MIC22600 1MHz 6A Integrated Switch Synchronous Buck Regulator General Description Features The Micrel MIC22600 is a high efficiency 6A Integrated synchronous buck (step-down) regulator. The MIC22600 is optimized for highest efficiency and achieves more than 90% efficiency, while still switching at 1MHz over a broad load range with only 1µH inductor and down to 47µF output capacitor. The ultra-high speed control loop keeps the output voltage within regulation even under extreme transient load swings commonly found in FPGAs and low voltage ASICs. The output voltage can be adjusted down to 0.7V to address all low voltage power needs. A full range of sequencing and tracking options is available with the MIC22600. The Enable/Delay pin combined with the Power Good/POR pin allows multiple outputs to be sequenced in any way during turn-on and turn-off. The RC (Ramp Control™) pin allows the device to be connected to another product in the MIC22xxx and/or MIC68xxx family, to keep the output voltages within a certain ∆V on start up. The MIC22600 is available in a 24-pin 4mm x 4mm MLF® with a junction operating range from –40°C to +125°C. Data sheets and support documentation can be found on Micrel’s web site at: www.micrel.com. • • • • • • • • • • • • • Input voltage range: 2.6V to 5.5V Output voltage adjustable down to 0.7V Output current up to 6A Full sequencing and tracking ability Power on Reset/Power Good Efficiency > 90% across a broad load range Ultra fast transient response – Easy RC compensation 100% maximum duty cycle Fully integrated MOSFET switches Micropower shutdown Thermal shutdown and current limit protection 24-pin 4mm x 4mm MLF® –40°C to +125°C junction temperature range Applications • • • • • • High power density point of load conversion Servers and routers DVD recorders Computing peripherals Base stations FPGAs, DSP and low voltage ASIC power Typical Application Efficiency V O - 3.3V 100 95 90 85 80 75 70 65 60 55 50 0 MIC22600 6A 1MHz Synchronous Output Converter VIN - 5V 1 2 3 4 5 6 OUTPUT CURRENT (A) 7 Sequencing & Tracking Ramp Control is a trademark of Micrel, Inc. MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc. Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com September 2007 M9999-092007-A Micrel, Inc. MIC22600 Ordering Information Part Number Voltage MIC22600YML Junction Temp. Range Adj. –40° to +125°C Package 24-Pin 4x4 MLF Lead Finish ® Pb-Free PGND SW SW SW SW PGND Pin Configuration PVIN PVIN EN SVIN DELAY SGND RC COMP FB POR/PG PVIN PGND SW SW SW SW PGND PVIN 24-Pin 4mm x 4mm MLF (ML) Pin Description Pin Number Pin Name 1, 6, 13, 18 PVIN Power Supply Voltage (Input): Requires bypass capacitor to GND. 17 SVIN Signal Power Supply Voltage (Input): Requires bypass capacitor to GND. 2 EN Enable/Delay (Input): This pin has a 1.2x band gap reference. When the pin is pulled higher that this the part will start up. Below this voltage the device is in its low quiescent current mode. The pin has a 1µA current source charging it to VIN. By adding a capacitor to this pin a delay may easily be generated. The enable function will not operate with an input voltage lower than the min specified. 4 RC Ramp Control: Capacitor to ground from this pin determines slew rate of output voltage during start-up. This can be used for tracking capability as well as soft start. 14 FB Feedback: Input to the error amplifier, connect to the external resistor divider network to set the output voltage. 15 COMP Compensation pin (Input): Place a RC to GND to compensate the device, see applications section. 5 POR/PG 7, 12, 19, 24 PGND Power Ground (Signal): Ground 16 SGND Signal Ground (Signal): Ground 3 DELAY Delay (Input): Capacitor to ground sets internal delay timer. Timer delays poweron reset (POR) output at turn-on and ramp down at turn-off. 8, 9, 10, 11, 20, 21, 22, 23 SW EP GND September 2007 Description Power On Reset (Output): Open-drain output device indicates when the output is out of regulation and is active after the delay set by the delay pin. Switch (Output): Internal power MOSFET output switches. Exposed Pad (Power): Must make a full connection to a GND plane for full output power to be realized. 2 M9999-092007-A Micrel, Inc. MIC22600 Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage (VIN) .........................................................6V Output Switch Voltage (VSW) ............................................6V Output Switch Current (ISW).......................Internally Limited Logic Input Voltage (VEN, VLQ)........................... VIN to –0.3V Storage Temperature (Ts) .........................–65°C to +150°C ESD Rating(3) .................................................................. 2kV Supply Voltage (VIN)......................................... 2.6V to 5.5V Junction Temperature (TJ) ..................–40°C ≤ TJ ≤ +125°C Thermal Resistance 4x4 MLF-24 (θJC) ...............................................14°C/W 4x4 MLF-24 (θJA) ...............................................40°C/W Electrical Characteristics(4) TA = 25°C with VIN = VEN = 3.3V; VOUT = 1.8V, unless otherwise specified. Bold values indicate –40°C< TJ < +125°C. Parameter Supply Voltage Range Under-Voltage Lockout Threshold UVLO Hysteresis Quiescent Current, PWM Mode Shutdown Current [Adjustable] Feedback Voltage FB Pin Input Current Current Limit in PWM Mode Output Voltage Line Regulation Output Voltage Load Regulation Maximum Duty Cycle Switch ON-Resistance PFET Switch ON-Resistance NFET Oscillator Frequency EN/DLY Threshold Voltage EN/DLY Source Current Condition Min (turn-on) 2.6 2.4 VEN =>1.34V; VFB = 0.9V (not switching) VEN = 0V ± 1% ± 2% (over temperature) VFB = 0.5*VNOM VOUT 1.8V; VIN = 2.6 to 5.5V, ILOAD= 100mA 100mA < ILOAD < 6000mA, VIN = 3.3V VFB ≤ 0.5V ISW = 1000mA; VFB=0.5V ISW = 1000mA; VFB=0.9V 0.693 0.686 6.5 VIN = 2.6 to VIN = 5.5V RC Pin IRAMP Ramp Control Current 0.7 Power On Reset IPG(LEAK) VPORH = 5.5V; POR = High Power On Reset VPG(LO) Output Logic-Low Voltage (undervoltage condition), IPOR = 5mA Threshold, % of VOUT below nominal Hysteresis Over-Temperature Shutdown Over-Temperature Shutdown Hysteresis 2.5 280 850 5 0.7 1 9 0.2 0.2 Max Units 5.5 2.6 1.2 1.34 1.3 V V mV µA µA V V nA A % % % Ω Ω MHz V µA 1.3 1 2 µA µA µA 1300 10 0.707 0.714 100 11.5 100 0.8 1.14 0.7 Power On Reset VPG Typ 0.03 0.025 1 1.24 1 1 130 7.5 10 mV 12.5 % 2 % 160 20 °C °C Notes: 1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Devices are ESD sensitive. Handling precautions recommended. 4. Specification for packaged product only. September 2007 3 M9999-092007-A Micrel, Inc. MIC22600 Typical Characteristics 10 Shutdown Current vs. Input Voltage 10 8 8 6 6 1.0 0.9 1 2 3 4 5 INPUT VOLTAGE (V) 6 0.8 0.2 VIN = 3.3V 0 2 TEMPERATURE (°C) Operating Current vs. Temperature 0.710 0.708 0.7 0.6 0.704 0.702 0.5 0.4 0.700 0.698 0.3 0.696 VIN = 3.3V Not switching FB = 1V Reference Voltage vs. Input Voltage Enable Voltage vs. Input Voltage Reference Voltage vs. Temperature 0.710 0.702 0.700 25°C 0.698 0.696 0.694 0.694 0.692 0.690 2 1.30 Not switching FB = 1V 3 4 5 6 INPUT VOLTAGE (V) 0.708 0.706 0.704 3 4 5 INPUT VOLTAGE (V) TEMPERATURE (°C) 1.30 25°C 0.4 0 0.706 0 1.0 2 0.8 0.2 0.1 1.2 4 2 0 0 Operating Current vs. Input Voltage 0.6 25°C 4 Shutdown Current vs. Temperature 6 Enable Voltage vs. Temperature 0.692 0.690 VIN = 3.3V TEMPERATURE (°C) Enable Hystersis vs. Temperature 24 20 1.25 1.25 16 1.20 1.20 25°C 12 VIN - 5V 8 1.15 1.15 VIN - 3V 4 1.10 2 1100 1080 1060 1040 1020 1000 980 960 940 920 900 3 4 5 INPUT VOLTAGE (V) Frequency vs. Temperature TEMPERATURE (°C) September 2007 6 0 1.10 50 45 40 35 30 25 20 15 10 5 0 2 TEMPERATURE (°C) TEMPERATURE (°C) P-Channel RDSON vs. Input Voltage Output Voltage vs. Ramp Control Voltage 2.0 1.8 1.6 1.4 90°C 1.2 1.0 0.8 0.6 0.4 2.5 3 3.5 4 4.5 5 INPUT VOLTAGE (V) 4 VIN - 5V 5.5 0.2 0 0 VIN - 2.5V 0.2 0.4 0.6 0.8 1.0 1.2 RAMP CONTROL VOLTAGE (V) M9999-092007-A Micrel, Inc. MIC22600 Typical Characteristics (continued) Efficiency VO - 1.2V Efficiency VO - 1.8V 100 95 VIN - 3.6V 90 VIN - 2.5V 85 80 VIN - 5V 75 70 65 60 55 50 0 1 2 3 4 5 6 OUTPUT CURRENT (A) 100 VIN - 2.5V 95 VIN - 3.6V 90 85 VIN - 5V 80 75 70 65 60 55 50 0 1 2 3 4 5 6 OUTPUT CURRENT (A) 50 40 30 20 10 0 -10 -20 -30 -40 -50 1 Bode Plot (VIN - 3.3V, VO - 1.8V) Gain 6A Phase 6A 10 100 FREQUENCY (kHz) September 2007 7 180 144 108 72 36 0 -36 -72 -108 -144 -180 1000 50 40 30 20 10 0 -10 -20 -30 -40 -50 1 Bode Plot (VIN - 5V, VO - 1.8V) Gain 6A Phase 6A 10 100 FREQUENCY (kHz) 5 Efficiency V O - 3.3V 100 95 90 85 80 75 70 65 60 55 50 0 7 180 144 108 72 36 -36 -72 -108 -144 -180 1000 50 VIN - 5V 1 2 3 4 5 6 OUTPUT CURRENT (A) Bode Plot (VIN - 5V, VO - 3.3V) 7 180 40 30 20 144 108 72 10 0 36 -10 -20 -30 -40 -50 1 -36 -72 -108 Gain 6A Phase 6A 10 100 FREQUENCY (kHz) -144 -180 1000 M9999-092007-A Micrel, Inc. MIC22600 Functional Characteristics September 2007 6 M9999-092007-A Micrel, Inc. MIC22600 Functional Characteristics (continued) September 2007 7 M9999-092007-A Micrel, Inc. MIC22600 Typical Circuits and Waveforms Sequencing Circuit and Waveform Tracking Circuit and Waveform September 2007 8 M9999-092007-A Micrel, Inc. MIC22600 Functional Diagram Figure 1. MIC22600 Block Diagram September 2007 9 M9999-092007-A Micrel, Inc. MIC22600 Functional Description PVIN PVIN is the input supply to the internal 30mΩ P-Channel Power MOSFET. This should be connected externally to the SVIN pin. The supply voltage range is from 2.6V to 5.5V. A 10µF ceramic is recommended for bypassing each PVIN supply. FB The feedback pin provides the control path to control the output. A resistor divider connecting the feedback to the output is used to adjust the desired output voltage. Refer to the feedback section in the “Applications Information” for more detail. EN/DLY This pin is internally fed with a 1µA current source to VIN. A delayed turn on is implemented by adding a capacitor to this pin. The delay is proportional to the capacitor value. The internal circuits are held off until EN/DLY reaches the enable threshold of 1.24V. POR This is an open drain output. A 47k resistor can be used for a pull up to this pin. POR is asserted high when output voltage reaches 90% of nominal set voltage and after the delay set by CDELAY. POR is asserted low without delay when enable is set low or when the output goes below the -10% threshold. For a Power Good (PG) function, the delay can be set to a minimum. This can be done by removing the Delay capacitor. RC RC allows the slew rate of the output voltage to be programmed by the addition of a capacitor from RC to ground. RC is internally fed with a 1µA current source and VOUT slew rate is proportional to the capacitor and the 1µA source. SW This is the connection to the source of the internal PChannel MOSFET and drain of the N-Channel MOSFET. This is a high frequency high power connection; therefore traces should be kept as short and as wide as practical. Delay Adding a capacitor to this pin allows the delay of the POR signal. When VOUT reaches 90% of its nominal voltage, the Delay pin current source (1µA) starts to charge the external capacitor. At 1.24V, POR is asserted high. SGND Internal signal ground for all low power sections. PGND Internal ground connection to the source of the internal N-Channel MOSFETs. Comp The MIC22600 uses an internal compensation network containing a fixed frequency zero (phase lead response) and pole (phase lag response) which allows the external compensation network to be much simplified for stability. The addition of a single capacitor and resistor will add the necessary pole and zero for voltage mode loop stability using low value, low ESR ceramic capacitors. September 2007 10 M9999-092007-A Micrel, Inc. MIC22600 the SW nodes of the IC. For this reason, the heat of the inductor is somewhat coupled to the IC, so it offers some level of protection if the inductor gets too hot. It is important to test all operating limits before settling on the final inductor choice. The size requirements refer to the area and height requirements that are necessary to fit a particular design. Please refer to the inductor dimensions on their datasheet. DC resistance is also important. While DCR is inversely proportional to size, DCR can represent a significant efficiency loss. Refer to the “Efficiency Considerations” below for a more detailed description. Application Information The MIC22600 is a 6A Synchronous step down regulator IC with a fixed 1 MHz, voltage mode PWM control scheme. The other features include tracking and sequencing control for controlling multiple output power systems, power on reset. Component selection Input Capacitor A minimum 10µF ceramic is recommended on each of the PVIN pins for bypassing. X5R or X7R dielectrics are recommended for the input capacitor. Y5V dielectrics, aside from losing most of their capacitance over temperature, they also become resistive at high frequencies. This reduces their ability to filter out high frequency noise. Enable/DLY Capacitor Enable/DLY sources 1µA out of the IC to allow a startup delay to be implemented. The delay time is simply the time it takes 1µA to charge CDLY to 1.25V. Therefore: Output Capacitor The MIC22600 was designed specifically for the use of ceramic output capacitors. 100µF can be increased to improve transient performance. Since the MIC22600 is in voltage mode, the control loop relies on the inductor and output capacitor for compensation. For this reason, do not use excessively large output capacitors. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors, aside from the undesirable effect of their wide variation in capacitance over temperature, become resistive at high frequencies. Using Y5V or Z5U capacitors can cause instability in the MIC22600. TDLY = Inductance • Rated current value • Size requirements ⎛V ×I Efficiency % = ⎜⎜ OUT OUT ⎝ VIN × I IN ⎞ ⎟⎟ × 100 ⎠ Maintaining high efficiency serves two purposes. It decreases power dissipation in the power supply, reducing the need for heat sinks and thermal design considerations and it decreases consumption of current for battery powered applications. Reduced current draw from a battery increases the devices operating time, critical in hand held devices. There are mainly two loss terms in switching converters: static losses and switching losses. Static losses are simply the power losses due to VI or I2R. For example, power is dissipated in the high side switch during the on cycle. Power loss is equal to the high side MOSFET RDS(ON) multiplied by the RMS Switch Current squared (ISW2). During the off cycle, the low side N-Channel MOSFET conducts, also dissipating power. Similarly, the inductor’s DCR and capacitor’s ESR also contribute to the I2R losses. Device operating current also reduces efficiency by the product of the quiescent (operating) current and the supply voltage. The current required to drive the gates on and off at a constant 1MHz or 4Mhz frequency and the switching transitions make up the switching losses. • DC resistance (DCR) The MIC22600 is designed for use with a 0.47µH to 4.7µH inductor. Maximum current ratings of the inductor are generally given in two methods: permissible DC current and saturation current. Permissible DC current can be rated either for a 40°C temperature rise or a 10% loss in inductance. Ensure the inductor selected can handle the maximum operating current. When saturation current is specified, make sure that there is enough margin that the peak current will not saturate the inductor. The ripple can add as much as 1.2A to the output current level. The RMS rating should be chosen to be equal or greater than the Current Limit of the MIC22600 to prevent overheating in a fault condition. For best electrical performance, the inductor should be placed very close to September 2007 1.10 − 6 Efficiency considerations Efficiency is defined as the amount of useful output power, divided by the amount of power consumed. Inductor Selection Inductor selection will be determined by the following (not necessarily in the order of importance): • 1.24 ⋅ C DLY 11 M9999-092007-A Micrel, Inc. MIC22600 Figure 2 shows an efficiency curve. The portion, from 0A to 1A, efficiency losses are dominated by quiescent current losses, gate drive and transition losses. In this case, lower supply voltages yield greater efficiency in that they require less current to drive the MOSFETs and have reduced input power consumption. Efficiency vs. Inductance 95 90 85 L = µF 80 L = 4.7µF 75 Efficiency 3V - 1.8V 70 95 65 90 60 85 55 80 50 0 75 200 400 600 800 OUTPUT CURRENT (mA) 70 Figure 3. Efficiency vs. Inductance 65 60 Compensation The MIC22600 has a combination of internal and external stability compensation to simplify the circuit for small, high efficiency designs. In such designs, voltage mode conversion is often the optimum solution. Voltage mode is achieved by creating an internal 1MHz ramp signal and using the output of the error amplifier to modulate the pulse width of the switch node, thereby maintaining output voltage regulation. With a typical gain bandwidth of 100-200 kHz, the MIC22600 is capable of extremely fast transient responses. The MIC22600 is designed to be stable with a typical application using a 1µH inductor and a 47µF ceramic (X5R) output capacitor. These values can be varied dependant upon the tradeoff between size, cost and efficiency, keeping the LC natural frequency 55 50 0 1 2 3 4 5 6 OUTPUT CURRENT (A) 7 Figure 2. Efficiency Curve The region, 1A to 6A, efficiency loss is dominated by MOSFET RDS(ON) and inductor DC losses. Higher input supply voltages will increase the Gate-to-Source voltage on the internal MOSFETs, reducing the internal RDS(ON). This improves efficiency by decreasing DC losses in the device. All but the inductor losses are inherent to the device. In which case, inductor selection becomes increasingly critical in efficiency calculations. As the inductors are reduced in size, the DC resistance (DCR) can become quite significant. The DCR losses can be calculated as follows; LPD = IOUT2 × DCR From that, the loss in efficiency due to inductor resistance can be calculated as follows: ⎡ ⎛ VOUT ⋅ I OUT Efficiency Loss = ⎢1 − ⎜⎜ ( V ⎣⎢ ⎝ OUT ⋅ I OUT ) + LPD ( 1 2⋅Π ⋅ L ⋅C ) ideally less than 26 kHz to ensure stability can be achieved. The minimum recommended inductor value is 0.47µH and minimum recommended output capacitor value is 22µF. The tradeoff between changing these values is that with a larger inductor, there is a reduced peak-to-peak current which yields a greater efficiency at lighter loads. A larger output capacitor will improve transient response by providing a larger hold up reservoir of energy to the output. The integration of one pole-zero pair within the control loop greatly simplifies compensation. The optimum values for CCOMP (in series with a 20k resistor) are shown below. ⎞⎤ ⎟⎥ × 100 ⎟ ⎠⎦⎥ Efficiency loss due to DCR is minimal at light loads and gains significance as the load is increased. Inductor selection becomes a trade-off between efficiency and size in this case. Alternatively, under lighter loads, the ripple current due to the inductance becomes a significant factor. When light load efficiencies become more critical, a larger inductor value maybe desired. Larger inductances reduce the peak-to-peak inductor ripple current, which minimize losses. The following graph in Figure 3 illustrates the effects of inductance value at light load. CÆ 22-47µF 47µF100µF 100µF470µF 0*-10pF 22pF 33pF LÈ 0.47µH † 1µH 0 -15pF 15-22pF 33pF 2.2µH 15-33pF 33-47pF 100-220pF † * VOUT > 1.2V, VOUT > 1V September 2007 12 M9999-092007-A Micrel, Inc. MIC22600 Feedback The MIC22600 provides a feedback pin to adjust the output voltage to the desired level. This pin connects internally to an error amplifier. The error amplifier then compares the voltage at the feedback to the internal 0.7V reference voltage and adjusts the output voltage to maintain regulation. The resistor divider network for a desired VOUT is given by: R2 = Enable/DLY pin The Enable pin contains a trimmed, 1µA current source which can be used with a capacitor to implement a fixed desired delay in some sequenced power systems. The threshold level for power on is 1.24V with a hysteresis of 20mV. Delay Pin The Delay pin also has a 1µA trimmed current source and a 1µA current sink which acts with an external capacitor to delay the operation of the Power On Reset (POR) output. This can be used also in sequencing outputs in a sequenced system, but with the addition of a conditional delay between supplies; allowing a first up, last down power sequence. After Enable is driven high, VOUT will start to rise (rate determined by RC capacitor). As the FB voltage goes above 90% of its nominal set voltage, Delay begins to rise as the 1µA source charges the external capacitor. When the threshold of 1.24V is crossed, POR is asserted high and Delay continues to charge to a voltage VDD. When FB falls below 90% of nominal, POR is asserted low immediately. However, if enable is driven low, POR will fall immediately to the low state and Delay will begin to fall as the external capacitor is discharged by the 1µA current sink. When the threshold of VDD1.24V is crossed, VOUT will begin to fall at a rate determined by the RC capacitor. As the voltage change in both cases is 1.24V, both rising and falling delays are 1.24 ⋅ C DLY matched at TPOR = 1.10 − 6 R1 ⎛ VOUT ⎞ ⎜⎜ − 1⎟⎟ ⎝ VREF ⎠ where VREF is 0.7V and VOUT is the desired output voltage. A 10kΩ or lower resistor value from the output to the feedback is recommended since large feedback resistor values increase the impedance at the feedback pin, making the feedback node more susceptible to noise pick-up. A small capacitor (50pF – 100pF) across the lower resistor can reduce noise pick-up by providing a low impedance path to ground. PWM Operation The MIC22600 is a voltage mode, pulse width modulation (PWM) controller. By controlling the ratio of on-to-off time, or duty cycle, a regulated DC output voltage is achieved. As load or supply voltage changes, so does the duty cycle to maintain a constant output voltage. In cases where the input supply runs into a dropout condition, the MIC22600 will run at 100% duty cycle. The MIC22600 provides constant switching at 1MHz with synchronous internal MOSFETs. The internal MOSFETs include a high-side P-Channel MOSFET from the input supply to the switch pin and an N-Channel MOSFET from the switch pin-to-ground. Since the low-side NChannel MOSFET provides the current during the off cycle, a freewheeling Schottky diode from the switch node-to-ground is not required. PWM control provides fixed frequency operation. By maintaining a constant switching frequency, predictable fundamental and harmonic frequencies are achieved. Other methods of regulation, such as burst and skip modes, have frequency spectrums that change with load that can interfere with sensitive communication equipment. RC pin The RC pin provides a trimmed 1µA current source/sink similar to the Delay Pin for accurate ramp up (soft start) and ramp down control. This allows the MIC22600 to be used in systems requiring voltage tracking or ratio-metric voltage tracking at startup. There are two ways of using the RC pin: 1. Externally driven from a voltage source 2. Externally attached capacitor sets output ramp up/down rate In the first case, driving RC with a voltage from 0V to VREF will program the output voltage between 0 and 100% of the nominal set voltage. In the second case, the external capacitor sets the ramp up and ramp down time of the output voltage. The time 0.7 ⋅ C RC is given by TRAMP = where TRAMP is the time 1.10 − 6 from 0 to 100% nominal output voltage. Sequencing and tracking The MIC22600 provides additional pins to provide up/down sequencing and tracking capability for connecting multiple voltage regulators together. September 2007 13 M9999-092007-A Micrel, Inc. MIC22600 Normal Tracking: Sequencing & Tracking examples There are four distinct variations which are easily implemented using the MIC22600. The two sequencing variations are Delayed and Windowed. The two tracking variants are Normal and Ratio Metric. The following diagrams illustrate methods for connecting two MIC22600’s to achieve these requirements. Sequencing: Ratio Metric Tracking: September 2007 14 M9999-092007-A Micrel, Inc. MIC22600 Current Limit The MIC22600 is protected against overload in two stages. The first is to limit the current in the P-channel switch; the second is over temperature shutdown. Current is limited by measuring the current through the high side MOSFET during its power stroke and immediately switching off the driver when the preset limit is exceeded. The circuit in Figure 4 describes the operation of the current limit circuit. Since the actual RDSON of the PChannel MOSFET varies part-to-part, over temperature and with input voltage, simple IR voltage detection is not employed. Instead, a smaller copy of the Power MOSFET (Reference FET) is fed with a constant current which is a directly proportional to the factory set current limit. This sets the current limit as a current ratio and thus, is not dependant upon the RDSON value. Current limit is set to 9A nominal. Variations in the scale factor K between the Power PFET and the reference PFET used to generate the limit threshold account for a relatively small inaccuracy. An alternative method here shows an example of a VDDQ & VTT solution for a DDR memory power supply. Note that POR is taken from Vo1 as POR2 will not go high. This is because POR is set high when FB > 0.9⋅VREF. In this example, FB2 is regulated to ½⋅VREF. Figure 4. Current Limit Detail Thermal Considerations The MIC22600 is packaged in the MLF® 4mm x 4mm, a package that has excellent thermal performance equaling that of the larger TSSOP packages. This maximizes heat transfer from the junction to the exposed pad (ePAD) which connects to the ground plane. The size of the ground plane attached to the exposed pad determines the overall thermal resistance from the junction to the ambient air surrounding the printed circuit board. The junction temperature for a given ambient temperature can be calculated using: TJ = TAMB + PDISS · RθJA Where • September 2007 15 PDISS is the power dissipated within the MLF® package and is typically 1.5W at 6A load. This has been calculated for a 1µH inductor and details can be found in table 1 below for reference. M9999-092007-A Micrel, Inc. • MIC22600 RθJA is a combination of junction to case thermal resistance (RθJC) and Case-to-Ambient thermal resistance (RθCA), since thermal resistance of the solder connection from the ePAD to the PCB is negligible; RθCA is the thermal resistance of the ground plane to ambient, so RθJA = RθJC + RθCA. Example: The Evaluation board has 2 copper planes contributing to an RθJA of approximately 25°C/W. The worst case RθJC of the MLF 4x4 is 14oC/W. RθJA = RθJC + RθCA RθJA = 14 + 25 = 39oC/W To calculate the junction temperature for a 50°C ambient: VINÆ 3 3.5 4 4.5 5 1 1.47 1.50 1.52 1.54 1.56 1.2 1.45 1.47 1.49 1.51 1.54 1.8 1.46 1.45 1.45 1.47 1.48 2.5 1.61 1.53 1.49 1.47 1.47 3.3 – 1.70 1.62 1.56 1.53 VOUT @6AÈ TJ = TAMB+PDISS . RθJA TJ = 50 + (1.5 x 39) TJ = 109°C This is below the maximum of 125°C. Table 1. Power Dissipation (W) for 6A output • TAMB is the Operating Ambient temperature. September 2007 16 M9999-092007-A Micrel, Inc. MIC22600 Package Information 24-Pin 4mm x 4mm MLF® (ML) MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2007 Micrel, Incorporated. September 2007 17 M9999-092007-A