MIC3838/3839 Micrel MIC3838/3839 Flexible Push-Pull PWM Controller General Description Features The MIC3838 and MIC3839 are a family of complementary Dual output drive stages in push-pull configuration output push-pull PWM control ICs that feature high speed and Configurable for current-mode or voltage-mode control low power consumption. The MIC3838/9 are ideal for telecom Easily implements volt-second clamp level (36V to 75V) isolated step down dc/dc conversion Leading edge current-sense blanking applications where high output current, small size, and high 3V reference output available efficiency are required 130A typical start-up current 1mA typical run current The MIC3838/9 are designed for high flexibility with minimum Operation to 1MHz pin-count. The devices are easily configurable for either On-chip error amplifier with 4MHz gain bandwidth voltage-mode or current-mode control. Additionally, the product MIC3838/9 can easily implement a volt-second clamp that Internal soft start automatically limits the duty cycle during input transients, On-chip V clamping allowing designers to use the smallest possible transformers DD Output drive stages capable of 500mA peak source and power components. A 3V reference output is also avail- current, 1A peak sink current able that eliminates the need for an external reference. Applications The dual-ended push-pull architecture of the MIC3838/9 allows more efficient utilization of the transformer than single- High efficiency brick power supply modules ended topologies, allowing smaller size dc/dc solutions. Half bridge converters Additionally, the out-of-phase push-pull topology allows a Full bridge converters higher effective duty cycle, reducing input and output ripple Push-pull converters as well as stress on the external components. The dead-time Voltage-fed push-pull converters between the two outputs is adjustable between 60ns to Telecom equipment and power supplies 200ns, limiting the duty cycle of each output stage to less than Industrial power supplies 50%. 42V automotive power supplies The MIC3838 has a turn-on threshold of 12.5V whereas the Base stations MIC3839 has a lower turn-on threshold of 4.3V. Both devices Networking power supplies are available in a small size MSOP-10 package with an operating range of 40C to +85C. Data sheets and support documentation can be found on Micrels web site at www.micrel.com. Typical Application V IN V 36V to 75V OUT 12V 100W Start-Up Circuitry MIC3838x-x VREF VDD RC OUTA GND OUTB COMP RAMP Feed Forward Ramp/ Volt Sec Clamp FB ILIM Reference & Isolation Voltage-Mode Half-Bridge Converter CIrcuit Micrel, Inc. 2180 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 MIC3838/3839 Micrel Ordering Information Part Number Turn On Turn Off Standard Lead-Free Threshold Threshold Temperature Range Package MIC3838BMM MIC3838YMM 12.5V 8.3V 40C to +85C 10-Pin MSOP MIC3839BMM MIC3839YMM 4.3V 4.1V 40C to +85C 10-Pin MSOP Pin Configuration COMP 1 10 VREF FB 2 9 VDD ILIM 3 8 OUTA RAMP 4 7 OUTB RC 5 6 GND MSOP-10 (MM) Pin Description Pin Number Pin Name Pin Function 1 COMP COMP is the output of the error amplifier and the input of the PWM comparator. The error amplifier in the MIC3838 is a true low-output imped- ance, 4MHz operational amplifier. As such, the COMP pin can both source and sink current. However, the error amplifier is internally current limited, so that zero duty cycle can be externally forced by pulling COMP to GND. The MIC3838 family features built-in full cycle soft start. Soft start is imple- mented as a clamp on the maximum COMP voltage. 2 FB The inverting input to the error amplifier. 3 ILIM The input to the peak current, and overcurrent comparators. The overcurrent comparator is only intended for fault sensing. Exceeding the overcurrent threshold will cause a soft start cycle. An internal MOSFET discharges the current sense filter capacitor to improve dynamic perfor- mance of the power converter. 4 RAMP Input to the PWM comparator. Sawtooth ramp for PWM control. Allows for either current-mode or voltage-mode control. An internal MOSFET dis- charges the current sense filter capacitor. 5 RC The oscillator programming pin. Only two components are required to program the oscillator, a resistor (tied between V and RC), and a capaci- DD tor (tied between RC and GND). The approximate oscillator frequency is 1.41 F = determined by the simple formula: OSCILLATOR RC The recommended range of timing resistors is between 7k and 200k and range of timing capacitors is between 100pF and 1000pF. Timing resistors less than 7k should be avoided. For best performance, keep the leads between components as short as possible. Separate ground and VDD traces to the external timing network are encouraged. 6 GND Ground. Return path for signal and gate drive functions. MIC3838/3839 2 April 2005