MIC2159 M9999-030509-B MIC2159YMME MSOP-10 MSOP10 MIC2169A - Datasheet Archive

 

SYNCHRONOUS-ittyTM Step-Down Converter IC General Description Features The MIC2159 is a high efficiency, simple to use

 

MIC2159 MIC2159 SYNCHRONOUS-ittyTM Step-Down Converter IC General Description Features The MIC2159 MIC2159 is a high efficiency, simple to use synchronous buck controller ICs housed in a 10-pin MSOP ePAD package. The MIC2159 MIC2159 switches at 400kHz, allowing the smallest possible external components and is designed to drive loads up to 30A. The devices feature high output driver capability, combined with an all nchannel synchronous architecture. The MIC2159 MIC2159 operates from a 3V to 14.5V input and can be configured to generate output voltages as low as 0.8V. Efficiencies of over 95% can be achieved within the smallest possible printed circuit board space area. The MIC2159 MIC2159 is available in a thermally capable 10-pin ePAD MSOP package, with a junction operating range from ­40°C to +125°C. · · · · · · · · · · · Small footprint 10-lead ePAD MSOP 3V to 14.5V input voltage range Adjustable output voltage down to 0.8V 400KHz operation Drives two n-channel MOSFETs - Built-in 3 drivers Simple control: voltage-mode PWM Fast transient response - Externally compensated Over-current protection Hiccup mode short-circuit protection Dual function COMP & EN pin - ISD = 50µA Short minimum ON time - 30ns - Very low duty cycle possible Applications · Point-of-Load DC/DC conversion · High Current Power Supplies · Telecom/datacom and Networking Power Supplies · Servers and Workstations · Graphics Cards and other PC Peripherals _ Typical Application MIC2159 MIC2159 Adjustable Output 400kHz Converter MLF and MicroLead Frame is a registered trademark 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 March 2009 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 Ordering Information Part Number Output Voltage Frequency Juction Temperature Range Package Adj 400KHz -40°C to +125°C 10-pin ePAD MSOP MIC2159YMME MIC2159YMME Pin Configuration 10-lead e-PAD MSOP (MME) Pin Description Pin Number (MSOP-10 MSOP-10) Pin Name Pin Function 1 VIN Supply voltage (Input): 3V to 14.5V 2 VDD 5V Internal Linear Regulator (Output): VDD is the external MOSFET gate drive supply voltage and an internal supply bus for the IC. When Vin is 5V, VDD should be connected to Vin. 3 CS Current Sense. Current-limit comparator non-inverting input. The current limit is sensed across the FET during the ON time. The current can be set by the resistor in series with the CS pin. 4 COMP/EN 5 FB 6 GND Ground (Return) 7 LSD Low-Side Drive (Output): High-current driver output for external synchronous MOSFET. 8 VSW Switch (Return): High-side MOSFET driver return 9 HSD High-Side Drive (Output): High current output-driver for the high-side MOSFET. Compensation (Input): Dual function pin. Pin for external compensation. If this pin is pulled below 0.2V, with the reference fully up the device shuts down (50A typical current draw) Feedback (Input): Input to error amplifier. Regulates error amplifier to 0.8V. When Vin is below 5v, 2.5v threshold FETs should be used. At Vin > 5V, 4.5V threshold FETs should be used. 10 March 2009 BST Boost (Input): Provides the drive voltage for the high-side MOSFET driver. The gate drive voltage is higher than the source voltage by VIN minus a diode drop. 2 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 Absolute Maximum Rating(1) Operating Ratings(2) Supply Voltage (VIN) .15.5V Booststrapped Voltage (VBST) . VIN +5V Junction Temperature Range.­40°C TJ +125°C Ambient Storage Temp .­65°C to +150°C Supply Voltage. +3V to +14.5V Output Voltage Range . 0.8V to VIN*DMAX Package Thermal Resistance JA 10-lead ePAD MSOP.63°C/W Electrical Characteristics Tj = 25ºC, bold values indicate -40ºC 5V 15 ns Source, VIN= 4.5V 2.5 3 Sink, VIN = 4.5V 2.5 3 Source, VIN= 3V 3 4 Sink, VIN = 3V 3 4 Note 3 50 Output Driver Impedance Driver Non-overlap Time ns Notes: 1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Guaranteed by design 4. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. 5. Specification for packaged product only. March 2009 4 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 Typical Characteristics March 2009 5 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 Block Diagram MIC2159 MIC2159 Block Diagram Functional Description The MIC2159 MIC2159 is a voltage mode, synchronous stepdown switching regulator controller designed for high power. Current limit is implemented without the use of an external sense resistor. It includes an internal soft-start function which reduces the power supply input surge current at start-up by controlling the output voltage rise time, a PWM generator, a reference voltage, two MOSFET drivers, and short-circuit current limiting circuitry to form a complete 400kHz switching regulator. amplifier. To illustrate the control loop, assume the output voltage drops due to sudden load turn-on, this would cause the inverting input of the error amplifier, which is the divided down version of VOUT, to be slightly less than the reference voltage, causing the output voltage of the error amplifier to go high. This will cause the PWM comparator to increase tON time of the top side MOSFET, causing the output voltage to go up and bringing VOUT back in regulation. If this sudden load transient was large enough to cause a 3% change in output voltage, then the output of the Hysteretic comparator will bypass the PWM comparator and drive the LSD and HSD outputs at full Duty cycle in order to recover the nominal output voltage in the fastest manner possible whilst fixed frequency PWM switching is maintained during normal loading. Theory of Operation The MIC2159 MIC2159 is a voltage mode step-down regulator. The figure above illustrates the block diagram for the voltage control loop. The output voltage variation due to load or line changes will be sensed by the inverting input of the transconductance error amplifier via the feedback resistors R3, and R2 and compared to a reference voltage at the non-inverting input. This will cause a small change in the DC voltage level at the output of the error amplifier which is the input to the PWM comparator. The other input to the comparator is a 1v triangular waveform. The comparator generates a rectangular waveform whose width tON is equal to the time from the start of the clock cycle t0 until t1, the time the triangle crosses the output waveform of the error March 2009 Soft-Start The COMP/EN pin on the MIC2159 MIC2159 is used for the following three functions: 1. Disables the part by grounding this pin. 2. External compensation to stabilize the voltage control loop. 3. Soft-start. 6 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 For better understanding of the soft-start feature, assume VIN = 12V and the MIC2159 MIC2159 is allowed to power-up by un-grounding the COMP/EN pin. The COMP pin has an internal 8.5µA current source that charges the external compensation capacitor. As soon as this voltage rises to 180mV (t = Cap_COMP × 0.18V/8.5µA), the MIC2159 MIC2159 allows the internal VDD linear regulator to power up and as soon as it crosses the under-voltage lockout of 2.6V, the chip's internal oscillator starts switching. At this point in time, the COMP pin current source increases to 40µA and an internal 12-bit counter starts counting, which takes approximately 2ms to complete. During counting, the COMP voltage is clamped at 0.65V. After this counting cycle the COMP current source is reduced to 8.5µA and the COMP pin voltage rises from 0.65V to 0.95V, the bottom edge of the saw-tooth oscillator. This is the beginning of 0% duty cycle and it increases slowly causing the output voltage to rise slowly. The MIC2159 MIC2159 has two hysteretic comparators that are enabled when VOUT is outside ±3% of steady state. When the output voltage reaches 97% of programmed output voltage then the gm error amplifier is enabled along with the hysteretic comparator. From this point onwards, the voltage control loop (gm error amplifier) is fully in control and will regulate the output voltage. Soft-start time can be calculated approximately by adding the following four time frames: Figure 1. MIC2159 MIC2159 Current Limiting Circuit The current limiting resistor RCS is calculated by the following equation: R CS = 1 IRIPPLE 2 Where: IRIPPLE = VOUT VIN VIN - VOUT FSWITCHING L FSWITCHING = 400kHz 200µA is the internal sink current to program the MIC2159 MIC2159 current limit. The MOSFET RDS(ON) varies 30% to 40% with temperature; therefore, it is recommended to add a 50% margin to the load current (ILOAD) in the above equation to avoid false current limiting due to increased MOSFET junction temperature rise. It is also recommended to connect RCS resistor directly to the drain of the top MOSFET Q1, and the RSW resistor to the source of Q1 to accurately sense the MOSFETs RDS(ON). To make the MIC2159 MIC2159 insensitive to board layout and noise generated by the switch node, a 1.4 resistor and a 1000pF capacitor is recommended between the switch node and GND. A 0.1µF capacitor in parallel with RCS should be connected to filter some of the switching noise. VOUT 0.5 Cap_COMP VIN 8.5. Soft-Start Time(Cap_COMP=100nF) = t1 + t2 + t3 + t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms Current Limit The MIC2159 MIC2159 uses the RDS(ON) of the top power MOSFET to measure output current. Since it uses the drain to source resistance of the power MOSFET, it is not very accurate. This MOSFET scheme is adequate to protect the power supply and external components during a fault condition by cutting back the time the top MOSFET is on if the feedback voltage is greater than 0.67V. In case of a hard short when feedback voltage is less than 0.67V, the MIC2159 MIC2159 discharges the COMP capacitor to 0.65V, resets the digital counter and automatically shuts off the top gate drive, and the gm error amplifier and the +/- 3% hysteretic comparators are completely disabled and the soft-start cycles restarts. This mode of operation is called the "hiccup mode" and its purpose is to protect the down stream load in case of a hard short. The circuit in Figure 1 illustrates the MIC2159 MIC2159 current limiting circuit. March 2009 Equation (1) 200 A IL = ILOAD + t1 = Cap_COMP × 0.18V/8.5µA t2 = internal counter, approx 2ms t3 = Cap_COMP × 0.3V/8.5µA t4 = R DS(ON) IL Internal VDD Supply The MIC2159 MIC2159 controller internally generates VDD for self biasing and to provide power to the gate drives. This VDD supply is generated through a low-dropout regulator and generates 5V from VIN supply greater than 5V. For supply voltage less than 5V, the VDD linear regulator is approximately 200mV in dropout. Therefore, it is recommended to short the VDD supply to the input supply through a 5 resistor for input supplies between 2.9V to 5V. 7 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 The drive voltage is derived from the internal 5V VDD bias supply. The nominal low-side gate drive voltage is 5V and the nominal high-side gate drive voltage is approximately 4.5V due the voltage drop across D1. An approximate 50ns delay between the low-side(off) to high-side(on) driver transitions is used to prevent current from simultaneously flowing unimpeded through both MOSFETs (shoot-through). Adaptive gate drive is implemented on the high-side(off) to low-side(on) driver transition to reduce losses in the flywheel diode and to prevent shoot-through. This is operated by detecting the VSW pin; once this pin is detected to reach 1.5v, the high-side MOSFET can be assumed to be off and the low side driver is enabled. MOSFET Gate Drive The MIC2159 MIC2159 high-side drive circuit is designed to switch an N-Channel MOSFET. The Functional Block Diagram shows a bootstrap circuit, consisting of D1 and CBST. This circuit supplies energy to the high-side drive circuit. Capacitor CBST circuit is charged while the lowside MOSFET is on and the voltage on the VSW pin is approximately 0V. When the high-side MOSFET driver is turned on, energy from CBST is used to turn the MOSFET on. As the MOSFET turns on, the voltage on the VSW pin increases to approximately VIN. Diode D1 is reversed biased and CBST floats high while continuing to keep the high-side MOSFET on. When the low-side switch is turned back on, CBST is recharged through D1. March 2009 8 M9999-030509-B M9999-030509-B Micrel, Inc. MIC2159 MIC2159 the on resistance times the total gate charge RDS(ON) × QG. Lower numbers translate into higher efficiency. Low gate-charge logic-level MOSFETs are a good choice for use with the MIC2159 MIC2159. Parameters that are important to MOSFET switch selection are: Application Information MOSFET Selection The MIC2159 MIC2159 controller works from input voltages of 3V to 14.5V and has an internal 5V regulator to provide power to turn the external N-Channel power MOSFETs for high- and low-side switches. For applications where VIN < 5V, the internal VDD regulator operates in dropout mode, and it is necessary that the power MOSFETs used are sub-logic level and are in full conduction mode for VGS of 2.5V. For applications when VIN > 5V; logiclevel MOSFETs, whose operation is specified at VGS = 4.5V must be used. For the lower (