SC4525ESETRT

SC4525E 28V 3A Step-Down Switching Regulator POWER MANAGEMENT Features Description Wide Input Voltage Range: 3V to 28V 3A Output Current 200kHz to 2MHz Programmable Frequency Precision 1V Feedback Voltage Peak Current-Mode Control Cycle-by-Cycle Current Limiting Hiccup Overload Protection with Frequency Foldback Soft-Start and Enable Thermal Shutdown Thermally Enhanced 8-pin SOIC Package Fully RoHS and WEEE Compliant            Applications XDSL and Cable Modems Set Top Boxes Point of Load Applications CPE Equipment DSP Power Supplies LCD and Plasma TVs Automotive Car Audio        The SC4525E is a constant frequency peak current-mode step-down switching regulator capable of producing 3A output current from an input ranging from 3V to 28V. The switching frequency of the SC4525E can be programmed up to 2MHz for component miniaturization or it can be set at lower frequencies to accommodate high step-down ratios. The SC4525E is suitable for next generation XDSL modems, high-definition TVs and various point of load applications. Peak current-mode PWM control employed in the SC4525E achieves fast transient response with simple loop compensation. Cycle-by-cycle current limiting and hiccup overload protection reduces power dissipation during output overload. Soft-start function reduces input startup current and prevents the output from overshooting during power-up. The SC4525E is available in SOIC-8 EDP package. S C 4 52 5A Typical Application Circuit Efficiency IN C4 4 .7mF 90 D1 10 V – 28 V 1N 4148 C1 0 .33 mF L1 BST IN SW S C 4 5 2 5E S S /E N 5.2mH 80 OUT R4 33 .2 k 5 V /3 A FB COMP C7 22 nF C8 22 pF ROSC R7 12 .7k GND R5 15.8k D2 20 B Q 030 R6 8.25k C5 2.2nF L1: C oiltronics C D 1- 5R 2 C2 10 mF X 3 Efficiency (%) V V IN = 24V V IN = 12V 70 60 50 40 C 2: M urata G R M 31C R 60 J106 K C 4: M urata G R M 32 E R 71 H 475 K 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Load Current (A) Figure 1. 1MHz 10V -28V to 5V/3A Step-down Converter Rev. 2.1  E fficien c y of th e 1 M H z 1 0 V -2 8 V to 5 V /3 A S tep -D ow n C on ve © 2013 Semtech Corporation SC4525E Pin Configuration Ordering Information SW 1 8 BST IN 2 7 FB ROSC 3 6 COMP GND 4 5 S S /E N 9 Device Package SC4525ESETRT(1)(2) SOIC-8 EDP SC4525EEVB Evaluation Board Notes: (1) Available in tape and reel only. A reel contains 2,500 devices. (2) Available in lead-free package only. Device is fully WEEE and RoHS compliant and halogen-free. (8 - Pin SOIC - EDP) Marking Information yyww=Date code (Example: 0752) xxxxx=Semtech Lot No. (Example: E9010)  SC4525E Absolute Maximum Ratings Thermal Information VIN Supply Voltage ……………………………… -0.3 to 32V Junction to Ambient (1) ……………………………… 36°C/W BST Voltage ……………………………………………… 42V Junction to Case (1) ………………………………… BST Voltage above SW …………………………………… 34V Maximum Junction Temperature……………………… 150°C 5.5°C/W Storage Temperature ………………………… -65 to +150°C SS Voltage ……………………………………………-0.3 to 3V Lead Temperature (Soldering) 10 sec ………………… 300°C FB Voltage ………………………………………….... -0.3 to 7V Recommended Operating Conditions SW Voltage ………………………………………… -0.6 to VIN SW Transient Spikes (10ns Duration)……… -2.5V to VIN +1.5V Input Voltage Range ……………………………… 3V to 28V Peak IR Reflow Temperature …………………………. Maximum Output Current ……………………………… 3A 260°C ESD Protection Level ………………………………… 2000V (2) Operating Ambient Temperature …………… -40 to +105°C Operating Junction Temperature …………… -40 to +125°C 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. NOTES(1) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards. (2) Tested according to JEDEC standard JESD22-A114-B. Electrical Characteristics Unless otherwise noted, VIN = 12V, VBST = 15V, VSS = 2.2V, -40°C < TA = TJ < 125°C, ROSC = 12.1kΩ. Parameter Conditions Min Typ Max Units 28 V 2.95 V Input Supply Input Voltage Range VIN Start Voltage 3 VIN Rising 2.70 VIN Start Hysteresis VIN Quiescent Current VIN Quiescent Current in Shutdown 2.82 225 mV VCOMP = 0 (Not Switching) 2 2.6 mA VSS/EN = 0, VIN = 12V 40 52 µA 1.000 1.020 V Error Amplifier Feedback Voltage Feedback Voltage Line Regulation FB Pin Input Bias Current 0.980 VIN = 3V to 28V 0.005 VFB = 1V, VCOMP = 0.8V -170 %/V -340 nA Error Amplifier Transconductance 300 µΩ-1 Error Amplifier Open-loop Gain 60 dB 15.2 A/V VFB = 0.9V 2.35 V COMP Source Current VFB = 0.8V, VCOMP = 0.8V 17 COMP Sink Current VFB = 1.2V, VCOMP = 0.8V 25 COMP Pin to Switch Current Gain COMP Maximum Voltage µA Internal Power Switch Switch Current Limit Switch Saturation Voltage (Note 1) ISW = -3.9A 3.9 5.1 6.6 A 380 600 mV  SC4525E Electrical Characteristics (Cont.) Unless otherwise noted, VIN = 12V, VBST = 15V, VSS = 2.2V, -40°C < TA = TJ < 125°C, ROSC = 12.1kΩ. Parameter Conditions Min Typ Max Units Minimum Switch On-time VIN = 10V, RSW=10Ω 70 120 230 ns Minimum Switch Off-time VIN = 6V, RSW=6Ω 30 75 130 ns 10 µA Switch Leakage Current Minimum Bootstrap Voltage ISW = -3.9A 1.8 2.3 V BST Pin Current ISW = -3.9A 100 150 mA Oscillator Switching Frequency Foldback Frequency ROSC = 12.1kΩ 1.04 1.3 1.56 MHz ROSC = 73.2kΩ 230 300 370 kHz ROSC = 12.1kΩ, VFB = 0 100 ROSC = 73.2kΩ, VFB = 0 35 60 90 0.2 0.3 0.4 V 0.95 1.2 1.4 V 250 kHz Soft Start and Overload Protection SS/EN Shutdown Threshold SS/EN Switching Threshold Soft-start Charging Current VFB = 0 V VSS/EN = 0 V VSS/EN = 1.5 V 1.9 1.6 Soft-start Discharging Current 2.4 3.2 µA 1.5 µA Hiccup Arming SS/EN Voltage VSS/EN Rising 2.15 V Hiccup SS/EN Overload Threshold VSS/EN Falling 1.9 V Hiccup Retry SS/EN Voltage VSS/EN Falling 0.6 1.0 1.2 V Over Temperature Protection Thermal Shutdown Temperature 165 °C Thermal Shutdown Hysteresis 10 °C Note 1: Switch current limit does not vary with duty cycle.  SC4525E Pin Descriptions SO-8 Pin Name Pin Function 1 SW Emitter of the internal NPN power transistor. Connect this pin to the inductor, the freewheeling diode and the bootstrap capacitor. 2 IN Power supply to the regulator. It is also the collector of the internal NPN power transistor. It must be closely bypassed to the ground plane with a capacitor. 3 ROSC An external resistor from this pin to ground sets the oscillator frequency. 4 GND Ground pin 5 SS/EN Soft-start and regulator enable pin. A capacitor from this pin to ground provides soft-start and overload hiccup functions. Hiccup can be disabled by overcoming the internal soft-start discharging current with an external pullup resistor connected between the SS/EN and the IN pins. Pulling the SS/EN pin below 0.2V completely shuts off the regulator to low current state. 6 COMP The output of the internal error amplifier. The voltage at this pin controls the peak switch current. A RC compensation network at this pin stabilizes the regulator. 7 FB The inverting input of the error amplifier. If VFB falls below 0.8V, then the switching frequency will be reduced to improve short-circuit robustness (see Applications Information for details). 8 BST Supply pin to the power transistor driver. Tie to an external diode-capacitor bootstrap circuit to generate drive voltage higher than VIN in order to fully enhance the internal NPN power transistor. 9 Exposed Pad The exposed pad serves as a thermal contact to the circuit board. While the exposed pad is electrically isolated, it is suggested to be soldered to the ground plane of the PC board.  SC4525E Block Diagram IN SLO PE COMP COMP 6 FB 7 + 2 S + + IS E N 3 .5 3m W + EA + OC IL IM + 1 8m V - BST V1 8 + PW M - S R FREQ UENCY F O LD B A C K ROSC Q POW ER T R A N S IS T O R CLK O S C IL L A T O R 3 1.2 V 1 R R SW O VERLO AD - PW M A1 + S S /E N 5 1V 1 .9V REFERENCE & THERM AL SHUTDO W N FAULT S O F T -S T A R T AND O VERLO AD H IC C U P CONTROL GND 4 Figure 2 — SC4525E Block Diagram 1.9 V S S /E N IC 2 .4mA B4 + S B1 O VERLO AD S OC R PW M R B2 1V /2 .1 5V FAULT Q ID 3 .9 mA _ Q B3 Figure 3 — Soft-start and Overload Hiccup Control Circuit  SC4525E Typical Characteristics Efficiency VO = 5V 85 70 65 VO = 1 .5V 60 55 45 1 .0 1 V O = 3 .3 V 75 VO = 2 .5 V 70 65 60 55 1M H z, V IN = 12 V D 2 = B 320 A 50 V IN = 1 2 V VO = 5 V 80 VO = 2 .5V 75 Efficiency (%) Efficiency (%) 80 1 .0 2 85 V O = 3 .3V Feedback Voltage vs Temperature Efficiency 90 VFB (V) 90 1M H z, VIN = 24 V 50 40 40 0 0 .5 1 1.5 2 Load Current (A) 2 .5 0 3 Frequency Setting Resistor vs Frequency 1000 0 .5 1 1 .5 2 Load Current (A) 2 .5 -5 0 Frequency vs Temperature 0 25 50 75 100 125 Foldback Frequency vs VFB 1 .2 5 Normalized Frequency 1 .1 R O S C = 7 3.2 k 1 .0 R O S C = 1 2.1 k 0 .9 -2 5 Temperature (oC) 1 .2 Normalized Frequency ROSC (k) 10 0 .9 7 3 V IN = 1 2V 100 0 .9 9 0 .9 8 D 2 = B 330 A 45 1 .0 0 1 R O S C = 7 3.2 k 0 .7 5 0 .5 TA = 2 5o C 0 .2 5 R O S C = 1 2 .1 k 1 0 0 .8 0 0 .2 5 0.5 0 .7 5 1 1 .2 5 1 .5 1 .7 5 2 -5 0 -2 5 0 Frequency (MHz) 300 -4 0 oC 200 150 0 .2 0 0 .4 0 0 .6 0 0 .8 0 1 .0 0 VF B (V) Switch Current Limit vs Temperature 5 .2 Current Limit (A) V CESAT (mV) 350 0 .0 0 125 1 0 0.0 BST Pin Current vs Switch Current V IN = 1 2 V V BST =15V 4 .8 1 2 5 oC 2 5 oC 100 Temperature ( C) 450 250 75 O Switch Saturation Voltage vs Switch Current 400 50 BST Pin Current (mA) 500 25 4 .4 4 .0 3 .6 7 5.0 -4 0 o C 5 0.0 1 2 5oC 2 5.0 100 3 .2 50 0 .0 0.5 1 .0 1 .5 2 .0 2 .5 3 .0 3 .5 4 .0 4 .5 Switch Current (A) 0.0 -5 0 -2 5 0 25 50 75 Temperature ( o C) 100 125 0 0 .5 1 1 .5 2 2 .5 3 3 .5 4 Switch Current (A)  SC4525E Curve 12 Curve 11 Typical Characteristics (Cont.) S S 270 RE V 6-7 S S 270 RE V 6-7 S S 270 RE V 6-7 VIN Supply Current vs Soft-Start Voltage VIN Thresholds vs Temperature 2.5 S ta rt 2.8 2.7 2.6 80 -40 o C 1.5 1.0 Curve 14 2.5 0.5 2.4 0.0 U VL O -50 -25 0 Curve 15 25 50 75 100 0 125 0.5 125 o C 40 0 1 1.5 S S 270 R E V 6-7 0 2 5 10 20 25 30 Soft-Start Charging Current vs Soft-Start Voltage SS Shutdown Threshold vs Temperature 0.40 125 o C 15 VIN (V) S S 270 RE V 6-7 VIN Quiescent Current vs VIN 2.5 -40 o C VSS (V) Temperature ( C) S S 2 7 0 R E V 6 -7 60 20 o 0.0 -0.5 o SS Threshold (V) -40 C 1.5 1.0 0.5 0.35 Current (uA) 2.0 Current (mA) V SS = 0 125 o C 2.0 Current (mA) VIN Threshold (V) 2.9 Current (uA) 3.0 VIN Shutdown Current vs VIN 100 0.30 15 VIN (V) 20 25 -2.0 -3.0 0.20 10 -40 o C -1.5 -2.5 0.0 5 125 o C 0.25 V C O MP = 0 0 -1.0 30 -50 -25 0 25 50 75 o Temperature ( C) 100 125 0 0.5 1 1.5 2 VSS (V)  SC4525E Applications Information Operation The SC4525E is a constant-frequency, peak current-mode, step-down switching regulator with an integrated 28V, 3.9A power NPN transistor. Programmable switching frequency makes the regulator design more flexible. With the peak current-mode control, the double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop. This simplifies loop compensation and achieves fast transient response with a simple Type-2 compensation network. As shown in Figure 2, the switch collector current is sensed with an integrated 3.53mW sense resistor. The sensed current is summed with a slope-compensating ramp before it is compared with the transconductance error amplifier (EA) output. The PWM comparator trip point determines the switch turn-on pulse width. The current-limit comparator ILIM turns off the power switch when the sensed signal exceeds the 18mV current-limit threshold. Driving the base of the power transistor above the input power supply rail minimizes the power transistor saturation voltage and maximizes efficiency. An external bootstrap circuit (formed by the capacitor C1 and the diode D1 in Figure 1) generates such a voltage at the BST pin for driving the power transistor. shown in Figure 3). As the SS/EN voltage exceeds 0.4V, the internal bias circuit of the SC4525E turns on and the SC4525E draws 2mA from VIN. The 1.9µA charging current turns off and the 2.4µA current source IC in Figure 3 slowly charges the soft-start capacitor. The error amplifier EA in Figure 2 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 1V reference and the other non-inverting input is tied to the output of the amplifier A1. Amplifier A1 produces an output V1 = 2(VSS/EN -1.2V). V1 is zero and COMP is forced low when VSS/EN is below 1.2V. During start up, the effective non-inverting input of EA stays at zero until the soft-start capacitor is charged above 1.2V. Once VSS/EN exceeds 1.2V, COMP is released. The regulator starts to switch when VCOMP rises above 0.4V. If the soft-start interval is made sufficiently long, then the FB voltage (hence the output voltage) will track V1 during start up. VSS/EN must be at least 1.83V for the output to achieve regulation. Proper soft-start prevents output overshoot. Current drawn from the input supply is also well controlled. Overload / Short-Circuit Protection Table 2 lists various fault conditions and their corresponding protection schemes in the SC4525E. Table 2: Fault conditions and protections Shutdown and Soft-Start The SS/EN pin is a multiple-function pin. An external capacitor connected from the SS pin to ground sets the soft-start and overload shutoff times of the regulator (Figure 3). The effect of VSS/EN on the SC4525E is summarized in Table 1. Table 1: SS/EN operation modes SS/EN Mode Supply Current 2.15V Switching & hiccup armed Load dependent Pulling the SS/EN pin below 0.2V shuts off the regulator and reduces the input supply current to 18µA (VIN = 5V). When the SS/EN pin is released, the soft-start capacitor is charged with an internal 1.9µA current source (not Condition Cause of Fault Protective Action Cycle-by-cycle limit at IL>ILimit, V FB>0.8V Over current IL>ILimit, V FB V IN + 2 .5 V D1 BST C1 VOUT SW SC4525E D2 GND (e) D2 GND D1 IN VOUT (d) BST V IN C1 SW IN (c) V S > 2 .5 V D2 (b) (a) V IN VOUT SW IN SW IN C1 V IN VOUT SW IN SC4525E GND D2 (f) Figure 6(a)-(f). Methods of Bootstrapping the SC4525E 13 SC4525E Applications Information (Cont.) If VIN(MAX) + VOUT > 42V, then a Zener diode D3 can be used in series with D1 to lower the BST voltage [Figure 6(c)]. The following inequality gives a suitable range for the Zener voltage VZ: 9287   ! 9= ! 9,10$;   9287    The SC4525E can also be bootstrapped from the input [Figure 6(b)]. This configuration is not as efficient as Figure 6(a). However this may be the only option if the output voltage is less than 2.5V and there is no other supply with voltage higher than 2.5V. Voltage stress at the BST pin can be somewhat higher than 2VIN. The BST pin voltage should not exceed its absolute maximum rating of 42V. loaded or VIN needs to be increased. Using larger soft-start capacitor CSS will also help the bootstrap circuit to run because there will be current in the inductor over a longer period of time. Figures 7(a) and 7(b) show the minimum input voltage required to start bootstrap and to run before dropping out as a function of the load current. The minimum start-up VIN decreases with higher dVIN/dt or larger soft-start capacitor CSS. The lines labeled “dropout” in these graphs show that once started, the bootstrap circuit is able to sustain itself down to zero load. To reduce BST voltage stress when stepping down from high VIN (>20V) to low VOUT (