOUT + VD 1− (3) VIN ratio of V . Equation (4) over-estimates the maximum OUT where VCESAT is the switch saturation voltage and VD is the voltage drop across the rectifying diode. output current at high frequencies (>1MHz) since switching losses are neglected in its derivation. Nevertheless it is a useful first-order approximation. Using VCESAT = 0.3V, VD = 0.5V and ILIM = 1.4A in (3) and (4), the maximum output currents for three VIN and VOUT combinations are shown in Table 1. Maximum Output Current In a boost switching regulator the inductor is connected to the input. The DC inductor current is the input current. When the power switch is turned on, the inductor current flows through the switch. When the power switch is off, the inductor current flows through the rectifying diode to the output. The maximum output current is the average diode current. The diode current waveform is trapezoidal with pulse width (1 – D)T (Figure 4). The output current available from a boost converter therefore depends on the converter operating duty cycle. The power switch current in the SC4502/SC4502H is internally limited to 2A. This is also the maximum inductor or the input current. By estimating the conduction losses in both the switch and the rectifying diode, an expression of the maximum available output current of a boost converter can be derived as follows: IOUTMAX = ILIM VIN D VD − D(VD − VCESAT ) − 1 − VOUT 45 VIN (4) Switch Current (1-D)T OFF ON IOUT ON OFF 12 0.820 0.25 3.3 5 0.423 0.80 5 12 0.615 0.53 ON DMIN Figure 4. Current Waveforms in a Boost Regulator 2005 Semtech Corp. 2.5 Example: Determine the maximum operating frequency of a Li-ion cell to 5V converter using the SC4502. Assuming that VD=0.5V, VCESAT=0.3V and VIN=2.6V - 4.2V, the minimum duty ratio can be found using (3). Diode Current DT IOUTMAX ( A ) The operating duty cycle of a boost converter decreases as VIN approaches VOUT. The PWM modulating ramp in a current-mode switching regulator is the sensed current signal. This current ramp is absent unless the switch is turned on. The intersection of this ramp with the output of the voltage feedback error amplifier determines the switch pulse width. The propagation delay time required to immediately turn off the switch after it is turned on is the minimum switch on time. Regulator closed-loop measurement shows that the SC4502/SC4502H has a minimum on time of about 150ns at room temperature. The power switch in the SC4502/SC4502H is either not turned on at all or on for at least 150ns. If the required switch on time is shorter than the minimum on time, the regulator will either skip cycles or it will start to jitter. Inductor Current ON D Considerations for High Frequency Operation IIN OFF VOUT ( V ) Table 1. Calculated Maximum Output Current [ Equation (4)] where ILIM is the switch current limit. ON VIN ( V ) 8 4.2 5 + 0.5 = 0.25 = 0.3 1− 5 + 0.5 1− www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information operating in continuous-conduction mode is The absolute maximum operating frequency of the D⋅ (VIN − VCESAT) (5) f ⋅L where f is the switching frequency and L is the inductance. DMIN 0.25 = = 1.67MHz . The 150ns 150ns actual operating frequency needs to be lower to allow for modulating headroom. ∆IL = converter is therefore Substituting (3) into (5) and neglecting VCESAT , The power transistor inside the SC4502/SC4502H is turned off every switching cycle for an interval determined by the discharge time of the oscillator ramp plus the propagation delay of the power switch. This minimum off time limits the maximum duty cycle of the regulator at a given switching frequency. A boost converter with high ∆IL = VIN VIN 1 − f ⋅ L VOUT + VD (6) In peak current-mode control, the slope of the modulating (sensed switch current) ramp should be steep enough to lessen jittery tendency but not so steep that large flux swing decreases efficiency. Inductor ripple current DIL between 25%-40% of the peak inductor current limit is a good compromise. Inductors so chosen are optimized in size and DCR. Setting ∆IL = 0.3•(1.4A) = 0.42A, VD=0.5V in (6), VOUT VIn ratio requires long switch on time and high duty cycle. If the required duty cycle is higher than the attainable maximum, the converter will operate in dropout. (Dropout is the condition in which the regulator cannot attain its set output voltage below current limit.) L= The minimum off times of closed-loop boost converters set to various output voltages were measured by lowering their input voltages until dropout occurs. It was found that the minimum off time of the SC4502/SC4502H ranged from 80ns to 110ns at room temperature. VIN f ⋅ ∆IL VIN VIN VIN 1 − = 1 − (7) VOUT + VD 0.42A ⋅ f VOUT + 0.5V where L is in µH and f is in MHz. Equation (6) shows that for a given VOUT, ∆IL is the highest (VOUT + VD ) Beware of dropout while operating at very low input voltages (1.5V-2V) with off time approaching 110ns. Shorten the PCB trace between the power source and the device input pin, as line drop may be a significant percentage of the input voltage. A regulator in dropout may appear as if it is in current limit. The cycle-by-cycle current limit of the SC4502/SC4502H is duty-cycle and input voltage invariant and is typically 2A. If the switch current limit is not at least 1.4A, then the converter is likely in dropout. The switching frequency should then be lowered to improve controllability. when VIN = Both the minimum on time and the minimum off time reduce control range of the PWM regulator. Bench measurement showed that reduced modulating range started to be a problem at frequencies over 2MHz. Although the oscillator is capable of running well above 2MHz, controllability limits the maximum operating frequency. The input current in a boost converter is the inductor current, which is continuous with low RMS current ripples. A 2.2µF-4.7µF ceramic input capacitor is adequate for most applications. Inductor Selection Both ceramic and low ESR tantalum capacitors can be used as output filtering capacitors. Multi-layer ceramic capacitors, due to their extremely low ESR (<5mΩ), are the best choice. Use ceramic capacitors with stable . If VIN varies over a wide range, then 2 choose L based on the nominal input voltage. The saturation current of the inductor should be 20%30% higher than the peak current limit (2A). Low-cost powder iron cores are not suitable for high-frequency switching power supplies due to their high core losses. Inductors with ferrite cores should be used. Input Capacitor Output Capacitor The inductor ripple current ∆I L of a boost converter 2005 Semtech Corp. 9 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information temperature and voltage characteristics. One may be tempted to use Z5U and Y5V ceramic capacitors for output filtering because of their high capacitance and small sizes. However these types of capacitors have high temperature and high voltage coefficients. For example, the capacitance of a Z5U capacitor can drop below 60% of its room temperature value at –25°C and 90°C. X5R ceramic capacitors, which have stable temperature and voltage coefficients, are the preferred type. The diode current waveform in Figure 4 is discontinuous with high ripple-content. In a buck converter, the inductor ripple current ∆IL determines the output ripple voltage. The output ripple voltage of a boost regulator is however much higher and is determined by the absolute value of the inductor current. Decreasing the inductor ripple current does not appreciably reduce the output ripple voltage. The current flowing in the output filter capacitor is the difference between the diode current and the output current. This capacitor current has a RMS value of: IOUT VOUT −1 VIN (8) If a tantalum capacitor is used, then its ripple current rating in addition to its ESR will need to be considered. When the switch is turned on, the output capacitor supplies the load current IOUT (Figure 4). The output ripple voltage due to charging and discharging of the output capacitor is therefore: ∆VOUT = IOUT ⋅ D ⋅ T COUT (9) For most applications, a 10µF - 22µF ceramic capacitor is sufficient for output filtering. It is worth noting that the output ripple voltage due to discharging of a 10µF ceramic capacitor (9) is higher than that due to its ESR. Rectifying Diode For high efficiency, Schottky barrier diodes should be used as rectifying diodes for the SC4502/SC4502H. These diodes should have a RMS current rating between 0.5A and 1A with a reverse blocking voltage of at least a few 2005 Semtech Corp. Volts higher than the output voltage. For switching regulators operating at low duty cycles (i.e. low output voltage to input voltage conversion ratios), it is beneficial to use rectifying diodes with somewhat higher RMS current ratings (thus lower forward voltages). This is because the diode conduction interval is much longer than that of the transistor. Converter efficiency will be improved if the voltage drop across the diode is lower. The rectifying diodes should be placed close to the SW pins of the SC4502/SC4502H to minimize ringing due to trace inductance. Surface-mount equivalents of 1N5817, 1N5819, MBRM120, MBR0520 (ON Semi) and 10BQ015, 10BQ040 (IRF) are all suitable. Soft-Start Soft-start prevents a DC-DC converter from drawing excessive current (equal to the switch current limit) from the power source during start up. If the soft-start time is made sufficiently long, then the output will enter regulation without overshoot. An external capacitor from the SS pin to the ground and an internal 1.5µA charging current source set the soft-start time. The soft-start voltage ramp at the SS pin clamps the error amplifier output. During regulator start-up, COMP voltage follows the SS voltage. The converter starts to switch when its COMP voltage exceeds 0.7V. The peak inductor current is gradually increased until the converter output comes into regulation. If the shutdown pin is forced below 1.1V or if a fault situation is detected, then the soft-start capacitor will be discharged to ground immediately. The SS pin can be left open if soft-start is not required. Shutdown The input voltage and shutdown pin voltage must be greater than 1.4V and 1.1V respectively to enable the SC4502/ SC4502H. Forcing the shutdown pin below 1.1V stops the SC4502/SC4502H from switching. Pulling this pin below 0.1V completely shuts off the SC4502/SC4502H. The total VIN shutdown current decreases to 10µA at 2V. Figure 5 shows several ways of interfacing the control logic to the shutdown pin. Beware that the shutdown pin is a high impedance pin. It should always be driven from a low-impedance source or tied to a resistive divider. Floating the shutdown pin will result in undefined voltage. In Figure 5(c) the shutdown pin is driven from a logic 10 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information IN IN SC4502 SC4502H SC4502 SC4502H SHDN SHDN (a) VIN (b) IN IN SC4502 SC4502H 1N4148 SC4502 SC4502H SHDN SHDN (c) (d) Figure 5. Methods of Driving the Shutdown Pin (a) Directly Driven from a Logic Gate (b) Driven from an Open-drain N-channel MOSFET or an Open-Collector NPN Transistor (VOL < 0.1V) (c) Driven from a Logic Gate with VOH > VIN (d) Combining Shutdown with Programmed UVLO (See Section Below). gate whose VOH is higher than the supply voltage of the SC4502/SC4502H. The diode clamps the maximum shutdown pin voltage to one diode voltage above the input power supply. Programming Undervoltage Lockout The SC4502/SC4502H has an internal VIN undervoltage lockout (UVLO) threshold of 1.4V. The transition from idle to switching is abrupt but there is no hysteresis. If the input voltage ramp rate is slow and the input bypass is limited, then sudden turn on of the power transistor will cause a dip in the line voltage. Switching will stop if VIN falls below the internal UVLO threshold. The resulting output voltage rise may be non-monotonic. The 1.1V disable threshold of the SC4502/SC4502H can be used 2005 Semtech Corp. in conjunction with a resistive voltage divider to raise the UVLO threshold and to add an UVLO hysteresis. Figure 6 shows the scheme. Both VH and VL (the desired upper and the lower UVLO threshold voltages) are determined by the 1.1V threshold crossings, VH and VL are therefore: R VH = 1 + 3 ⋅ (1.1V ) R4 VL = VH − VHYS = VH − IHYSR3 (10) Re-arranging, R3 = R4 = 11 VHYS IHYS R3 VH −1 1.1V (11) (12) www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information VL = VH − VHYS = 2.75V − 0.69V = 2.06V > 1.4V . IN Frequency Compensation 6/8 Figure 7 shows the equivalent circuit of a boost converter using the SC4502/SC4502H. The output filter capacitor and the load form an output pole at frequency: I HYS 4.6 µA R3 SWITCH CLOSED WHEN Y = “1” ωp2 = SHDN 3 (13) + where C2 is the output capacitance and ROUT = Y - 1.1V R4 2 ⋅ IOUT 2 = VOUT ⋅ C2 ROUT ⋅ C2 COMPARATOR VOUT is IOUT the equivalent load resistance. SC4502/SC4502H The zero formed by C2 and its equivalent series resistance (ESR) is neglected due to low ESR of the ceramic output capacitor. Figure 6. Programmable Hysteretic UVLO Circuit There is also a right half plane (RHP) zero with angular frequency: with VL > 1.4 V . ROUT ⋅ (1 − D) L 2 Example: Increase the turn on voltage of a VIN = 3.3V boost converter from 1.4V to 2.75V. ωZ2 = ωz2 decreases with increasing duty cycle D and increasing IOUT. Using the 5V to 12V boost regulator (1.4MHz) in Figure 1(a) as an example, Using VH = 2.75V and R4 = 100KΩ in (12), R3 = 150K Ω . The resulting UVLO hysteresis is: R OUT ≥ VHYS = IHYSR3 = 4.6µA • 150K Ω = 0.69V 5V = 10Ω 0.5A The turn off voltage is: I V IN (14) OUT POWER STAGE VOUT C5 R1 ESR R OUT C2 COMP Gm - FB + R3 RO C6 C4 1.242V R2 VOLTAGE REFERENCE Figure 7. Simplified Block Diagram of a Boost Converter 2005 Semtech Corp. 12 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information ωp1 = 5 12 + 0.5 = 0.62 D= 0.3 1− 12 + 0.5 1− = 380 rads −1 ⇒ 60Hz C4 and R3 also forms a zero with angular frequency: Therefore ωp2 ≤ ωz1 = The poles p1, 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, the crossover frequency 10Ω ⋅ (1 − 0.62 ) = 272 Krads −1 ⇒ 43.4KHz 5.3µH 2 The spacing between p2 and z2 is the closest when the converter is delivering the maximum output current from the lowest V IN . This represents the worst-case compensation condition. Ignoring C 5 and C 6 for the moment, C 4 forms a low frequency pole with the equivalent output resistance RO of the error amplifier: RO = 1 1 = R3C4 59KΩ • 560pF = 30.3 Krads −1 ⇒ 4.8 KHz 2 = 20Krads−1 ⇒ 3.18KHz (10Ω)⋅ (10µF) and ωz2 ≥ 1 1 = ROC4 4.7MΩ • 560pF Amplifier Open Loop Gain 49dB = = 4.7MΩ Transconductance 60µΩ −1 z2 . Placing z1 near p2 nulls its 3 effect and maximizes loop bandwidth. Thus should not be higher than R3C4 ≈ VOUT ⋅ C2 2 ⋅ IOUT(MAX) (15) R3 determines the mid-band loop gain of the converter. Increasing R 3 increases the mid-band gain and the crossover frequency. However it reduces the phase margin. The values of R 3 and C 4 can be determined GND R3 C3 R4 C4 C6 R2 U1 C1 SHDN R1 L1 C5 C2 D1 VOUT VIN Figure 8. Suggested PCB Layout for the SC4502/SC4502H. Notice that there is no via directly under the device. All vias are 12mil in diameter. 2005 Semtech Corp. 13 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Applications Information empirically by observing the inductor current and the output voltage during load transient. Compensation is optimized when the largest R3 and the smallest C4without producing ringing or excessive overshoot in its inductor current and output voltage are found. Figures 9(b), 10(c), 11(b) and 11(c) show load transient responses of empirically optimized DC-DC converters. In a batteryoperated system, compensating for the minimum VIN and the maximum load step will ensure stable operation over the entire input voltage range. C5 adds a feedforward zero to the loop response. In some cases, it improves the transient speed of the converter. C6 rolls off the gain at high frequency. This helps to stabilize the loop. C5 and C6 are often not needed. Board Layout Considerations In a step-up switching regulator, the output filter capacitor, the main power switch and the rectifying diode carry switched currents with high di/dt. For jitter-free operation, the size of the loop formed by these components should be minimized. Since the power switch is integrated inside the SC4502/SC4502H, grounding the output filter capacitor next to the SC4502/SC4502H ground pin minimizes size of the high di/dt current loop. The input bypass capacitors should also be placed close to the input pins. Shortening the trace at the SW node reduces the parasitic trace inductance. This not only reduces the EMI but also decreases the sizes of the switching voltage spikes and glitches. Figure 8 shows how various external components are placed around the SC4502/SC4502H. The frequencysetting resistor should be placed near the ROSC pin with a short ground trace on the PC board. These precautions reduce switching noise pickup at the ROSC pin. To achieve a junction to ambient thermal resistance (θJA) of 40°C/W, the exposed pad of the SC4502/SC4502H should be properly soldered to a large ground plane. Use only 12mil diameter vias in the ground plane if necessary. Avoid using larger vias under the device. Molten solder may seep through large vias during reflow, resulting in poor adhesion, poor thermal conductivity and low reliability. Typical Application Circuits D1 VIN L1 3.3V 5.6µH 8 OFF ON C1 2.2µF 10BQ015 6,7 IN 3 SHDN 10 SW FB SC4502 SS COMP GND C3 47nF 4,5 VOUT 12V, 0.3A R1 174K 2 C2 10µF 1 ROSC 9 R4 9.31K R3 40.2K R2 20K C4 1.8nF 40µs/div Upper Trace : Output Voltage, AC Coupled, 1V/div Lower Trace : Inductor Current, 0.5A/div L1: Sumida CR43 Figure 9(a). 1.35 MHz All Ceramic Capacitor 3.3V to 12V Boost Converter. 2005 Semtech Corp. Figure 9(b). Load Transient Response of the Circuit in Figure 9(a). ILOAD is switched between 0.1A and 0.3A at 1A/µs. 14 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Typical Application Circuits Efficiency vs Load Current 95 2.5µH 8 OFF ON 3 1-CELL LI-ION C1 2.2µF 10 6,7 IN SW SHDN FB SC4502 SS COMP GND C3 47nF 10BQ015 4,5 90 5V, 0.5A 85 R1 301K 2 C2 10µF 1 ROSC 9 VOUT R4 7.68K R3 34.8K R2 100K Efficiency (%) D1 L1 2.6 - 4.2V 1.5MHz 80 VIN = 4.2V 75 70 65 VIN = 3.6V VIN = 2.6V 60 VOUT = 5V 55 C4 1nF 50 0.001 0.010 L1: Sumida CDRH5D28 0.100 1.000 Load Current (A) Figure 10(a). 1.5 MHz All Ceramic Capacitor Single Li-ion Cell to 5V Boost Converter. Figure 10(b). Efficiency of the Single Li-ion Cell to 5V Boost Converter in Figure 10(a). VIN=2.6V 40µs/div Upper Trace : Output Voltage, AC Coupled, 0.5V/div Lower Trace : Inductor Current, 0.5A/div Figure 10(c). Load Transient Response of the Circuit in Figure . 10(a). ILOAD is switched between 90mA and 0.5A at 1A/µs. 2005 Semtech Corp. 15 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Typical Application Circuits 4-CELL 3.6 - 6V C6 L1 3.3µH 8 OFF ON 3 C1 2.2µF 10 10BQ015 6,7 IN SW FB SC4502 SS COMP GND C3 47nF 2.2µF SHDN 4,5 R1 60.4K 2 C2 10µF 1 R3 35.7K ROSC 9 VOUT 5V, 0.5A D1 R4 7.68K C4 1.5nF L2 3.3µH C6 22pF R2 20K L1 and L2: Coiltronics DRQ73-3R3 Figure 11(a). 1.5 MHz All Ceramic Capacitor 4-Cell to 5V SEPIC Converter. VIN=3.6V VIN=6V 40µs/div 40µs/div Upper Trace : Output Voltage, AC Coupled, 0.5V/div Lower Trace : Input Inductor Current, 0.2A/div Upper Trace : Output Voltage, AC Coupled, 0.5V/div Lower Trace : Input Inductor Current, 0.2A/div Figure 11(b). Load Transient Response of the Circuit in Figure 11(a). I LOAD is switched between 150mA and 450mA at 1A/µs. 2005 Semtech Corp. Figure 11(c). Load Transient Response of the Circuit in Figure 11(a). I LOAD is switched between 250mA and 700mA at 1A/µs. 16 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Typical Application Circuits D2 D3 D4 D5 C5 0.1µF C6 0.1µF C7 0.1µF L1 3.3V D1 3.3µH R5 150K 8 3 C1 2.2µF 10 R6 100K 10BQ015 6,7 IN SW SHDN FB COMP GND C3 47nF 4,5 23V (10mA) C8 1µF OUT1 8V (0.4A) R1 274K 2 SC4502 SS OUT2 C9 0.1µF 1 ROSC 9 R4 7.68K R3 33.2K C2 10µF R2 49.9K C4 1.5nF D7 L1 : Coiltronics SD18-3R3 D2 - D7 : BAT54S D6 OUT3 -8V (10mA) C10 1µF Figure 12(a). 1.5MHz Triple-Output TFT Power Supply. CH4 CH4 CH1 CH1 CH2 CH3 CH2 CH3 2ms/div CH1 : OUT1 Voltage, 5V/div CH2 : OUT2 Voltage, 10V/div CH3 : OUT3 Voltage, 5V/div CH4 : SHDN Voltage, 2V/div 4ms/div CH1 : OUT1 Voltage, 5V/div CH2 : OUT2 Voltage, 10V/div CH3 : OUT3 Voltage, 5V/div CH4 : Input Voltage, 2V/div Figure 12(c). TFT Power Supply Start-up Transient as the SHDN Pin is stepped from 0 to 2V. Figure 12(b). TFT Power Supply VIN Start-up Transient. 2005 Semtech Corp. 17 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Typical Application Circuits D1 14V 8 OFF ON 3 SHDN C1 2.2µF 10 SC4502H SS COMP GND C3 47nF 4,5 ROSC 80 2 FB C2 10µF 1 R3 5.11K R2 649 75 1.0MHz 10µH 70 65 60 9 R4 1.4MHz 6.8µH 85 R1 17.4K 6,7 SW 90 35V SS14 IN Efficiency at 14V input VOUT Efficiency (%) L1 VIN 55 C4 1.5nF 50 0 Figure 13(a). All Ceramic Capacitor High Voltage Application f (MHz ) R 4(KΩ) L1 1.0 15.8 10uH IHLP-2525BD _01 1.4 10 6.8uH IHLP-2525BD _01 2005 Semtech Corp. 0.05 0.1 Load (A) 0.15 0.2 Figure 13(b). Efficiency of the All Ceramic Capacitor High Voltage Application 18 www.semtech.com SC4502/SC4502H POWER MANAGEMENT Outline Drawing - MLPD-10, 3 x 3mm A E B DIM A A1 A2 b C D E e L N aaa bbb E PIN 1 INDICATOR (LASER MARK) A aaa C A1 .039 .031 .002 .000 (.008) .007 .009 .011 .074 .079 .083 .042 .048 .052 .114 .118 .122 .020 BSC .012 .016 .020 10 .003 .004 0.80 1.00 0.00 0.05 (0.20) 0.18 0.23 0.30 1.87 2.02 2.12 1.06 1.21 1.31 2.90 3.00 3.10 0.50 BSC 0.30 0.40 0.50 10 0.08 0.10 SEATING PLANE C A2 C 1 DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX 2 LxN D N bxN bbb e C A B NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS TERMINALS. Land Pattern - MLPD-10, 3 x 3mm K DIM (C) H G Y X Z C G H K P X Y Z DIMENSIONS INCHES MILLIMETERS (.112) .075 .055 .087 .020 .012 .037 .150 (2.85) 1.90 1.40 2.20 0.50 0.30 0.95 3.80 P NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804 2005 Semtech Corp. 19 www.semtech.com